Friday, April 10, 2015

low-noise block downconverter (or LNB)


A low-noise block downconverter (or LNB) is the receiving device mounted on Satellite dishes  used for satellite TV reception, which collects the radio waves from the dish. Also called a low-noise block

The LNB gets its power from the receiver or STB inside the house. This power  is sent "up" the same coaxial cable that carries the received signals "down" to the receiver, eliminating the need for a separate power cable.



Amplification and noise
The signal received by the LNB is extremely weak and it has to be amplified before downconversion. The low noise amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal.

Explanation diagram showing how an LNB works, with input waveguide, low noise amplifier, mixer, band-pass filters, local oscillator and output L band amplifier
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 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 receive frequency band, LNB local oscillator frequency and output frequency band are shown below.
C band is 3.4 - 4.8 GHz.   Ku band is 10.7 - 12.75 GHz.  Ka band is 19.2 - 21.2 GHz.


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 Temprature  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.


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 !

Correcting satellite dish distortion


LNB Outputs
As we said above, each satellite receiver needs its own feed from an LNB. This picture shows a Quad LNB – an LNB with four outputs:
Quad LNB - with four outputs

Quad LNB – with four outputs

Twin Tuner boxes? Some satellite receivers, such as Sky+, Sky+HD and the Foxsat Freesat receiver, have two tuners – these let you record one satellite channel while watching a different satellite channel. This means that they need two feeds from the LNB, not one.
Sky+ HD Box

Sky+ boxes need two LNB connections

Before picking an LNB, you need to work out how many boxes you want to feed. The most you can feed is 8, using an Octo LNB. An Octo LNB will feed 4 twin-tuner boxes or 8 standard satellite boxes (or any combination).

Can I split one satellite feed into two feeds?
This is a fairly common question. What should you do if you only have one feed from the dish, and two sockets on a box? Sadly, you can’t split a single feed from an LNB to let it feed two tuners at the same time.
Why? Satellite receivers send voltage and signals to the LNB to get the LNB to change frequency and ‘polarisation’ (some channels are Vertical and some are Horizontal). If you have two receivers both trying to switch a single LNB, one box will win, and one will lose.
You can get an LNB switch box that will let you switch your single feed to either of two receivers – just not to feed both at the same time. These are no substitute for having two dedicated feeds from the LNB though.


Need an LNB, cables or connectors?
We recommend Maplin.co.uk.

LNB Problems?
Sometimes, but rarely, LNBs develop a fault. Typically, this may mean that you can’t get any channels, or you can only get channels from one polarity (Horizontal-only, or Vertical-only).
If you’re with Sky and can’t get your channels, call them for advice – they may need to come out and replace your LNB. Otherwise you can replace an LNB yourself. You can buy LNBs suitable for Sky and Freesat from Maplin.co.uk.


http://www.radioandtelly.co.uk/


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