Oscilloscope

The Agilent InfiniiVision 7000 Series is a digital storage oscilloscope which can capture and analyze analog and digital signals

An oscilloscope (commonly abbreviated to scope or O-scope) is a type of electronic test instrument that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis). Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change, or change slowly. The oscilloscope is one of the most versatile and widely-used electronic instruments. ^[1]

Oscilloscopes are widely used when it is desired to observe the exact wave shape of an electrical signal. In addition to the amplitude of the signal, an oscilloscope can measure the frequency, show distortion, show the time between two events (such as pulse width or pulse rise time), and show the relative timing of two related signals. Some better modern digital oscilloscopes can analyze and display the spectrum of a repetitive event. Special-purpose oscilloscopes, called spectrum analyzers, have sensitive inputs and can display spectra well into the GHz range. A few oscilloscopes that accept plug-ins can display spectra in the audio range.

Oscilloscopes are used in the sciences, medicine, engineering, telecommunications, and industry. General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as adjusting an automotive ignition system, or to display the waveform of the heartbeat.

Originally all oscilloscopes used cathode ray tubes as their display element and linear amplifiers for signal processing, but modern oscilloscopes can have LCD or LED screens, high-speed analog-to-digital converters and digital signal processors. Although not as commonplace, some oscilloscopes used storage CRTs to capture single events and display them for a limited time. Oscilloscope peripheral modules for general purpose laptop or desktop personal computers use the computer's display, and can turn them into useful and flexible test instruments.

Features and uses

Description

Display and general external appearance

A typical oscilloscope has a display screen, numerous input connectors, and control knobs and buttons on the front panel. Portable instruments are small enough to carry to a work site and may even be battery operated. Laboratory grade 'scopes, especially old instruments using vacuum tubes, are bench-top devices. Special purpose 'scopes may be permanently mounted in a rack. To aid measurement, a grid called the graticule is superimposed on the face of the screen. Each square in the graticule is known as a (major) division.

Size and portability

Large bench-top oscilloscopes were sometimes mounted on carts to allow sharing one expensive instrument among several work areas. Miniaturized oscilloscopes were of great value for field service equipment repair. Today even a very capable laboratory instrument can be lifted by a single person, and hand-held digital oscilloscopes are made by several manufacturers.

Inputs

The signal to be measured is fed to one of the input connectors, which is usually a coaxial connector such as a BNC or UHF type. Binding posts or banana plugs may be used for lower frequencies. If the signal source has its own coaxial connector, then a simple coaxial cable is used; otherwise, a specialised cable called a "scope probe", supplied with the oscilloscope, is used. In general, for routine use, an open wire test lead for connecting to the point being observed is not satisfactory, and a probe is generally necessary. General-purpose oscilloscopes have a standardised input resistance of 1 megohm in parallel with a capacitance of around 20 picofarads. This allows the use of standard oscilloscope probes. Scopes for use with very high frequencies may have 50-ohm inputs, which must be either connected directly to a 50-ohm signal source or used with Z₀ or active probes.

Less-frequently-used inputs include one (or two) for triggering the sweep, horizontal deflection for X-Y mode displays, and trace brightening/darkening, sometimes called "Z-axis" inputs.

Probes

Open wire test leads are likely to pick up interference, and their capacitance at the probing end is likely to disturb the circuit/device being examined. They are appropriate only for low frequencies and low-impedance devices. Nearly always, probes made for 'scope use are the ordinary means of connecting to the device being examined. The probe cable is a special coaxial type (with a resistive center conductor to damp out ringing), with quite-effective shielding. Its capacitance is greater than that of an open wire, and in some cases, such a probe is satisfactory.

However, a typical 'scope probe contains a 9-megohm series resistor shunted by a low-value capacitor; combined with the input resistance and capacitance of a standard 'scope input, the probe and the 'scope input form a fairly-accurate 10:1 attenuator that (up to a certain bandwidth) is frequency-independent. This degrades the 'scope's sensitivity by a factor of 10, but the capacitance at the probe tip as only a few pF (picofarads), which is not enough to disturb many typical circuits. (Nevertheless, the reactance of even that few pF is significantly low at high frequencies within the probe and 'scope's bandwidth.) In the great majority of cases, the loss of sensitivity in order to gain less disturbance to the circuit being observed is very worth while.

Attenuator probes do not necessarily match the input of a given 'scope, and their capacitance needs to be adjusted if they are connected to a different 'scope. As well, they should be checked periodically even when not moved. They are checked and if necessary adjusted by looking at a square wave with a quite-flat top and bottom. When properly adjusted, the horizontal trace of the square wave does not tilt either upward or downward. Because the probe, combined with the 'scope input, forms a frequency-compensated attenuator, this procedure is often called "compensating" a probe. Any decent 'scope has an output jack that provides a known-amplitude square wave with excellent shape for checking and adjusting probes.

Probes with 10:1 attenuation are by far the most common; for large signals (and slightly-less capacitive loading), 100:1 probes are not rare. There are also probes that contain switches to select 10:1 or direct (1:1) ratios, but one must be aware that the 1:1 setting has significant capacitance (tens of pF) at the probe tip, because the whole cable's capacitance is now directly connected.

Good 'scopes allow for probe attenuation, easily showing effective sensitivity at the probe tip. Some of the best ones have indicator lamps behind translucent windows in the panel to prompt the user to read effective sensitivity. The probe connectors (modified BNC's) have an extra contact to define the probe's attenuation. (A certain value of resistor, connected to ground, "encodes" the attenuation.)

There are special high-voltage probes which also form compensated attenuators with the 'scope input; the probe body is physically large, and one made by Tektronix requires partly filling a canister surrounding the series resistor with volatile liquid fluorocarbon to displace air. At the 'scope end is a box with several waveform-trimming adjustments. For safety, a barrier disc keeps one's fingers distant from the point being examined. Maximum voltage is in the low tens of kV. (Observing a high-voltage ramp can create a staircase waveform with steps at different points every repetition, until the probe tip is in contact. Until then, a tiny arc charges the probe tip, and its capacitance holds the voltage (open circuit). As the voltage continues to climb, another tiny arc charges the tip further.)

There are also current probes, with cores that surround the conductor carrying current to be examined. One type has a hole for the conductor, and requires that the wire be passed through the hole; it's for semi-permanent or permanent mounting. However, other types, for testing, have a two-part cores that permit them to be placed around a wire. Inside the probe, a coil wound around the core provides a current into a appropriate load, and the voltage across that load is proportional to current. However, this type of probe can sense AC, only.

A more-sophisticated probe (originally made by Tektronix) includes a magnetic flux sensor (Hall-effect) in the magnetic circuit. The probe connects to an amplifier, which feeds (low frequency) current into the coil to cancel the sensed field; the magnitude of that current provides the low-frequency part of the current waveform, right down to DC. The coil still picks up high frequencies. There is a combining network akin to a loudspeaker crossover network.

The trace

In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds or decimal fractions of a second per division. If the input voltage departs from zero, the trace is deflected either upwards (normally for positive polarity) or downwards (negative). Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a plot of voltage against time, with the more distant past on the left and the more recent past on the right.

Front Panel Controls

Focus Control

This control adjusts CRT focus to obtain the sharpest, most-detailed trace. In practice, focus needs to be adjusted slightly when observing quite-different signals, which means that it needs to be an external control. Flat-panel displays do not need a focus control; their sharpness is always optimum. .....

Intensity Control

This adjusts trace brightness. Slow traces on CRT 'scopes need less, and fast ones, especially if they don't repeat very often, require more. On flat panels, however, trace brightness is essentially independent of sweep speed, because the internal signal processing effectively synthesizes the display from the digitized data.

Beam Finder

Modern 'scopes have direct-coupled deflection amplifiers, which means the trace could be deflected off-screen. They also might have their CRT beam blanked without the operator knowing it. In such cases, the screen is blank. To help in restoring the display quickly and without experimentation, the beam finder circuit overrides any blanking and ensures that the beam will not be deflected off-screen; it limits the deflection. With a display, it's usually very easy to restore a normal display. (While active, beam-finder circuits might temporarily distort the trace severely, but that's OK.)

Graticule

The graticule is a grid of squares that serve as reference marks for measuring the displayed trace. These markings, whether located directly on the screen or on a removable plastic filter, usually consist of a 1 cm grid with closer tick marks (often at 2 mm) on the centre vertical and horizontal axis. One expects to see ten major divisions across the screen; the number of vertical major divisions varies. Comparing the grid markings with the waveform permits one to measure both voltage (vertical axis) and time (horizontal axis). Frequency can also be determined by measuring the waveform period and calculating its reciprocal.

