A low-noise block downconverter (LNB) 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 (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low-noise amplifier (LNA). The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and intermediate frequency (IF) 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 (IF). 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 (although some dish designs have the LNB on or behind the reflector). 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 (BUC), is used at the satellite earth station (uplink) 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 (or sometimes noise temperature). 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 (dB) 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.
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, 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 (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:
C-band
f IF = f LO − f recv {\displaystyle f_{\text{IF}}=f_{\text{LO}}-f_{\text{recv}}}
Ku-band
f IF = f recv − f LO {\displaystyle f_{\text{IF}}=f_{\text{recv}}-f_{\text{LO}}}
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