Low Noise Block Downconverter
Have you ever wondered what is an LNB and what is an LNB LO frequency?
The abbreviation LNB stands for “Low Noise Block doawnconverter”. It is the device on the front of a satellite dish that receives the very low level microwave signal from the satellite, amplifies it, changes the signal frequency band to a lower frequency band and sends it down the cable to the indoor receiver, demodulator/modem.
The expression low noise refers the quality of the first stage input amplifier transistor. The quality is measured by its Noise Temperature, Noise Figure or Noise Factor. Both Noise Figure and Noise Factor may be converted into Noise Temperature. The lower the Noise Temperature the better. So an LNB with Noise Temperature = 100K is twice as good as one with 200K. C band LNBs tend have the lowest noise temperature performance while Ka LNBs have the highest (worst).
The expression “Block” refers to the conversion of a block of microwave frequencies as received from the satellite being down-converted to a lower (block) range of frequencies in the cable to the receiver. Satellites broadcast mainly in the range 4 to 12 to 21 GHz.
If both input probes have their own LNB amplifiers etc you have effectively two LNBs in the same module, which will have two output cables, one for each polarisation. Many variants on this theme exist, with options also for multiple bands. Such a “Quad LNB” might thus have 4 outputs, for each polarisation and each of two bands. Such an arrangement is attractive for a block of flats, head end antenna, which need to feed multiple indoor satellite TV receivers with the viewers all wanting all permutations of the two polarisations and two frequency bands.
LNB Frequency Stability
All LNBs used for satellite TV reception use dielectric resonator stabilised local oscillators. The DRO is just a pellet of material which resonates at the required frequency. Compared with quartz crystal a DRO is relatively unstable with temperature and frequency accuracies may be +/- 250 kHz to as much as +/- 2 MHz at Ku band. This variation includes both the initial value plus variations of temperature over the full extremes of the operating range. Fortunately most TV carriers are quite wide bandwidth (like 27 MHz) so even with 2 MHz error the indoor receiver will successfully tune the carrier and capture it within the automatic frequency control capture range.
If you want the LNB for the reception of narrow carriers, say 50 kHz wide, you have a problem since the indoor receiver may not find the carrier at all or may even find the wrong one. In which case you need a rather clever receiver that will sweep slowly over a range like +/- 2 MHz searching for the carrier and trying to recognise it before locking on to it. Alternatively it is possible to buy Phase Lock Loop LNBs which have far better frequency accuracy. Such PLL LNBs have in internal crystal oscillator or rely on an external 10 MHz reference signal sent up the cable by the indoor receiver. PLL LNBs are more expensive. The benefit of using an external reference PLL LNB is that the indoor reference oscillator is easier to maintain at a stable constant temperature. Ka band LNBs operate at such high frequency that they can need phase look loop frequency control unless the wanted carriers are very large bandwidth. An internal PLL uses a crystal oscillator in the LNB. An external reference PLL uses a 10 MHz reference supply from the customer’s indoor modem or receiver.
LNB Supply Voltages
The DC voltage power supply is fed up the cable to the LNB. Often by altering this voltage it is possible to change the polarization or, less commonly, the frequency band. Voltages are normally 13 volts or 19 volts.
Perfect weatherproofing of the outdoor connector is essential, otherwise corrosion is rapid. Note that both the inner and outer conductors must make really good electrical contact. High resistance can cause the LNB to switch permanently into the low voltage state. Very peculiar effects can occur if there poor connections amongst multiple cables to say an LNB and to a transmit BUC module as the go and return DC supplies may become mixed up and the wrong voltage applied across the various items. The electrical connections at the antennas between the LNB and the BUC chassis are often indeterminate and depend of screws in waveguide flanges etc. Earth loop currents may also be a problem – it is possible to find 50 Hz or 60 Hz mains currents on the outer conductors – so be careful. Such stray currents and induced RF fields from nearby transmitters and cell phones may interfere with the wanted signals inside the cables. The quality and smoothing of the the DC supplies used for the LNBs is important.
LNB Transmit Reject Filter
Some LNBs incorporate a receive band pass, transmit band reject filter at the front end. This provides both good image reject response for the receive function but also protects the LNB from spurious energy from the transmitter, which may destroy the LNB.