On old and lower-cost CRT 'scopes the graticule is a sheet of plastic, often with light-diffusing markings and concealed lamps at the edge of the graticule. The lamps had a brightness control. Higher-cost instruments have the graticule marked on the inside face of the CRT, to eliminate parallax errors; better ones also had adjustable edge illumination with diffusing markings. (Diffusing markings appear bright.) Digital 'scopes, however, generate the graticule markings on the display in the same way as the trace.

External graticules also protect the glass face of the CRT from accidental impact. Some CRT 'scopes with internal graticules have an unmarked tinted sheet plastic light filter to enhance trace contrast; this also serves to protect the faceplate of the CRT.

Accuracy and resolution of measurements using a graticule is relatively limited; better 'scopes sometimes have movable bright markers on the trace that permit internal circuits to make more refined mesaurements.

Both calibrated vertical sensitivity and calibrated horizontal time are set in 1 - 2 - 5 - 10 steps. This leads, however, to some awkward interpretations of minor divisions. At 2, each of the five minor divisions is 0.4, so one has to think 0.4, 0.8, 1.2, and 1.6, which is rather awkward. One Tektronix plug-in used a 1 - 2.5 - 5 - 10 sequence, which simplified estimating. The "2.5" didn't look as "neat", but was very welcome.

Timebase Controls

These select the horizontal speed of the CRT's spot as it creates the trace; this process is commonly referred to as the sweep. In all but the least-costly modern 'scopes, the sweep speed is selectable and calibrated in units of time per major graticule division. Quite a wide range of sweep speeds is generally provided, from seconds to as fast as picoseconds (in the fastest 'scopes) per division. Usually, a continuously-variable control (often a knob in front of the calibrated selector knob) offers uncalibrated speeds, typically slower than calibrated. This control provides a range somewhat greater than that of consecutive calibrated steps, making any speed available between the extremes.

Holdoff Control

Found on some better analog 'scopes, this varies the time (holdoff) during which the sweep circuit ignores triggers. It provides a stable display of some repetitive events in which some triggers would create confusing displays. It is usually set to minimum, because a longer time decreases the number of sweeps per second, resulting in a dimmer trace.

Vertical Sensitivity, Coupling, and Polarity Controls

To accommodate a wide range of input amplitudes, a switch selects calibrated sensitivity of the vertical deflection. Another control, often in front of the calibrated-selector knob, offers a continuously-variable sensitivity over a limited range from calibrated to less-sensitive settings.

Often, but not always, the observed signal is offset by a steady component, and only the changes are of interest. A switch (AC position) connects a capacitor in series with the input that passes only the changes (provided that they are not too slow -- "slow" would mean visible). However, when the signal has a fixed offset of interest, or changes quite slowly, the input is connected directly (DC switch position). Any decent 'scope displays DC. For convenience, to see where zero volts input currently shows on the screen, many 'scopes have a third switch position (GND) that disconnects the input and grounds it. Often, in this case, the user centers the trace with the Vertical Position control.

Better 'scopes have a polarity selector. Normally, a positive input moves the trace upward, but this permits inverting -- positive deflects the trace downward.

Horizontal Sensitivity Control

This control is found only on more elaborate 'scopes; it offers adjustable sensitivity for external horizontal inputs.

Vertical Position Control

Moves the whole displayed trace up and down. Often used to set the no-input trace exactly on the center line of the graticule, but permits offsetting vertically by a limited amount. With direct coupling, can compensate for a limited DC component of an input.

Horizontal Position Control

Moves the display sidewise. Usually sets the left end of the trace at the left edge of the graticule, but can displace the whole trace when desired. Also moves X-Y mode traces sidewise in some 'scopes, and can compensate for a limited DC component as for vertical position.

Dual-Trace Controls

* (Please see Dual and Multiple-trace Oscilloscopes, below.)

Each input channel usually has its own set of sensitivity, coupling, and position controls, although some four-trace 'scopes have only mimimal controls for their third and fourth channels.

Dual-trace 'scopes have a mode switch to select either channel alone, both channels, or (in some 'scopes) an X-Y display, which uses the second channel for X deflection. When both channels are displayed, the type of channel switching can be selected on some 'scopes; on others, the type depends upon timebase setting. If manually selectable, channel switching can be free-running (asynchronous), or between consecutive sweeps. Some Philips dual-trace analog 'scopes had a fast analog multiplier, and provided a display of the product of the input channels.

Multiple-trace 'scopes have a switch for each channel to enable or disable display of that trace's signal.

Delayed-Sweep Controls

* (Please see Delayed Sweep, below.)

These include controls for the delayed-sweep timebase, which is calibrated, and often also variable. The slowest speed is several steps faster than the slowest main sweep speed, although the fastest is generally the same. A calibrated multiturn delay time control offers wide range, high resolution delay settings; it spans the full duration of the main sweep, and its reading corresponds to graticule divisions (but with much finer precision). Its accuracy is also superior to that of the display.

A switch selects display modes: Main sweep only, with a brightened region showing when the delayed sweep is advancing, delayed sweep only, or (on some 'scopes) a combination mode.

Good CRT 'scopes include a delayed-sweep intensity control, to allow for the dimmer trace of a much-faster delayed sweep that nevertheless occurs only once per main sweep. Such 'scopes also are likely to have a trace separation control for multiplexed display of both the main and delayed sweeps together.

Sweep Trigger Controls

* (Please see Triggered Sweep, below.)

A switch selects the Trigger Source. It can be an external input, one of the vertical channels of a dual or multiple-trace 'scope, or the AC line (mains) frequency. Another switch enables or disables Auto trigger mode, or selects single sweep, if provided in the 'scope. Either a spring-return switch position or a pushbutton arms single sweeps.

A Level control varies the voltage on the waveform which generates a trigger, and the Slope switch selects positive-going or negative-going polarity at the selected trigger level.

Basic types of sweeps

Triggered sweeps

To display events with unchanging or slowly (visibly) changing waveforms, but occurring at times that may or may not be evenly spaced, modern oscilloscopes have triggered sweeps. Compared to simpler 'scopes with sweep oscillators that are always running, triggered-sweep 'scopes are markedly more versatile.

A triggered sweep starts at a selected point on the signal, providing a stable display. In this way, triggering allows the display of periodic signals such as sine waves and square waves, as well as nonperiodic signals such as single pulses, or pulses that don't recur at a fixed rate.

With triggered sweeps, the scope will blank the beam and start to reset the sweep circuit each time the beam reaches the extreme right side of the screen. For a period of time, called holdoff, (extendable by a front-panel control on some better 'scopes), the sweep circuit resets completely and ignores triggers. Once holdoff expires, the next trigger starts a sweep. The trigger event is usually the input waveform reaching some user-specified threshold voltage (trigger level) in the specified direction (going positive or going negative -- trigger polarity).

In some cases, variable holdoff time can be really useful to make the sweep ignore interfering triggers that occur before the events one wants to observe. In the case of repetitive, but quite-complex waveforms, variable holdoff can create a stable display that can't otherwise practically be obtained.

Automatic sweep mode

Triggered sweeps can offer a blank screen if there are no triggers. To avoid this, these sweeps include a a timing circuit (millisecond range) that generates free-running triggers to provide a trace. Once triggers arrive, this timer stops providing pseudo-triggers. For observing low repetition rates, this mode can be de-selected.

Recurrent sweeps

If the input signal is periodic, the sweep repetition rate can be adjusted to display a few cycles of the waveform. Early (tube) 'scopes and lowest-cost 'scopes have sweep oscillators that run continuously, and are uncalibrated. Such oscilloscopes are very simple, comparatively inexpensive, and were useful in radio servicing and some TV servicing. Measuring voltage or time is possible, but only with extra equipment, and is quite inconvenient. They are primarily qualitative instruments.

They have a few (widely spaced) frequency ranges, and relatively wide-range continuous frequency control within a given range. In use, the sweep frequency is set to slightly lower than some submultiple of the input frequency, to display typically at least two cycles of the input signal (so all details are visible). A very simple control feeds an adjustable amount of the vertical signal (or possibly, a related external signal) to the sweep oscillator. The signal triggers beam blanking and a sweep retrace sooner than it would occur free-running, and the display becomes stable.

Single Sweeps

Some 'scopes offer these -- the sweep circuit is manually armed (typically by a pushbutton or equivalent) "Armed" means it's ready to respond to a trigger. Once the sweep is complete, it resets, and will not sweep until re-armed. This mode, combined with a 'scope camera, captures single-shot events.

