Low-noise block downconverter

K_u-band LNB with both sides uncovered.

A low noise block-downconverter (or LNB) is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception. The device is sometimes called an LNA (for low noise amplifier), LNC (for low noise converter) or even LND (for low noise downconverter) but as block-downconversion is the principal function of the device, LNB is the preferred term, although this acronym is often incorrectly expanded to the incomplete descriptions, low noise block or low noise block converter.

It is functionally equivalent to the dipole antenna used for most terrestrial TV reception, although it is actually waveguide based. Inside the LNB waveguide a metal pin, or probe, protrudes into the waveguide at right angles to the axis and this acts as an aerial, collecting the signal travelling down the waveguide.

LNBF disassembled (All Parts)

The LNB is usually fixed on the satellite dish framework, at the focus of the reflector, and it derives its power from the connected receiver, sent "up" the same cable that carries the received signals "down" to the receiver. The corresponding component in the transmit link uplink to a satellite is called a Block upconverter (BUC).

Block-downconversion

Satellites use comparatively high radio frequencies (microwaves) to transmit their TV signals. As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, satellite antennas are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent through coaxial cables, the higher the frequency, the more losses occur in the cable per unit of length. The signals used for satellite are of such high frequency (in the multiple gigahertz range) that special (costly) cable types or waveguides would be required and any significant length of cable leaves very little signal at the receiving end.

The purpose of the LNB is to use the superheterodyne principle to take a block (or band) of relatively high frequencies and convert them to similar signals carried at a much lower frequency (called the intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation, so there is much more signal left at the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission.

The frequency conversion is performed by mixing a fixed frequency produced by a local oscillator inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver:

IF frequency = received frequency - local oscillator frequency

The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver. For example, to downconvert the incoming signals from Astra 1KR, which transmits in a frequency block of 10.70GHz-11.70GHz, to within a standard European receiver’s IF tuning range of 950MHz-2150MHz, a 9.75GHz local oscillator frequency is used, producing a block of signals in the band 950MHz-1950MHz.

For the block of higher transmission frequencies used by Astra 2A and 2B (11.70GHz-12.75GHz), a different local oscillator frequency converts the block of incoming frequencies. Typically, a local oscillator frequency of 10.60GHz is used to downconvert the block to 1100MHz-2150MH, which is, again, within the receiver’s 950MHz-2150MHz IF tuning range.

For the reception of wideband satellite television carriers, typically 27 MHz wide, the accuracy of the frequency of the LNB local oscillator need only be in the order of ±500 kHz, so low cost dielectric oscillators (DRO) may be used. For the reception of narrow bandwidth carriers or ones using advanced modulation techniques, such as 16-QAM, highly stable and low phase noise LNB local oscillators are required. These use an internal crystal oscillator or an external 10 MHz reference from the indoor unit and a phase-locked loop (PLL) oscillator.

Amplification and Noise

To compensate for the inevitable attenuation of the extremely weak signal received, by its transmission along the cable to the receiver, and to keep introduced noise (unwanted signals) at a low level relative to the signal, the LNB provides considerable amplification, both of the signal from the waveguide probe and of the converted IF signal. Special electronic engineering techniques are used to ensure the signal has less noise on the output than would be possible with less stringent engineering. If low noise engineering techniques were not used, the sound and picture of satellite TV would be of very low quality, if it could be received at all without a much larger dish reflector.

The low noise quality of an LNB is expressed as the noise figure (or sometimes noise temperature). This is the ratio of the amount of noise in the output to the amount in the input, in decibels (dB). The ideal LNB would have a noise figure of 0dB. In fact, every LNB introduces some noise, although clever design, expensive components and even individual tweaking of the LNB after manufacture, can reduce noise levels to very low levels.

Every LNB off the production line has a different noise figure because of manufacturing tolerances. When you choose an LNB, the noise figure quoted in the specifications is important to determine its suitability but the figure quoted is usually representative of neither that particular LNB nor the performance across the whole frequency range.

