Low-noise block downconverter

A low-noise block downconverter is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter, or even low-noise downconverter, the device is sometimes inaccurately called a low-noise amplifier.
The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and intermediate frequency amplifier. It serves as the RF front end of the satellite receiver, receiving the microwave signal from the satellite collected by the dish, amplifying it, and downconverting the block of frequencies to a lower block of intermediate frequencies. This downconversion allows the signal to be carried to the indoor satellite TV receiver using relatively cheap coaxial cable; if the signal remained at its original microwave frequency it would require an expensive and impractical waveguide line.
The LNB is usually a small box suspended on one or more short booms, or feed arms, in front of the dish reflector, at its focus. The microwave signal from the dish is picked up by a feedhorn on the LNB and is fed to a section of waveguide. One or more metal pins, or probes, protrude into the waveguide at right angles to the axis and act as antennas, feeding the signal to a printed circuit board inside the LNB's shielded box for processing. The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects.
The LNB gets its power from the receiver or set-top box, using the same coaxial cable that carries signals from the LNB to the receiver. This phantom power travels to the LNB; opposite to the signals from the LNB.
A corresponding component, called a block upconverter, is used at the satellite earth station dish to convert the band of television channels to the microwave uplink frequency.

Amplification and noise

The signal received by the LNB is extremely weak and it has to be amplified before downconversion. The low-noise amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal.
The low-noise quality of an LNB is expressed as the noise figure. This is the signal-to-noise ratio at the input divided by the signal-to-noise ratio at the output. It is typically expressed as a decibels value. The ideal LNB, effectively a perfect amplifier, would have a noise figure of 0 dB and would not add any noise to the signal. Every LNB introduces some noise but clever design techniques, expensive high-performance low-noise components such as HEMTs and even individual tweaking of the LNB after manufacture, can reduce some of the noise contributed by the LNB's components. Active cooling to very low temperatures can help reduce noise too, and is often used in scientific research applications.
Every LNB off the production line has a different noise figure because of manufacturing tolerances. The noise figure quoted in the specifications, important for determining the LNB's suitability, is usually representative of neither that particular LNB nor the performance across the whole frequency range, since the noise figure most often quoted is the typical figure averaged over the production batch.
Image:Universal-euro-sat-lnb.jpg|thumb|right|120px|K_u-band linear-polarized LNBF

Block downconversion

Satellites use comparatively high radio frequencies to transmit their TV signals. As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, it is preferable for satellite antennas to be mounted outdoors. However, plastic glazing is transparent to microwaves and residential satellite dishes have successfully been hidden indoors looking through acrylic or polycarbonate windows to preserve the external aesthetics of the home.
The purpose of the LNB is to use heterodyning to take a block of relatively high frequencies and convert them to similar signals carried at a much lower frequency. 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 is amplified and sent down the cable to the receiver:
;C-band:
;K_u-band:
where is a 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 10.70–11.70 GHz, to within a standard European receiver's IF tuning range of 950–2,150 MHz, a 9.75 GHz local oscillator frequency is used, producing a block of signals in the band 950–1,950 MHz.
For the block of higher transmission frequencies used by Astra 2A and 2B, a different local oscillator frequency converts the block of incoming frequencies. Typically, a local oscillator frequency of 10.60 GHz is used to downconvert the block to 1,100–2,150 MHz, which is still within the receiver's 950–2,150 MHz IF tuning range.
In a C-band antenna setup, the transmission frequencies are typically 3.7–4.2 GHz. By using a local oscillator frequency of 5.150 GHz the IF will be 950–1,450 MHz which is, again, in the receiver's 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 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 oscillator.

Low-noise block feedhorns (LNBFs)

With the launch of the first DTH broadcast satellite in Europe by SES in 1988, antenna design was simplified for the anticipated mass market. In particular, the feedhorn and the polarizer were combined with the LNB itself into a single unit, called an LNB-feed or LNB-feedhorn, 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.
Image:sky lnb.jpg|thumb|right|120px|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 flange around the input waveguide mouth which is bolted to a matching flange around the output of the feedhorn or polarizer unit.

Polarization

It is common to polarize satellite TV signals because it provides a way of transmitting more TV channels using a given block of frequencies. This approach requires the use of receiving equipment that can filter incoming signals based on their polarization. Two satellite TV signals can then be transmitted on the same frequency and provided that they are polarized differently, the receiving equipment can still separate them and display whichever one is currently required.
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 linearly polarized signals so the converted signals can be treated the same as in systems that use linear polarization for transmission.
Image:old flange lnb.jpg|thumb|right|120px|A 1980s K_u-band LNB without built-in polarization selection and with a WR75 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, 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 rotates the incoming signal with an electromagnet around the waveguide or rotates an intermediate probe within the waveguide using a servo motor 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 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.