Types of trigger include:

external trigger, a pulse from an external source connected to a dedicated input on the scope.
edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction. These are the most-common types of triggers; the level control sets the threshold voltage, and the slope control selects the direction (negative or positive-going). (The first sentence of the description also applies to the inputs to some digital logic circuits; thoes inputs have fixed threshold and polarity response.)
video trigger, a circuit that extracts synchronizing pulses from video formats such as PAL and NTSC and triggers the timebase on every line, a specified line, every field, or every frame. This circuit is typically found in a waveform monitor device, although some better 'scopes include this function.
delayed trigger, which waits a specified time after an edge trigger before starting the sweep. As described under delayed sweeps, a trigger delay circuit (typically the main sweep) extends this delay to a known and adjustable interval. In this way, the operator can examine a particular pulse in a long train of pulses.

Some recent designs of 'scopes include more sophisticated triggering schemes; these are described toward the end of this article.

Delayed sweeps

These are found on more-sophisticated 'scopes, which contain a second set of timebase circuits for a delayed sweep. A delayed sweep provides a very-detailed look at some small selected portion of the main timebase. The main timebase serves as a controllable delay, after which the delayed timebase starts. This can start when the delay expires, or can be triggered (only) after the delay expires. Ordinarily, the delayed timebase is set for a faster sweep, sometimes much faster, such as 1000:1. At extreme ratios, jitter in the delays on consecutive main sweeps degrades the display, but delayed-sweep triggers can overcome that.

The display shows the vertical signal in one of several modes -- the main timebase, or the delayed timebase only, or a combination. When the delayed sweep is active, the main sweep trace brightens while the delayed sweep is advancing. In one combination mode, provided only on some 'scopes, the trace changes from the main sweep to the delayed sweep once the delayed sweep starts, although less of the delayed fast sweep is visible for longer delays. Another combination mode multiplexes (alternates) the main and delayed sweeps so that both appear at once; a trace separation control displaces them.

Dual and Multiple-trace Oscilloscopes

Oscilloscopes with two vertical inputs, referred to as dual-trace 'scopes, are extremely useful and commonplace. Usisg a single-beam CRT, they time-multiplex the inputs, usually switching between them fast enough to display two traces apparently at once. Less common are 'scopes with more traces; four inputs are common among these, but a few (Kikusui, for one) offered a display of the sweep trigger signal if desired. Some multi-trace 'scopes use the external trigger input as an optional vertical input, and some have third and fourth channels with only minimal controls. In all cases, the inputs, when independently displayed, are time-multiplexed, but dual-trace 'scopes often can add their inputs to display a real-time analog sum. (Inverting one channel provides a difference, provided that neither channel is overloaded. This difference mode can provide a moderate-performance differential input.)

Switching channels can be asynchronous, that is, free-running, with trace blanking while switching, or after each horizontal sweep is complete. Asynchronous switching is usually designated "Chopped", while sweep-synchronized is designated "Alt[ernate]". A given channel is alternately connected and disconnected, leading to the term "chopped". Multi-trace 'scopes also switch channels either in Chopped or Alt modes.

In general, Chopped mode is better for slower sweeps. It's possible for the internal chopping rate to be a multiple of the sweep repetition rate, creating blanks in the traces, but in practice this is rarely a problem; the gaps in one trace are overwritten by traces of thte following sweep. A few 'scopes had a modulated chopping rate to avoid this occasional problem. Alternate mode, however, is better for faster sweeps.

True dual-beam CRT 'scopes did exist, but were not common. One type (Cossor, U.K.) had a beam-splitter plate in its CRT, and single-eneded deflection following the splitter. (More details are near the end of this article; see "CRT Invention". Others had two complete electron guns, requiring tight control of axial (rotational) mechanical alignment in manufacturing the CRT. Beam-splitter types had horizontal deflection common to both vertical channels, but dual-gun 'scopes could have separate time bases, or use one time base for both channels. Multiple-gun CRTs (up to ten guns!) were made in past decades. With ten guns, the envelope (bulb) was cylindrical throughout its length.

The Vertical Amplifier

In an analog 'scope, the vertical amplifier acquires the signal[s] to be displayed. In better 'scopes, it delays them by a fraction of a microsecond, and provides a signal large enough to deflect the CRT's beam. That deflection is at least somewhat beyond the edges of the graticule, and more typically some distance off-screen. The amplifier has to have low distortion to display its input accurately (it must be linear), and it has to recover quickly from overloads. As well, its time-domain response has to represent transients accurately -- minimal overshoot, rounding, and tilt of a flat pulse top.

A vertical input goes to a frequency-compensated step attenuator to reduce large signals to prevent overload. The attenuator feeds a low-level stage (or a few), which in turn feed gain stages (and a delay-line driver if there is a delay). Following are more gain stages, up to the final output stage which develops a large signal swing (tens of volts, sometimes over 100 volts) for CRT electrostatic deflection.

In dual and multiple-trace 'scopes, an internal electronic switch selects the relatively low-level output of one channel's amplifiers and sends it to the following stages of the vertical amplifier, which is only a single channel, so to speak, from that point on.

In free-running ("chopped") mode, the oscillator (which may be simply a different operating mode of the switch driver) blanks the beam before switching, and unblanks it only after the switching transients have settled.

Part way through the amplifier is a feed to the sweep trigger circuits, for internal triggering from the signal. This feed would be from an individual channel's amplifier in a dual or multi-trace 'scope, the channel depending upon the setting of the trigger source selector.

This feed precedes the delay (if there is one), which allows the sweep circuit to unblank the CRT and start the forward sweep, so the CRT can show the triggering event. High-quality analog delays add a modest cost to a 'scope, and are omitted in 'scopes that are cost-sensitive.

The delay, itself, comes from a special cable with a pair of conductors wound around a flexible magnetically-soft core. The coiling provides distributed inductance, while a conductive layer close to the wires provides distributed capacitance. The combination is a wideband transmission line with considerable delay per unit length. Both ends of the delay cable require matched impedances to avoid reflections.

Bandwidth

Bandwidth is a measure of the range of frequencies that can be displayed; it refers primarily to the vertical amplifier, although the horizontal deflection amplifier has to be fast enough to handle the fastest sweeps. The bandwidth of the 'scope is limited by the vertical amplifiers and the CRT (in analog instruments) or by the sampling rate of the analog to digital converter in digital instruments. The bandwidth is defined as the frequency at which the sensitivity is 0.707 of the sensitivity at lower frequency (a drop of 3 dB). The rise time of the fastest pulse that can be resolved by the scope is related to its bandwidth approximately:

Bandwidth in Hz x rise time in seconds = 0.35 ^[2]

For example, a 'scope intended to resolve pulses with a rise time of 1 nanosecond would have a bandwidth of 350 MHz.

For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten times the highest frequency desired to resolve; for example a 20 megasample/second rate would be applicable for measuring signals up to about 2 megahertz.

X-Y mode

Most modern oscilloscopes have several inputs for voltages, and thus can be used to plot one varying voltage versus another. This is especially useful for graphing I-V curves (current versus voltage characteristics) for components such as diodes, as well as Lissajous patterns. Lissajous figures are an example of how an oscilloscope can be used to track phase differences between multiple input signals. This is very frequently used in broadcast engineering to plot the left and right stereophonic channels, to ensure that the stereo generator is calibrated properly. Historically, stable Lissajous figures were used to show that two sine waves had a relatively simple frequency relationship, a numerically-small ratio. They also indicated phase difference between two sine waves of the same frequency.

Complete loss of signal in an X-Y display means that the CRT's beam strikes a small spot, which risks burning the phosphor. Older phosphors burned more easily. Some dedicated X-Y displays reduce beam current a lot, or blank the display entirely, if there are no inputs present.

Other features

Some oscilloscopes have cursors, which are lines that can be moved about the screen to measure the time interval between two points, or the difference between two voltages. A few older 'scopes simply brightened the trace at movable locations. These cursors are more accurate than visual estimates referring to graticule lines.

Better quality general purpose oscilloscopes include a calibration signal for setting up the compensation of test probes; this is (often) a 1 kHz square-wave signal of a definite peak-to-peak voltage available at a test terminal on the front panel. Some better 'scopes also have a squared-off loop for checking and adjusting current probes.

Sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes, known as "storage scopes", preserve the most recent sweep on the screen. This was originally achieved by using a special CRT, a "storage tube", which would retain the image of even a very brief event for a long time.

