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.
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 !
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
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+ 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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