A low-noise block converter (LNB, for low-noise block, or sometimes LNC, for low-noise converter) is the (receiving, or downlink) antenna of what is commonly called the parabolic (actually paraboloid) satellite dish commonly used for satellite TV reception. It is functionally equivalent to the dipole antenna used for most other TV reception purposes, although it is actually waveguide based. Whereas the dipole antenna is unable to adapt itself to various polarization planes without being rotated, the LNB can be switched electronically between horizontal and vertical polarization reception. The LNB is usually fixed on or in the satellite dish, for the reasons outlined below. The corresponding component in the uplink transmit link is called a Block upconverter (BUC).
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 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 of 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 ±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.
Direct broadcast satellite (DBS) dishes use an LNBF (NB feedhorn), which integrates the antenna feedhorn with the LNB. Small diplexers are often used to distribute the resulting IF signal (usually 950 to 1450 MHz) piggybacked in the same cable TV wire that carries lower-frequency terrestrial television from an outdoor antenna. Another diplexer then separates the signals to the receiver of the TV set, and the integrated receiver/decoder (IRD) of the DBS set-top box.
Newer Ka band systems use additional IF blocks from the LNBF, one of which will cause interference to UHF and cable TV frequencies above 250 MHz, precluding the use of diplexers. The other block is higher than the original, up to 2.5 GHz, requiring the LNB to be connected to high-quality all-copper RG-6/U cables. This is in addition to higher electrical power and electrical current requirements for multiple dual-band LNBFs.
For some satellite Internet and free-to-air (FTA) signals, a universal LNB (Ku band) is recommended. Most North American DBS signals use circular polarization, instead of linear polarization, therefore requiring a different LNB type for proper reception. In this case, the polarization must be adjusted between clockwise and counterclockwise, rather than horizontal and vertical.
In the case of DBS, the voltage supplied by the set-top box to the LNB determines the polarisation setting. With multi-TV systems, a dual LNB allows both to be selected at once by a switch, which acts as a distribution amplifier. The amplifier then passes the proper signal to each box according to what voltage each has selected. The newest systems may select polarization and which LNBF to use by sending DiSEqC codes instead. The oldest satellite systems actually powered a rotating antenna on the feedhorn, at a time when there was typically only one LNB or LNA on a very large TVRO dish.
A universal LNB can receive both polarisations (Vertical and Horizontal) and the full range of frequencies in the satellite Ku band. Some models can receive both polarisations simultaneously (though rarely) through four different connectors Low/Hor, Low/Ver, High/Hor, High/Ver, and others are switchable (using 13V for Vertical and 17 volt for Horizontal) or fully adjustable in their polarisation though rarely as this would require a separate polarisor.
Here is an example of Universal LNB specifications:
By covering a smaller frequency range an LNB with a better noise figure can be produced. Pay TV operators can also supply a single fixed polarization LNBF to save a small amount of expense.
Here is an example of a Standard Linear LNB:
Here is an example of an LNB used for DBS:
Here is an example of a North American C-band LNB:
Two or four LNBs are in one unit to enable use of multiple receivers on one dish. Note the difference between a quad (or double twin) and a quatro. A quad LNB has four independent outputs, each of them has a separate switch for band/polarization. Then a quad LNB can drive up to four receivers independently. A quatro LNB has four outputs, each of them supplies only 1/4 of the available channels (Lo/Hi band and H/V polarization). A quatro LNB is suitable for a shared installation, using one or more multiswitches to deliver signals to any number of decoders.