Some digital oscilloscopes can sweep at speeds as slow as once per hour, emulating a strip chart recorder. That is, the signal scrolls across the screen from right to left. Most oscilloscopes with this facility switch from a sweep to a strip-chart mode at about one sweep per ten seconds. This is because otherwise, the scope looks broken: it's collecting data, but the dot cannot be seen.

In current 'scopes, digital signal sampling is more often used for all but the simplest models. Samples feed fast analog-to-digital converters, following which all signal processing (and storage) is digital.

Many oscilloscopes have different plug-in modules for different purposes, e.g., high-sensitivity amplifiers of relatively narrow bandwidth, differential amplifiers, amplifiers with four or more channels, sampling plugins for repetitive signals of very high frequency, and special-purpose plugins, including audio/ultrasonic spectrum analyzers, and stable-offset-voltage direct-coupled channels with relatively high gain.

Examples of use

One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment. One of the advantages of a scope is that it can graphically show signals: where a voltmeter may show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other cases the precise shape or timing of a pulse is important.

In a piece of electronic equipment, for example, the connections between stages (e.g. electronic mixers, electronic oscillators, amplifiers) may be 'probed' for the expected signal, using the scope as a simple signal tracer. If the expected signal is absent or incorrect, some preceding stage of the electronics is not operating correctly. Since most failures occur because of a single faulty component, each measurement can prove that half of the stages of a complex piece of equipment either work, or probably did not cause the fault.

Once the faulty stage is found, further probing can usually tell a skilled technician exactly which component has failed. Once the component is replaced, the unit can be restored to service, or at least the next fault can be isolated. This sort of troubleshooting is typical of radio and TV receivers, as well as audio amplifiers, but can apply to quite-different devices such as electronic motor drives.

Another use is to check newly designed circuitry. Very often a newly designed circuit will misbehave because of design errors, bad voltage levels, electrical noise etc. Digital electronics usually operate from a clock, so a dual-trace scope which shows both the clock signal and a test signal dependent upon the clock is useful. Storage scopes are helpful for "capturing" rare electronic events that cause defective operation.

Another use is for software engineers who must program electronics. Often a scope is the only way to see if the software is running the electronics properly.

^{Pictures of use}

Heterodyne
AC hum on sound.
Sum of a low-frequency and a high-frequency signal.
Bad filter on sine.
Dual trace, showing different time bases on each trace.

Selection

Oscilloscopes generally have a checklist of some set of the above features. The basic measure of virtue is the bandwidth of its vertical amplifiers. Typical scopes for general purpose use should have a bandwidth of at least 100 MHz, although much lower bandwidths are acceptable for audio-frequency applications. A useful sweep range is from one second to 100 nanoseconds, with triggering and delayed sweep.

The chief benefit of a quality oscilloscope is the quality of the trigger circuit. If the trigger is unstable, the display will always be fuzzy. The quality improves roughly as the frequency response and voltage stability of the trigger increase.

Analog oscilloscopes have been almost totally displaced by digital storage scopes except for the low bandwidth (< 60 MHz) segment of the market. Greatly increased sample rates have eliminated the display of incorrect signals, known as "aliasing", that was sometimes present in the first generation of digital scopes. The used test equipment market, particularly on-line auction venues, typically have a wide selection of older analog scopes available. However it is becoming more difficult to obtain replacement parts for these instruments and repair services are generally unavailable from the original manufacturer.

As of 2007^[update], a 350 MHz bandwidth (BW), 2.5 giga-samples per second (GS/s), dual-channel digital storage scope costs about US$7000 new. The current real-time analog bandwidth record, as of February 2007^[update], is held by the Tektronix DPO70000 and DSA70000 oscilloscope families with a 20 GHz BW (non-interleaved) and a sample rate of 50 GHz. The current equivalent time sampling bandwidth record for sampling digital storage oscilloscopes, as of June 2006^[update], is held by the LeCroy WaveExpert series with a 100 GHz bandwidth.

Software

Many oscilloscopes today provide one or more external interfaces to allow remote instrument control by external software. These interfaces (or buses) include GPIB, Ethernet, serial port, and USB.

How it works

Cathode-ray oscilloscope (CRO)

The earliest and simplest type of oscilloscope consisted of a cathode ray tube, a vertical amplifier, a timebase, a horizontal amplifier and a power supply. These are now called 'analog' scopes to distinguish them from the 'digital' scopes that became common in the 1990s and 2000s.

Before the introduction of the CRO in its current form, the cathode ray tube had already been in use as a measuring device. The cathode ray tube is an evacuated glass envelope, similar to that in a black-and-white television set, with its flat face covered in a fluorescent material (the phosphor). The screen is typically less than 20 cm in diameter, much smaller than the one in a television set. Older 'scopes had round screens or faceplates, while newer CRTs in better 'scopes have rectangular faceplates.

In the neck of the tube is an electron gun, which is a small heated metal cylinder with a flat end coated with electron-emitting oxides. Close to it is a much-larger-diameter cylinder carrying a disc at its cathode end with a round hole in it; it's called a "grid" (G1), by historic analogy with amplifier vacuum-tube grids. A small negative grid potential (referred to the cathode) is used to block electrons from passing through the hole when the electron beam needs to be turned off, as during sweep retrace or when no trigger events occur.

However, when G1 becomes less negative with respect to the cathode, another cylindrical electrode designated G2, which is hundreds of volts positive referred to the cathode, attracts electrons through the hole. Their trajectories converge as they pass through the hole, creating quite-small diameter "pinch" called the crossover. Following electrodes ("grids"), electrostatic lenses, focus this crossover onto the screen; the spot is an image of the crossover.

Typically, the CRT cathode runs at roughly -2 kV or so, and various methods are used to correspondingly offset the G1 voltage. Proceeding along the electron gun, the beam passes through the imaging lenses and first anode, emerging with an energy in electron-volts equal to that of the cathode. The beam passes through one set of deflection plates , then the other, where it is deflected as required to the phosphor screen.

The average voltage of the deflection plates is relatively close to ground, because they have to be directly connected to the vertical output stage.

By itself, once the beam leaves the deflection region, it can produce a usefully-bright trace. However, for higher bandwidth oscilloscopes where the trace may move more rapidly across the phosphor screen, a positive post-deflection acceleration ("PDA") voltage of over 10,000 volts is often used, increasing the energy (speed) of the electrons that strike the phosphor. The kinetic energy of the electrons is converted by the phosphor into visible light at the point of impact.

When switched on, a CRT normally displays a single bright dot in the center of the screen, but the dot can be moved about electrostatically or magnetically. The CRT in an oscilloscope always uses electrostatic deflection. Ordinary electrostatic deflection plates can typically move the beam roughly only 15 degrees or so off-axis, which means that 'scope CRTs have long, narrow funnels, and for their screen size, are usually quite long. It's the CRT length that makes CRT 'scopes "deep", from front to back. Modern flat-panel 'scopes have no need for such rather-extreme dimensions; their shapes tend to be more like one kind of rectangular lunchbox.

Between the electron gun and the screen are two opposed pairs of metal plates called the deflection plates. The vertical amplifier generates a potential difference across one pair of plates, giving rise to a vertical electric field through which the electron beam passes. When the plate potentials are the same, the beam is not deflected.

When the top plate is positive with respect to the bottom plate, the beam is deflected upwards; when the field is reversed, the beam is deflected downwards. The horizontal amplifier does a similar job with the other pair of deflection plates, causing the beam to move left or right. This deflection system is called electrostatic deflection, and is different from the electromagnetic deflection system used in television tubes. In comparison to magnetic deflection, electrostatic deflection can more readily follow random and fast changes in potential, but is limited to small deflection angles.

Common representations of deflection plates are misleading. For one, the plates for one deflection axis are closer to the screen than the plates for the other. Plates that are closer together provide better sensitivity, but they also need to be extend far enough along the CRT's axis to obtain adequate sensitivity. (The longer the time a given electron spends in the field, the farther it's deflected.) However, closely-spaced long plates would cause the beam to contact them before full amplitude deflection occurs, so the compromise shape has them relatively close together toward the cathode, and flared apart in a shallow vee toward the screen. They are not flat in any but quite-old CRTs!

The timebase is an electronic circuit that generates a ramp voltage. This is a voltage that changes continuously and linearly with time. When it reaches a predefined value the ramp is reset and settles to its starting value. When a trigger event is recognized, provided the reset process (holdoff) is complete, the ramp starts again. The timebase voltage usually drives the horizontal amplifier. Its effect is to sweep the screen end of the electron beam at a constant speed from left to right across the screen, then blank the beam and return its deflection voltages to the left, so to speak, in time to begin the next sweep. Typical sweep circuits can take significant time to reset; in some tube 'scopes, fast sweeps required more time to retrace than to sweep.