K_u-band linear-polarized LNBF

The noise figure quoted for most LNBs is the typical figure and should be an average of the production batch. Even if it's an accurate average (only a few samples from the batch are actually measured), the sample range is not given and so you cannot tell how far from the quoted average your LNB may lie. In addition, the figures measured are often at just one frequency so the quoted figure tells you nothing about the amount of noise introduced at other frequencies, which may be considerably higher.

LNBFs

With the launch of the first DTH broadcast satellite in Europe (Astra 1A) by SES Astra in 1988, antenna design was simplified for the anticipated mass-market. In particular, the feedhorn (which gathers the signal and directs it to the LNB) and the polarizer (which selects between differently polarized signals) were combined with the LNB itself into a single unit, called an LNB-feed or LNB-feedhorn (LNBF), or even an "Astra type" LNB. The prevalence of these combined units has meant that today the term LNB is commonly used to refer to all antenna units that provide the block-downconversion function, with or without a feedhorn.

LNBF for Sky Digital and Freesat in the UK

The Astra type LNBF that includes a feedhorn and polarizer is the most common variety, and this is fitted to a dish using a bracket that clamps a collar around the waveguide neck of the LNB between the feedhorn and the electronics package. The diameter of the LNB neck and collar is usually 40mm although other sizes are also produced. In the UK, the "minidish" sold for use with Sky Digital and Freesat uses an LNBF with an integrated clip-in mount.

LNBs without a feedhorn built-in are usually provided with a (C120) flange around the input waveguide mouth which is bolted to a matching flange around the output of the feedhorn or polarizer unit.

Polarization

Satellite TV signals are usually broadcast using alternating polarization to maximise the number of transmissions carried in a given frequency range as reception equipment can distinguish between transmissions using the same frequency but different polarization. Throughout the World, most satellite TV transmissions use vertical and horizontal linear polarization but in North America, DBS transmissions use left and right hand circular polarization. Within the waveguide of a North American DBS LNB a slab of dielectric material is used to convert left and right circular polarized signals to vertical and horizontal linear polarized signals so the converted signals can be treated the same.

A 1980s LNB (2.18dB noise figure) without built-in polarization selection and with a C120 fitting for separate feedhorn and polarizer

The probe inside the LNB waveguide collects signals that are polarized in the same plane as the probe. To maximise the strength of the wanted signals (and to minimise reception of unwanted signals of the opposite polarization), the probe is aligned with the polarization of the incoming signals. This is most simply achieved by adjusting the LNB's skew - its rotation about the waveguide axis. To remotely select between the two polarizations, and to compensate for inaccuracies of the skew angle, it used to be common to fit a polarizer in front of the LNB's waveguide mouth. This either rotated the incoming signal with an electromagnet around the waveguide (a magnetic polarizer) or rotated an intermediate probe within the waveguide using a servo motor (a mechanical polarizer) but such adjustable skew polarizers are rarely used today.

The simplification of antenna design that accompanied the first Astra DTH broadcast satellites in Europe to produce the LNBF extended to a simpler approach to the selection between vertical and horizontal polarized signals too. Astra type LNBFs incorporate two probes in the waveguide, at right angles to one another so that, once the LNB has been skewed in its mount to match the local polarization angle, one probe collects horizontal signals and the other vertical, and an electronic switch (controlled by the voltage of the LNB's power supply from the receiver : 13 volts for vertical and 18 volts for horizontal) determines which polarization is passed on through the LNB for amplification and block-downconversion.

Such LNBs can receive all the transmissions from a satellite with no moving parts and with just one cable connected to the receiver, and have since become the most common type of LNB produced.

