A low-noise block converter (LNB, for low-noise block, or sometimes LNC, for low-noise converter) is used in communications satellite (usually broadcast satellite) reception. The LNB is usually fixed on or in the satellite dish, for the reasons outlined below.
Satellites use comparatively high radio frequencies to transmit their 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 waveguide would be required and any significant length of cable leaves very little signal left on the receiving end.
The job of the LNB is to use the superheterodyne principle to take a wide block (or band) of relatively high frequencies, amplify and convert them to similar signals carried at a much lower frequency (called intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation of the signal, so there is much more signal left on 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 “low-noise” part means that special electronic engineering techniques are used, that the amplification and mixing takes place before cable attenuation and that the block is free of additional electronics like a power supply or a digital receiver. This all leads to a signal, which has less noise (unwanted signals) on the output than would be possible with less stringent engineering. Generally speaking, the higher the frequencies with which an electronic component has to operate, the more critical it is that noise be controlled. If low noise engineering techniques were not used, the sound and picture of satellite TV would be very low quality, if it could even be received at all without a much larger dish reflector. The low-noise quality of an LNB is expressed as the noise figure or noise temperature.
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 ±500kHz, 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.