Meanwhile, the vertical amplifier is driven by an external voltage (the vertical input) that is taken from the circuit or experiment that is being measured. The amplifier has a very high input impedance, typically one megohm, so that it draws only a tiny current from the signal source. Attenuator probes reduce the current drawn even more. The amplifier drives the vertical deflection plates with a voltage that is proportional to the vertical input. Because the electrons have already been accelerated by typically 2kV (roughly), this amplifier also has to deliver almost a hundred volts, and this with a very wide bandwidth. The gain of the vertical amplifier can be adjusted to suit the amplitude of the input voltage. A positive input voltage bends the electron beam upwards, and a negative voltage bends it downwards, so that the vertical deflection at any part of the trace shows the value of the input at that time.

^[3]

The response of any oscilloscope is much faster than that of mechanical measuring devices such as the multimeter, where the inertia of the pointer (and perhaps damping) slows down its response to the input.

Observing high speed signals, especially non-repetitive signals, with a conventional CRO is difficult, due to non-stable or changing triggering threshold which makes it hard to "freeze" the waveform on the screen. This often requires the room to be darkened or a special viewing hood to be placed over the face of the display tube. To aid in viewing such signals, special oscilloscopes have borrowed from night vision technology, employing a microchannel plate electron multiplier behind the tube face to amplify faint beam currents.

Although a CRO allows one to view a signal, in its basic form it has no means of recording that signal on paper for the purpose of documentation. Therefore, special oscilloscope cameras were developed to photograph the screen directly. Early cameras used roll or plate film, while in the 1970s Polaroid instant cameras became popular. A P11 CRT phosphor (visually blue) was especially effective in exposing film. Cameras (sometimes using single sweeps) were used to capture faint traces.

The power supply is an important component of the scope. It provides low voltages to power the cathode heater in the tube (isolated for high voltage!), and the vertical and horizontal amplifiers as well as the trigger and sweep circuits. Higher voltages are needed to drive the electrostatic deflection plates, which means that the output stage of the vertical deflection amplifier has to develop large signal swings. These voltages must be very stable, and amplifier gain must be correspondingly stable. Any significant variations will cause errors in the size of the trace, making the 'scope inaccurate.

Later analog oscilloscopes added digital processing to the standard design. The same basic architecture - cathode ray tube, vertical and horizontal amplifiers - was retained, but the electron beam was controlled by digital circuitry that could display graphics and text mixed with the analog waveforms. Display time for those was interleaved -- multiplexed -- with waveform display in basically much the same way that a dual/multitrace 'scope displays its channels. The extra features that this system provides include:

on-screen display of amplifier and timebase settings;
voltage cursors - adjustable horizontal lines with voltage display;
time cursors - adjustable vertical lines with time display;
on-screen menus for trigger settings and other functions.

Dual beam oscilloscope

A dual beam oscilloscope was a type of oscilloscope once used to compare one signal with another. There were two beams produced in a special type of CRT.

Unlike an ordinary "dual-trace" oscilloscope (which time-shared a single electron beam, thus losing about 50% of each signal), a dual beam oscilloscope simultaneously produced two separate electron beams, capturing the entirety of both signals. One type (Cossor, U.K.) had a beam-splitter plate in its CRT, and single-ended vertical deflection following the splitter. (There is more about this type of 'scope near the end of the this article.)

Other dual-beam 'scopes had two complete electron guns, requiring tight control of axial (rotational) mechanical alignment in manufacturing the CRT. In the latter type, two independent pairs of vertical plates deflect the beams. Vertical plates for channel A had no effect on channel B's beam. Similarly for channel B, separate vertical plates existed which deflected the B beam only.

On some dual-beam scopes the time base, horizontal plates and horizontal amplifier were common to both beams (the beam-splitter CRT worked this way). On more elaborate scopes like the Tektronix 556 there were two independent time bases and two sets of horizontal plates and horizontal amplifiers. Thus one could look at a very fast signal on one beam and a slow signal on another beam.

Most multichannel 'scopes do not have multiple electron beams. Instead, they display only one trace at a time, but switch the later stages of the vertical amplifier between one channel and the other either on alternate sweeps (ALT mode) or many times per sweep (CHOP mode). Very few true dual beam oscilloscopes were built.

With the advent of digital signal capture, true dual beam oscilloscopes became obsolete, as it was then possible to display two truly simultaneous signals from memory using either the ALT or CHOP display technique, or even possibly a raster display mode.

Analog storage oscilloscope

Trace storage is an extra feature available on some analog scopes; they used direct-view storage CRTs. Storage allows the trace pattern that normally decays in a fraction of a second to remain on the screen for several minutes or longer. An electrical circuit can then be deliberately activated to store and erase the trace on the screen.

The storage is accomplished using the principle of secondary emission. When the ordinary writing electron beam passes a point on the phosphor surface, not only does it momentarily cause the phosphor to illuminate, but the kinetic energy of the electron beam knocks other electrons loose from the phosphor surface. This can leave a net positive charge. Storage oscilloscopes then provide one or more secondary electron guns (called the "flood guns") that provide a steady flood of low-energy electrons traveling towards the phosphor screen. Flood guns cover the entire screen, ideally uniformly. The electrons from the flood guns are more strongly drawn to the areas of the phosphor screen where the writing gun has left a net positive charge; in this way, the electrons from the flood guns re-illuminate the phosphor in these positively-charged areas of the phosphor screen.

If the energy of the flood gun electrons is properly balanced, each impinging flood gun electron knocks out one secondary electron from the phosphor screen, thus preserving the net positive charge in the illuminated areas of the phosphor screen. In this way, the image originally written by the writing gun can be maintained for a long time -- many seconds to a few minutes. Eventually, small imbalances in the secondary emission ratio cause the entire screen to "fade positive" (light up) or cause the originally-written trace to "fade negative" (extinguish). It is these imbalances that limit the ultimate storage time possible.

Storage oscilloscopes (and large-screen storage CRT displays) of this type, with storage at the phosphor, were made by Tektronix. Other companies, notably Hughes, earlier made storage 'scopes with a more-elaborate and costly internal storage structure.

Some oscilloscopes used a strictly binary (on/off) form of storage known as "bistable storage". Others permitted a constant series of short, incomplete erasure cycles which created the impression of a phosphor with "variable persistence". Certain oscilloscopes also allowed the partial or complete shutdown of the flood guns, allowing the preservation (albeit invisibly) of the latent stored image for later viewing. (Fading positive or fading negative only occurs when the flood guns are "on"; with the flood guns off, only leakage of the charges on the phosphor screen degrades the stored image.

Analogue Sampling Oscilloscope

The principle of sampling was developed during the 1930's in Bell Laboratories by Nyquist, after whom the sampling theorem is named. The first sampling oscilloscope was, however, developed in the late 1950's at the Atomic Energy Research Establishment at Harwell in England by G.B.B. Chaplin, A.R. Owens and A.J. Cole. ["A Sensitive Transistor Oscillograph With DC to 300 Mc/s Response", Proc I.E.E. (London) Vol.106, Part B. Suppl., No. 16, 1959].

The first sampling oscilloscope was an analog instrument, originally developed as a front-end unit for a conventional oscilloscope.The need for this instrument grew out of the requirement of nuclear scientists at Harwell to capture the waveform of very fast repetitive pulses. The current state-of-the-art oscilloscopes -- with bandwidths of typically 20 MHz -- were not able to do this and the 300 MHz effective bandwidth of ther analog sampling oscilloscope represented a considerable advance.

A short series of these 'front-ends' was made at Harwell and found much use and Chaplin et al. patented the invention. Commercial exploitation of this patent was ultimately done by the Hewlett-Packard Company (later Agilent Technologies).

Digital oscilloscopes

While analog devices make use of continually varying voltages, digital devices employ binary numbers which correspond to samples of the voltage. In the case of digital oscilloscopes, an analog-to-digital converter (ADC) is used to change the measured voltages into digital information. Waveforms are taken as a series of samples. The samples are stored, accumulating until enough are taken in order to describe the waveform, which are then reassembled for display. Digital technology allows the information to be displayed with brightness, clarity, and stability. There are, however, limitations as with the performance of any oscilloscope. The highest frequency at which the oscilloscope can operate is determined by the analog bandwidth of the front-end components of the instrument and the sampling rate.

Digital oscilloscopes can be classified into three primary categories: digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes.