Universal LNB

A twin-output universal LNB with a C120 flange fitting for a separate feedhorn

In Europe, as SES Astra launched more satellites to the Astra 19.2°E orbital position in the 1990s, the range of downlink frequencies used in the FSS band (10.70-11.70GHz) grew beyond that catered for by the standard LNBs and receivers of the time. Reception of signals from Astra 1D required an extension of receivers’ IF tuning range from 950-1950MHz to 950-2150MHz and a change of LNBs’ local oscillator frequency from the usual 10GHz to 9.75GHz (so-called “Enhanced” LNBs).

The launch of Astra 1E and subsequent satellites saw the first use by Astra of the BSS band of frequencies (11.70-12.75GHz) for new digital services and required the introduction of an LNB that would receive the whole frequency range 10.70-12.75GHz - the “Universal” LNB.

A Universal LNB has a switchable local oscillator frequency of 9.75/10.60GHz to provide two modes of operation – low band reception (10.70-11.70GHZ) and high band reception (11.70-12.75GHz). The local oscillator frequency is switched in response to a 22kHz signal superimposed on the supply voltage from the connected receiver. Along with the supply voltage level used to switch between polarizations, this enables a Universal LNB to receive both polarizations (Vertical and Horizontal) and the full range of frequencies in the satellite K_u band under the control of the receiver, in four sub-bands:

Supply Voltage	Supply Tone	LO Frequency	Polarization Received	Frequency Band Received	IF Range Used
13V	0KHz	9.75GHz	Vertical	Low (10.70-11.70GHz)	950-1950MHz
18V	0KHz	9.75GHz	Horizontal	Low (10.70-11.70GHz)	950-1950MHz
13V	22KHz	10.60GHz	Vertical	High (11.70-12.75GHz)	1100-2150MHz
18V	22KHz	10.60GHz	Horizontal	High (11.70-12.75GHz)	1100-2150MHz

Example LNBs

Standard North America K_u band LNB

Here is an example of a standard linear LNB:

• Local oscillator: 10.75 GHz
• Frequency: 11.70-12.20 GHz
• Noise figure: 1 dB typical
• Polarization: Linear

Astra type LNBF

Standard European “Astra” LNB

Here is an example of a Universal LNB used in Europe:

• Local oscillator: 9.75/10.60GHz switchable
• Frequency: 10.70-12.75GHz
• Noise figure: 0.2dB typical
• Polarization: Linear

North America DBS LNB

Here is an example of an LNB used for DBS:

• Local oscillator: 11.25 GHz
• Frequency: 12.20-12.70 GHz
• Noise figure: 0.7 dB
• Polarization: Circular

C-band LNB

Here is an example of a North American C-band LNB:

• Local oscillator: 5.15 GHz
• Frequency: 3.40-4.20 GHz
• Noise figure: ranges from 25 to 100 kelvins (uses Kelvin ratings as opposed to dB rating).
• Polarization: Linear

Multi-Output LNBs

An eight-output Octo LNBF

Dual/Twin/Quad/Octo LNBs

An LNB with a single feedhorn but multiple outputs for connection to multiple tuners (in separate receivers or within the same receiver in the case of a twin-tuner PVR receiver). Typically, two, four or eight outputs are provided. Each output responds to the tuner’s band and polarization selection signals independently of the other outputs and "appears" to the tuner to be a separate LNB. Such an LNB usually may derive its power from a receiver connected to any of the outputs. Unused outputs may be left unconnected (but waterproofed for the protection of the whole LNB).

Note: In the US, an LNB with two outputs is termed a "dual LNB" but in the UK, the term "dual LNB" describes antennas for reception from two satellite positions, using either two separate LNBs or a single Monoblock LNB with two feedhorns. In the UK, the term "twin-output LNB", or simply "twin LNB", is usually used for an LNB with a single feedhorn but two independent outputs.

Quattro LNBs

A special type of LNB intended for use in a shared dish installation to deliver signals to any number of tuners. A quattro LNB has a single feedhorn and four outputs, which each supply just one of the K_u sub-bands (low band/horizontal polarization, high band/vertical polarization, low/vertical and high/horizontal) to a multiswitch or an array of multiswitches, which then delivers to each connected tuner whichever sub-band is required by that tuner.

