Low Noise Block-downconvertor

 (so called because it converts a whole band or "block" of frequencies to a lower band)

TOPICS:

Simple Table Conversions for Freeview

Dual-band LNBs

Dual polarisation LNBs

Multi-LNBs

LNB supply voltages

How to test an LNB:

Overloading an LNB

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 Noise 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.

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 polarization 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 through.

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 is inverted.

Examples of input frequency band, LNB local oscillator frequency and output frequency band are shown below. for the common New Zealand LNB's

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

Simple Table Conversions for Freeview Freq's

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 polarizations and it can be arranged for there to be two input probes at right angles, thus allowing two alternative polarizations to be selected (vertical or horizontal), either one or the other.

Dual polarization LNBs may commonly be switched remotely using two alternative DC supply voltages.  e.g. 13 volts makes it receive vertical polarization and 19 volts make it receive horizontal polarization.

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 polarization.  Many variants on this theme exist, with options also for multiple bands. 

Such a "Quad LNB" might thus have 4 outputs, for each polarization 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 polarizations and two frequency bands. 

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. 

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.

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.

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