Digital storage oscilloscope

File:MSO6014A.JPG

A digital storage oscilloscope manufactured by Agilent Technologies

The digital storage oscilloscope, or DSO for short, is now the preferred type for most industrial applications, although simple analog CROs are still used by hobbyists. It replaces the unreliable storage method used in analog storage scopes with digital memory, which can store data as long as required without degradation. It also allows complex processing of the signal by high-speed digital signal processing circuits.

The vertical input, instead of driving the vertical amplifier, is digitised by an analog to digital converter to create a data set that is stored in the memory of a microprocessor. The data set is processed and then sent to the display, which in early DSOs was a cathode ray tube, but is now more likely to be an LCD flat panel. DSOs with color LCD displays are common. The data set can be sent over a LAN or a WAN for processing or archiving. The screen image can be directly recorded on paper by means of an attached printer or plotter, without the need for an oscilloscope camera. The scope's own signal analysis software can extract many useful time-domain features (e.g. rise time, pulse width, amplitude), frequency spectra, histograms and statistics, persistence maps, and a large number of parameters meaningful to engineers in specialized fields such as telecommunications, disk drive analysis and power electronics.

Digital oscilloscopes are limited principally by the performance of the analog input circuitry and the sampling frequency. In general, the sampling frequency should be at least the Nyquist rate, double the frequency of the highest-frequency component of the observed signal, otherwise aliasing may occur.

Digital storage also makes possible another unique type of oscilloscope, the equivalent-time sample scope. Instead of taking consecutive samples after the trigger event, only one sample is taken. However, the oscilloscope is able to vary its timebase to precisely time its sample, thus building up the picture of the signal over the subsequent repeats of the signal. This requires that either a clock or repeating pattern be provided. This type of scope is frequently used for very high speed communication because it allows for a very high "sample rate" and low amplitude noise compared to traditional real-time scopes.

To sum this up: Advantages over the analog oscilloscope:

Brighter and bigger display with color to distinguish multiple traces
Equivalent time sampling and Average across consecutive samples or scans lead to higher resolution down to µV
Peak detection
Pre-trigger (events before the trigger occurs can be displayed)
Easy pan and zoom across multiple stored traces allows beginners to work without a trigger
- This needs a fast reaction of the display (some scopes have 1 ms delay)
- The knobs have to be large and turn smoothly
Also slow traces like the temperature variation across a day can be recorded
The memory of the oscilloscope can be arranged not only as a one-dimensional list but also as a two-dimensional array to simulate a phosphor screen. The digital technique allows a quantitative analysis (E.g. Eye diagram)
Allows for automation, though most models lock the access to their software

A disadvantage of digital oscilloscopes is the limited refresh rate of the screen. On an analog oscilloscope, the user can get an intuitive sense of the trigger rate simply by looking at the steadiness of the CRT trace. For a digital oscilloscope, the screen looks exactly the same for any signal rate which exceeds the screen's refresh rate. Additionally, it is sometimes hard to spot "glitches" or other rare phenomena on the black-and-white screens of standard digital oscilloscopes; the slight persistence of CRT phosphors on analog scopes makes glitches visible even if many subsequent triggers overwrite them. Both of these difficulties have been overcome recently by "digital phosphor oscilloscopes," which store data at a very high refresh rate and display it with variable intensity, to simulate the trace persistence of a CRT scope.

A related type of analog sampling 'scope for displaying very fast, repetitive waveforms sampled very quickly (fractional nanoseconds) and held the samples long enough to be displayed by a narrow-band vertical amplifier and a modest-performance CRT. A comparatively slow sweep on the CRT corresponded with progressive tiny advancing sample times, so that many samples created a waveform of the fast signal.

Later designs sampled at random times within the time span represented by one sweep; the samples were displayed at horizontal positions corresponding to the delay from sweep start.

Triggering used tunnel diodes and frequency dividers.

Mixed signal oscilloscopes

A mixed signal oscilloscope (or MSO) has two kinds of inputs, a small number (typically two or four) of analog channels, and a larger number (typically sixteen) of digital channels. These measurements are acquired with a single time base, they are viewed on a single display, and any combination of these signals can be used to trigger the oscilloscope.

An MSO combines all the measurement capabilities and the use model of a Digital Storage Oscilloscope (DSO) with some of the measurement capabilities of a logic analyzer. MSOs typically lack the advanced digital measurement capabilities and the large number of digital acquisition channels of full-fledged logic analyzers, but they are also much less complex to use. Typical mixed-signal measurement uses include the characterization and debugging of hybrid analog/digital circuits like: embedded systems, Analog-to-digital converters (ADCs), Digital-to-analog converters (DACs), and control systems.

Hand held oscilloscopes

Hand held oscilloscope are useful for many test and field service applications. Today, a hand held oscilloscope is usually a digital sampling oscilloscope, using a liquid crystal display. Typically, a hand held oscilloscope has two analog input channels, but four-input-channel versions are also available. Some instruments combine the functions of a digital multimeter with the oscilloscope.

PC-based oscilloscopes (PCO)

File:Visual Analyzer oscilloscope.png

Oscilloscope software running in Windows that uses the computer's sound card as a cheap ADC

Although most people think of an oscilloscope as a self-contained instrument in a box, a new type of "oscilloscope" is emerging that consists of a specialized signal acquisition board (which can be an external USB or Parallel port device, or an internal add-on PCI or ISA card). The hardware itself usually consists of an electrical interface providing isolation and automatic gain controls, several high-speed analog-to-digital converters and some buffer memory, or even on-board DSPs. Depending on the exact hardware configuration, the hardware could be best described as a digitizer, a data logger or as a part of a specialized automatic control system.

The PC provides the display, control interface, disc storage, networking and often the electrical power for the acquisition hardware. The viability of PC-based oscilloscopes depends on the current widespread use and low cost of standardized PCs. Since prices can range from as little as $100 to as much as $3000 depending on their capabilities, such instruments are particularly suitable for the educational market, where PCs are commonplace but equipment budgets are often low. The acquisition hardware, in certain cases, may only consist of a standard sound card or even a game port, if only audio and low-frequency signals are involved.

The PCO can transfer data to the computer in two main ways - streaming, and block mode. In streaming mode the data is transferred to the PC in a continuous flow without any loss of data. The way in which the PCO is connected to the PC (e.g. USB) will dictate the maximum achievable speed using this method. Block mode utilizes the on-board memory of the PCO to collect a block of data which is then transferred to the PC after the block has been recorded. The PCO hardware then resets and records another block of data. This process happens very quickly, but the time taken will vary according to the size of the block of data and the speed at which it can be transferred. This method enables a much higher sampling speed, but in many cases the hardware will not record data whilst it is transferring the existing block, meaning that some data loss will occur.

The advantages of PC-based oscilloscopes include:

Lower cost than a stand-alone oscilloscope, assuming the user already owns a PC. Professional-grade PCO hardware (e.g. with bandwidth in the MHz rather than in the kHz range) tends to be more expensive than e.g. a typical PCI sound card, and some can even cost more than a new PC [1].
Easy exporting of data to standard PC software such as spreadsheets and word processors.
Ability to control the instrument by running a custom program on the PC.
Use of the PC's networking and disc storage functions, which cost extra when added to a self-contained oscilloscope.
PCs typically have large high-resolution color displays which can be easier to read than the smaller displays found on conventional scopes. Color can be utilized to differentiate waveforms. It can also show increased information including more of the waveform or extras like automatic waveform measurements and simultaneous alternative views.
Portability when used with a laptop PC.
Some are much smaller physically than even handheld oscilloscopes.

There are also some disadvantages, which include:

Power-supply and electromagnetic noise from PC circuits, which requires careful and extensive shielding to obtain good low-level signal resolution.
Data transfer rates to the PC, which are dependent upon the connection method. This affects the maximum sampling speed achievable by the PCO when streaming.
Need for the owner to install oscilloscope software on the PC, which may not be compatible with the current release of the PC operating system.
Time for the PC to boot, compared with the almost instant start-up of a self-contained oscilloscope (although, as some modern oscilloscopes are actually PCs or similar machines in disguise, this distinction is narrowing).

As more processing power and data storage is included in oscilloscopes, the distinction is becoming blurred. Mainstream oscilloscope vendors manufacture large-screen, PC-based oscilloscopes, with very fast (multi-GHz) input digitizers and highly-customized user interfaces.

Software for a PC may use the sound card or game port to acquire analog signals, instead of dedicated signal acquisition hardware. However, these devices have very restricted input voltage ranges, limited precision, and very restricted frequency ranges. The ground reference for these inputs is the same as the ground for the PC logic and power supply; this may inject unacceptable amounts of noise into the circuit under test. However, these devices can be useful for demonstration or hobby use.

