Low noise block downconverter ( LNB )

Have you ever wondered what is an LNB and what an LNB LO frequency ?  Here is some information about LNBs that I hope will help explain matters.

The abbreviation LNB stands for Low Noise Block.   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 signals to a lower frequency band and sends them down the cable to the indoor receiver.

The expression low noise refers the the quality of the first stage input amplifier transistor.  The quality is measured in units called Noise Temperature, Noise Figure or Noise Factor.   Both Noise Figure and Noise Factor may be converted into Noise Temperature.  The lower the noice temperature the better.   So an LNB with Noise Temperature =  100K is twice as good as one with 200K.

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.

Low noise block downconverter (LNB) diagram

The diagram shows the input waveguide on the left which is connected to the collecting feed or horn.  As shown there is a vertical pin through the broad side of the waveguide that extracts the vertical polarisation signals as an electrical current.  The satellite signals first go through a band pass filter which only allows the intended band of microwave frequencies to pass thorough.  The signals are then amplified by a Low Noise Amplifier and thence to the Mixer.  At the Mixer all that has come through the band pass filter and amplifier stage is severely scrambled up by a powerful local oscillator signal to generate a wide range of distorted output signals.  These include additions, subtractions and multiples of the wanted input signals and the local oscillator frequency.    Amongst the mixer output products are the difference frequencies between the wanted input signal and the local oscillator frequencies.  These are the ones of interest.   The second band pass filter selects these and feeds them to the output L band amplifier and into the cable.  Typically the output frequency = input frequency - local oscillator frequency.  In some cases it is the other way round so that the output frequency = local oscillator frequency - input frequency.  In this case the output spectrum to be inverted.

Examples of input frequency band, LNB local oscillator frequency and output frequency band are shown below.

All the above illustrate a simple LNB, with one LNA and one LO frequency.

More complex LNBs exist, particularly for satellite TV reception where people wish to receive signals from multiple bands, alternative polarisations, and possibly simultaneously.

Dual-band LNBs

These will typically have two alternative local oscillator frequencies, for example 9.75 GHz and 10.6 GHz with the higher frequency option selected using a 22 kHz tone injected into the cable.  Such an LNB may be used to receive 10.7 - 11.7 GHz using the lower 9.75 GHz LO frequency or the higher band 11.7 - 12.75 GHz using the higher 10.6 GHz LO frequency.

Dual polarisation LNBs

The LNB shown above has one wire going into the waveguide to pick up vertical polarisation.  If the input waveguide is circular is can support two polarisations and it can be arranged for there to be two input probes at right angles, thus allowing two alternative polarisations to be selected (vertical or horizontal), either one or the other.  Dual polarisation LNBs may commonly be switched remotely using two alternative DC supply voltages.  e.g. 13 volts makes it receive vertical polarisation and 19 volts make it receive horizontal polarisation.

Multi-LNBs

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.

LNB Phase noise

All modern DRO LNBs are sold as 'digi-ready'.  What this means is that some attention has been paid in the design to keeping the phase noise down so as to facilitate the reception of digital TV carriers.  The phase noise of DRO LNBs is still far worse than for PLL LNBs.   What good phase noise performance is really needed for is for the reception of low bit rate digital carriers and for digital carriers using high spectral efficiency modulation methods like 8-PSK, 8-QAM or 16-QAM modulation, which reduce the bandwidth required but need more power from the satellite, a bigger receive dish and better quality PLL type oscillators in both the transmit and receive chains.

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 polarisation 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, such as those from Invacom, 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.

How to test an LNB:

Check with a current meter that it is drawing DC current from the power supply.  The approx number of milliamps will be given by the manufacturer.  Badly made or corroded F type connections are the most probable cause of faults.  Remember that the centre pin of the F connector plug should stick out about 2mm, proud of the surrounding threaded ring.

Use a satellite finder power meter.   If you point the LNB up at clear sky (outer space) then the noise temperature contribution from the surroundings will be negligible, so the meter reading will correspond to the noise temperature of the LNB, say 100K (K means degrees Kelvin, above the 0 K absolute zero temperature).   If you then point the LNB at your hand or towards the ground, which is at a temperature of approx 300K then the noise power reading on the meter should go up, corresponding to approx 400K (100K +300K).

Note that LNBs may fail on one polarisation or on one frequency band and that the failure mode may only occur at certain temperatures.

If you choose to try a replacement LNB in a VSAT system check the transmit reject filter and supply voltage - you don't want to be one of those people who keeps blowing up LNBs trying to find a good one !

Overloading an LNB

If you have a very large dish, say 7m diameter and point it at a satellite whose signals are intended for reception by small 70cm diameter antennas then the 20 dB increase in total power of the signals into the LNB may be sufficient to overload some of the transistor amplifier stages inside.  This is not always obvious.  Measuring the composite output power of the LNB using a power meter is suggested and comparing this with the -1 dB compression point in the manufacturer's specification.  An alternative is to do an antenna pattern test on both a high power and a low power satellite.  Any non linearity problem with the high power satellite is then clearly visible.  Special low gain or high power output level LNBs are available for use with large dishes.