- Low-noise block converter
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 theparabolic (actuallyparaboloid )satellite dish commonly used forsatellite TV reception. It is functionally equivalent to thedipole antenna used for most other TV reception purposes, although it is actuallywaveguide based. Whereas the dipole antenna is unable to adapt itself to variouspolarization planes without being rotated, the LNB can be switched electronically between horizontal and vertical polarization reception.The LNB is usually fixed on or in thesatellite dish , for the reasons outlined below. The corresponding component in theuplink transmit link is called aBlock upconverter (BUC).Satellites use comparatively high
radio frequencies to transmit their signals.As
microwave satellite signals do not easily pass throughwall s,roof s, or evenglass window s, satellite antennas are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent throughcoaxial cable s, 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 orwaveguide s 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 lessattenuation 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 theamplification and mixing takes place before cable attenuation and that the block is free of additional electronics like apower supply or adigital 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 thenoise figure ornoise temperature .For the reception of
wideband satellite television carriers, typically 27 MHz wide, theaccuracy of the frequency of the LNBlocal 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 aphase-locked loop (PLL)oscillator .LNBFs
Direct broadcast satellite (DBS) dishes use anLNBF (NB feedhorn), which integrates the antennafeedhorn with the LNB. Smalldiplexer s are often used to distribute the resulting IF signal (usually 950 to 1450 MHz) piggybacked in the samecable TV wire that carries lower-frequencyterrestrial television from an outdoor antenna. Another diplexer then separates the signals to the receiver of the TV set, and theintegrated receiver/decoder (IRD) of the DBSset-top box .Newer Ka band systems use additional IF blocks from the LNBF, one of which will cause
interference toUHF 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 andelectrical current requirements for multiple dual-band LNBFs.For some
satellite Internet andfree-to-air (FTA) signals, a universal LNB (Ku band) is recommended. Most North American DBS signals usecircular polarization , instead oflinear polarization , therefore requiring a different LNB type for proper reception. In this case, the polarization must be adjusted betweenclockwise andcounterclockwise , 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 aswitch , which acts as adistribution 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 sendingDiSEqC 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 largeTVRO dish.Universal LNB
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:
* LO: 9.75 / 10.6GHz . (the 10.6 Ghz Oscillator is selected by applying a 22 Khz Tone to the cable)
* Freq: 10.7 - 12.75 GHz
* NF: 0.7 dB (The Best LNB's have claimed values as low as 0.1 but it's unlikely that this is a true and reliable value)
* Polarisation: LinearStandard North America Ku-band LNB
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:
* Local oscillator: 10.75 GHz
* Frequency: 11.7-12.2 GHz
* Noise Figure: 0.5 dB
* Polarization: Linear
* Today, modern LNB'S give 0.3 - 0.2 and now also 0.1 dB (Noise levels during the transmission of signals)North America DBS LNB
Here is an example of an LNB used for DBS:
* Local oscillator: 11.25 GHz
* Frequency: 12.2-12.7 GHz
* Noise figure: 0.7 dB
* Polarization: CircularC-band LNB
Here is an example of a North American
C-band LNB:
* Local oscillator: 5.15 GHz
* Frequency: 3.4-4.2 GHz
* Noise figure: ranges from 15 to 100kelvin s (uses Kelvin ratings as opposed to dB rating).
* Polarization: LinearDual/Quad/Octo LNBs
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.
Monobloc LNBs
A monobloc LNB (also spelled "monoblock") is a unit consisting of two LNBs and is designed to receive satellites spaced close together, generally 6 degrees. For example in parts of Europe monoblocs designed to receive the
Hot Bird (13E) andAstra 1 (19E) satellites are popular because they enable reception of both satellites on a single dish without requiring an expensive and noisy rotator.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.
ee also
* Block Up Converter BUC
* Orthomode Transducer
*signal-to-noise ratio
*Bias tee External links
* [http://www.wsidigital.com/xtremeii.htm LNB Installation Diagram] .
* [http://www.satcure.co.uk/tech/lnb.htm LNB mysteries explained] .
* [http://www.satsig.net/lnb/explanation-description-lnb.htm Explanation and block diagram of LNB]
* [http://www.satsig.net/noise.htm Noise Temperature and Noise Figure]
* [http://www.xtendedplay.co.uk/techtip-quatro-quad-lnb-differences.html Article: Quad LNB v Quattro LNB - what's the difference?] .
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