If a sound card is used, frequency response is usually limited to the audio range, and DC signals cannot be measured. The number of inputs is limited by the number of recording channels and the inputs can handle only audio line-level voltages without the risk of damage.

If the game port is used as the acquisition hardware, the sampling frequency is very low, typically below 1 kHz, and the input voltages can only vary over a range of a couple of volts. In addition, the game port cannot easily be programmed for a specific sampling rate, nor can it be easily assigned a precise quantization step. These limitations only make it suitable for low-precision visualization of low frequency signals.

History

Hand-drawn oscillograms

The earliest method of creating an image of a waveform was through a laborious and painstaking process of measuring the voltage or current of a spinning rotor at specific points around the axis of the rotor, and noting the measurements taken with a galvanometer. By slowly advancing around the rotor, a general standing wave can be drawn on graphing paper by recording the degrees of rotation and the meter strength at each position.

This process was first partially automated by Jules François Joubert with his step-by-step method of wave form measurement. This consisted of a special single-contact commutator attached to the shaft of a spinning rotor. The contact point could be moved around the rotor following a precise degree indicator scale and the output appearing on a galvanometer, to be hand-graphed by the technician. ^[5] This process could only produce a very rough waveform approximation since it was formed over a period of several thousand wave cycles, but it was the first step in the science of waveform imaging.

A tiny tilting mirror

In the 1920s, a tiny tilting mirror attached to a diaphragm at the apex of a horn provided good response up to a few kHz, perhaps even 10 kHz. A time base, unsynchronized, was provided by a spinning mirror polygon, and a collimated beam of light from an arc lamp projected the waveform onto the lab wall or a screen.

Even earlier, audio applied to a diaphragm on the gas feed to a flame made the flame height vary, and a spinning mirror polygon gave an early glimpse of waveforms.

Automatic paper-drawn oscillograph


Schematic and perspective view of the Hospitalier Ondograph, which used a pen on a paper drum to record a waveform image built up over time, using a synchronous motor drive mechanism and a permanent magnet galvanometer. ^[6]^[7]

The first automated oscillographs used a galvanometer to move a pen across a scroll or drum of paper, capturing wave patterns onto a continuously moving scroll. Due to the relatively high-frequency speed of the waveforms compared to the slow reaction time of the mechanical components, the waveform image was not drawn directly but instead built up over a period of time by combining small pieces of many different waveforms, to create an averaged shape.

The device known as the Hospitalier Ondograph was based on this method of wave form measurement. It automatically charged a capacitor from each 100th wave, and discharged the stored energy through a recording galvanometer, with each successive charge of the capacitor being taken from a point a little farther along the wave. ^[8] (Such wave-form measurements were still averaged over many hundreds of wave cycles but were more accurate than hand-drawn oscillograms.)

Moving-paper oscillographs using UV-sensitive paper and advanced mirror galvanometers provided multi-channel recordings in the mid-20th century. Frequency response was into at least the low audio ratge.

Photographic Oscillograph



Top-Left: Duddell moving-coil oscillograph with mirror and two supporting moving coils on each side of it, suspended in an oil bath. The large coils on either side are fixed in place, and provide the magnetic field for the moving coil. (Permanent magnets were rather feeble at that time.) Top-Middle: Rotating shutter and moving mirror assembly for placing time-index marks next to the waveform pattern. Top-Right: Moving-film camera for recording the waveform. Bottom: Film recording of sparking across switch contacts, as a high-voltage circuit is disconnected.^[9]^[10]^[11]^[12]

In order to permit direct measurement of waveforms it was necessary for the recording device to use a very low-mass measurement system that can move with sufficient speed to match the motion of the actual waves being measured. This was done with the development of the moving-coil oscillograph by William Duddell which in modern times is also referred to as a mirror galvanometer. This reduced the measurement device to a small mirror that could move at high speeds to match the waveform.

To perform a waveform measurement, a photographic slide would be dropped past a window where the light beam emerges, or a continuous roll of motion picture film would be scrolled across the aperture to record the waveform over time. Although the measurements were much more precise than the built-up paper recorders, there was still room for improvement due to having to develop the exposed images before they could be examined.

Allen B. Du Mont Labs. made moving-film cameras, in which continuous film motion provided the time base. Horizontal deflection was probably disabled, although a very slow sweep would have spread phosphor wear. CRTs with P11 phosphor were either standard or available.

DuMont also made projection oscilloscopes, with multistage PDA, ultimately 25 kV or so.

CRT Invention

Cathode ray tubes (CRTs) were developed in the late 19th century. At that time, the tubes were intended primarily to demonstrate and explore the physics of electrons (then known as cathode rays). Karl Ferdinand Braun invented the CRT oscilloscope as a physics curiosity in 1897, by applying an oscillating signal to electrically charged deflector plates in a phosphor-coated CRT. Applying a reference oscillating signal to the horizontal deflector plates and a test signal to the vertical deflector plates produced transient plots of electrical waveforms on the small phosphor screen.

The first dual beam oscilloscope was developed in the late 1930s by the British company A.C.Cossor (later acquired by Raytheon). The CRT was not a true double beam type but used a split beam made by placing a third plate between the vertical deflection plates. It was widely used during WWII for the development and servicing of radar equipment. Although extremely useful for examining the performance of pulse circuits it was not calibrated so could not be used as a measuring device. It was, however, useful in producing response curves of IF circuits and consequently a great aid in their accurate alignment.

The triggered-sweep oscilloscope

Oscilloscopes became a much more useful tool in 1946 when Howard Vollum and Jack Murdock invented the triggered-sweep oscilloscope, Tektronix Model 511. Howard Vollum had first seen such 'scopes in Germany. Before triggered sweep came into use, the horizontal deflection of the oscilloscope beam was controlled by a free-running sawtooth waveform generator. If the period of the horizontal sweep did not match the period of the waveform to be obeserved, each subsequent trace would start at a different place in the waveform leading to a jumbled display or a moving image on the screen. The sweep could be synchronized with the period of the signal, but then the sweep speed was uncalibrated. Many oscilloscopes had a synchronization feature which fed a signal from the vertical deflection into the sweep generator circuit, but the equivalent of trigger level had at best a narrow range, and trigger polarity was not selectable.

Triggering allows stationary display of a repeating waveform, as multiple repetitions of the waveform are drawn over the exact same trace on the phosphor screen. A triggered sweep maintains the calibration of sweep speed, making it possible to measure properties of the waveform such as frequency, phase, rise time, and others, that would not otherwise be possible. ^[13]

More importantly, triggers can occur at varying intervals, and unless too closely spaced, each trigger creates an identical sweep. There is no requirement for a constant-frequency input to obtain stable traces.

During World War II, a few oscilloscopes used for radar development (and a few laboratory oscilloscopes) had so-called driven sweeps. These sweep circuits remained dormant, with the CRT beam cut off, until a drive pulse from an external device unblanked the CRT and started one constant-speed horizontal trace, which could have a calibrated speed, permitting measurement of time intervals. Once the sweep was complete, the sweep circuit blanked the CRT (turned off the beam) and the circuit reset itself, ready for the next drive pulse.

Long-persistence CRTs, sometimes used in 'scopes for displaying quite-slowly-changing voltages, used a phosphor such as P7, which comprised a double layer. The inner layer fluoresced bright blue from the electron beam, and its light excited a phosphorescent "outer" layer, directly visible inside the envelope (bulb). The latter stored the light, and released it with a yellowish glow with decaying brightness over tens of seconds. This type of phosphor was alos used in radar analog PPI CRT displays, which are a graphic decoration (rotating radial light bar) in some TV weather-report scenes.

Triggered-sweep oscilloscopes compare the vertical deflection signal (or rate of change of the signal) with an adjustable threshold, referred to as trigger level. As well, the trigger circuits also recognize the slope direction of the vertical signal when it crosses the threshold -- whether the vertical signal is positive-going or negative-going at the crossing. This is called trigger polarity. When the vertical signal crosses the set trigger level and in the desired direction, the trigger circuit unblanks the CRT and starts an accurate linear sweep. Each start can happen at any time after the preceding one (but not too soon) -- provided that the preceding sweep is complete, and the sweep circuit has completely reset itself to its initial state. (This dead time can be significant.) During the sweep, the sweep circuit itself ignores sweep-start signals from the trigger-processing circuits.

Having selectable trigger polarity and trigger level, along with the driven sweep, made oscilloscopes into exceptionally-valuable and exceedingly-useful test and measurement instruments. Early triggered-sweep oscilloscopes had calibrated time bases, as well as vertical (deflection) amplifiers with calibrated sensitivity. The trace speed across the screen was given in units of time per division of the graticule.