Although a quattro LNB typically looks similar to a quad LNB, it cannot (sensibly) be connected to receivers directly. Note again the difference between a quad and a quattro LNB: A quad LNB can drive four tuners directly, with each output providing signals from the entire K_u band. A quattro LNB is for connection to a multiswitch in a shared dish distribution system and each output provides only a quarter of the K_u band signals.

SCR LNB with three SCR taps for daisy-chaining multiple tuners

SCR/Unicable LNBs

Multiple tuners may also be fed from an SCR or Unicable LNB in a single cable distribution system. A Unicable LNB has one output connector but operates in a different way to standard LNBs so it can feed multiple tuners daisy-chained along a single coax cable.

Instead of block-downconverting the whole received spectrum, an SCR LNB downconverts a small section of the received signal (equivalent to the bandwidth of a single transponder on the satellite) selected according to a DiSEqC-compliant command from the receiver, to output at a fixed frequency in the IF. Up to 16 tuners can be allocated a different frequency in the IF range and for each, the SCR LNB downconverts the corresponding individually requested transponder.

Most SCR LNBs also include either a legacy mode of operation or a separate legacy output which provides the received spectrum block-downconverted to the whole IF range in the conventional way.

A optical fibre LNB (with fibre connection and conventional F-connector for power input)

Optical Fibre LNBs

LNBs for fibre satellite distribution systems operate in a similar way to conventional electrical LNBs, except that all four of the sub-bands in the entire Ku Band spectrum of 11.70 GHz-12.75 GHz across two signal polarisations are simultaneously block-downconverted (as in a Quattro LNB). The fours sub-bands’ IFs are stacked to create one IF with a range of 0.95 GHz-5.45 GHz (a bandwidth of 4500 MHz), which is modulated on an optical signal using a semiconductor laser, to send down the fibre cable.

At the receiver, the optical signal is converted back to the traditional electrical signal to “appear” to the receiver as a conventional LNB.

Monoblock LNBs

A twin-output monoblock LNBF for Astra 19.2°E and Hot Bird with a collar size adapter fitted

A Monoblock LNB (also spelled "monobloc") is a unit consisting of two LNBs and is designed to receive satellites spaced close together, generally 6°. For example in parts of Europe, monoblocks designed to receive the Hot Bird and Astra 19.2°E satellites are popular because they enable reception of both satellites on a single dish without requiring an expensive, slow and noisy motorised dish. A similar advantage is provided by the Duo LNB for simultaneous reception of signals from both the Astra 23.5°E and Astra 19.2°E positions.

Cold temperatures

It is possible for an LNB to physically freeze due to ice build-up in very low temperatures, obscuring the signal. This is only likely to occur when the LNB is not receiving power from the satellite receiver (i.e. no programmes are being watched). To combat this, many satellite receivers provide an option to keep the LNB powered while the receiver is on standby. In fact most LNBs are kept powered because this helps to stabilise the temperature and, thereby, the local oscillator frequency. In the case of UK BSkyB receivers, the LNB remains powered while in standby so that the receiver can receive firmware updates and Electronic Programme Guide updates. In the United States the LNB connected to Dish Network receivers remains powered as well as those receivers that receive software and firmware updates and guide information over the air at night.

See also

• Block upconverter (BUC)
• Orthomode transducer
• Signal-to-noise ratio
• Bias tee
• Duo LNB
• Single Cable Distribution
• Fibre satellite distribution
• Low noise amplifier (LNA)
• Transmit and receive integrated assembly (TRIA)

External links

• LNB mysteries explained.
• Explanation and block diagram of LNB
• Noise Temperature and Noise Figure
• SES Astra business website
• SES Astra consumer website
• Astra Recommendations (for satellite reception equipment including LNB types)
• VSAT Installation Manual with explanation of the Low-noise block converter