As oscilloscopes have become more powerful over time, enhanced triggering options allow capture and display of more complex waveforms. For example, trigger holdoff is a feature in most modern oscilloscopes that can be used to define a certain period following a trigger during which the oscilloscope will not trigger again. This makes it easier to establish a stable view of a waveform with multiple edges which would otherwise cause another trigger.

Tektronix

Vollum and Murdock went on to found Tektronix, the first manufacturer of calibrated oscilloscopes (which included a graticule on the screen and produced plots with calibrated scales on the axes of the screen). Later developments by Tektronix included the development of multiple-trace oscilloscopes for comparing signals either by time-multiplexing (via chopping or trace alternation) or by the presence of multiple electron guns in the tube. In 1963, Tektronix introduced the Direct View Bistable Storage Tube (DVBST), which allowed observing single pulse waveforms rather than (as previously) only repeating wave forms. Using micro-channel plates, a variety of secondary-emission electron multiplier inside the CRT and behind the faceplate, the most-advanced analog oscilloscopes (for example, the Tek 7104 mainframe) could display a visible trace (or allow the photography) of a single-shot event even when running at extremely fast sweep speeds. This 'scope went to 1 GHz.

In vacuum-tube 'scopes made by Tektronix, the vertical amplifier's delay line was a long frame, L-shaped for space reasons, that carried several dozen discrete inductors and a corresponding number of low-capacitance adjustable ("trimmer") cylindrical capacitors. These 'scopes had plug-in vertical input channels. For adjusting the delay line capacitors, a high-pressure gas-filled mercury-wetted reed switch created extremely-fast-rise pulses which went directly to the later stages of the vertical amplifier. With a fast sweep, any misadjustment created a dip or bump, and touching a capacitor made its local part of the waveform change. Adjusting the capacitor made its bump disappear. Eventually, a flat top resulted.

Vacuum-tube output stages in early wideband 'scopes used radio transmitting tubes, but they consumed a lot of power. Picofarads of capacitance to ground limited bandwidth. A better design, called a distributed amplifier, used multiple tubes, but their inputs (control grids) were connected along a tapped L-C delay line, so the tubes' input capacitances became part of the delay line. As well, their outputs (plates/anodes) were likewise connected to another tapped delay line, its output feeding the deflection plates. (This amplifier was push-pull, so there were four delay lines, two for input, and two for output.)

Digital oscilloscopes

The first Digital Storage Oscilloscope (DSO) was invented by Walter LeCroy (who founded the LeCroy Corporation, based in New York, USA) after producing high-speed digitizers for the research center CERN in Switzerland. LeCroy remains one of the three largest manufacturers of oscilloscopes in the world.

Starting in the 1980s, digital oscilloscopes became prevalent. Digital storage oscilloscopes use a fast analog-to-digital converter and memory chips to record and show a digital representation of a waveform, yielding much more flexibility for triggering, analysis, and display than is possible with a classic analog oscilloscope. Unlike its analog predecessor, the digital storage oscilloscope can show pre-trigger events, opening another dimension to the recording of rare or intermittent events and troubleshooting of electronic glitches. As of 2006 most new oscilloscopes (aside from education and a few niche markets) are digital.

Digital scopes rely on effective use of the installed memory and trigger functions: not enough memory and the user will miss the events they want to examine; if the scope has a large memory but does not trigger as desired, the user will have difficulty finding the event.

Washing

During the years when oscilloscopes were built using vacuum tubes (valves) and, therefore, a great deal of high voltage electronics, it was a recommended service procedure to wash the interior circuits of one's oscilloscope. This was advised to prevent the build up of dust that may have caused low resistance and tracking paths from high voltage terminals. Tektronix published the recommended procedure in their company magazine TekScope. It involved a gentle, low-pressure application of water and dish detergent, followed by careful rinsing and drying of the instrument. In this way, the service technician could remove dust and other conductive contaminants that might otherwise impair the correct calibration of the instrument. The pre-servicing interior washing was continued long after semiconductor circuits had replaced tubes. (Not all of the 'scope was washable; the fan motor was probably not, although the power transformer was(!) )

Use as props

In the 1950s and 1960s, oscilloscopes were frequently used in movies and television programs to represent generic scientific and technical equipment. The 1963–65 U.S. TV show The Outer Limits famously used an image of fluctuating Lissajous figures on an oscilloscope as the background to its opening credits ("There is nothing wrong with your television set....").

References

^ Frank Spitzer and Barry Howarth, Principles of modern Instrumentation, Holt, Rinehart and Winston, New York, 1972, ISBN 0-03-080208-3 pg. 110
^ Spitzer and Howarth page 119
^ Special purpose oscilloscopes called modulation monitors may directly apply a relatively large-voltage radio-frequency signal to the deflection plates with no intervening amplifier stage. In such instances, the waveform of the applied RF could generally not be shown, because the frequency was much too high. In such monitors, the CRT's bandwidth, which is typically a few hundred MHz, permits the envelope of the high-frequency RF to be displayed. The display is not a trace, but a solid triangle of light. Some bench-top oscilloscopes brought out terminals for the deflection plates for such uses. (Edited; basically from D. S. Evans and G. R. Jessup (ed), VHF-UHF Manual (3rd Edition), Radio Society of Great Britain, London, 1976 page 10.15)
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1844, Fig. 2589
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, pp. 1841-1846
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1850, Fig. 2597
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1851, Fig. 2598
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, pp. 1849-1851
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1858, Fig. 2607
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1855, Fig. 2620
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1866, Figs. 2621-2623
^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1867, Fig. 2625
^ Frank Spitzer and Barry Howarth, Principles of modern Instrumentation, Holt, Rinehart and Winston, New York, 1972, ISBN 0-03-080208-3 pg. 122

External links

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[1] Frank Spitzer and Barry Howarth, Principles of modern Instrumentation, Holt, Rinehart and Winston, New York, 1972, ISBN 0-03-080208-3 pg. 110

[2] Spitzer and Howarth page 119

[3] Special purpose oscilloscopes called modulation monitors may directly apply a relatively large-voltage radio-frequency signal to the deflection plates with no intervening amplifier stage. In such instances, the waveform of the applied RF could generally not be shown, because the frequency was much too high. In such monitors, the CRT's bandwidth, which is typically a few hundred MHz, permits the envelope of the high-frequency RF to be displayed. The display is not a trace, but a solid triangle of light. Some bench-top oscilloscopes brought out terminals for the deflection plates for such uses. (Edited; basically from D. S. Evans and G. R. Jessup (ed), VHF-UHF Manual (3rd Edition), Radio Society of Great Britain, London, 1976 page 10.15)

[4] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1844, Fig. 2589

[5] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, pp. 1841-1846

[6] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1850, Fig. 2597

[7] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1851, Fig. 2598

[8] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, pp. 1849-1851

[9] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1858, Fig. 2607

[10] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1855, Fig. 2620

[11] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1866, Figs. 2621-2623

[12] Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 6, Chapter 63: Wave Form Measurement, p. 1867, Fig. 2625

[13] Frank Spitzer and Barry Howarth, Principles of modern Instrumentation, Holt, Rinehart and Winston, New York, 1972, ISBN 0-03-080208-3 pg. 122

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Features and uses

Description

Display and general external appearance

Size and portability

Inputs

Probes

The trace

Front Panel Controls

Focus Control

Intensity Control

Beam Finder

Graticule

Timebase Controls

Holdoff Control

Vertical Sensitivity, Coupling, and Polarity Controls

Horizontal Sensitivity Control

Vertical Position Control

Horizontal Position Control

Dual-Trace Controls

Delayed-Sweep Controls

Sweep Trigger Controls

Basic types of sweeps

Triggered sweeps

Automatic sweep mode

Recurrent sweeps

Single Sweeps

Delayed sweeps

Dual and Multiple-trace Oscilloscopes

The Vertical Amplifier

Bandwidth

X-Y mode

Other features

Examples of use

Selection

Software

How it works

Cathode-ray oscilloscope (CRO)

Dual beam oscilloscope

Analog storage oscilloscope

Analogue Sampling Oscilloscope

Digital oscilloscopes

Digital storage oscilloscope

Mixed signal oscilloscopes

Hand held oscilloscopes

PC-based oscilloscopes (PCO)

History

Hand-drawn oscillograms

A tiny tilting mirror

Automatic paper-drawn oscillograph

Photographic Oscillograph

CRT Invention

The triggered-sweep oscilloscope

Tektronix

Digital oscilloscopes

Washing

Use as props

See also

References

External links