Hardware ComponentsLNBThere is great post by ShaneW at
http://techies.co.za/phpBB2/viewtopic.php?t=5368&start=0&postdays=0&postorder=asc&highlight=
Hi guys. I was asked to explain to a few colleagues how an LNB works so I wrote up a bit of an explanation, thought it might help a few people on here aswell.
Please let me know if you have any comments/questions/changes.
LNB stands for Low Noise Block Down converter. Its main function is to convert a Block(Group) of High frequency signals to a lower frequency block, amplify it and give it to the decoder/receiver.
Satellites transmit and receive signals in the 3GHz – 5GHZ (Cband) and 10GHZ- 13GHz (KU Band) range, bear in mind that 5 GHZ=5 000 MHz = 5 000 000 KHz = 5 000 000 000 Hz (cycles per second). When signals are in this range, a coax cable is not a very conductor and you would lose most of your signal. This is where an LNB comes in, it converts the high frequency signal, coming from the satellite, to a lower more manageable frequency. These frequencies are in the 950 MHz to 2150 MHz range (L Band) and this is referred to as IF (intermediate frequency). Some High-end satellite earth stations use a LNA (Low Noise Amplifier) which doesn’t perform the Down-conversion, the high frequency signals are transported on a line, but it is expensive as you would need a waveguide(hollow tube with the correct dimensions for that frequency.)
Most satellite decoders are designed to receive L – Band signals, this is why we can use a satellite decoder to receive either C-Band or KU-Band, because they are both in the same frequency range (L-Band), after being down-converted by the LNB.
The First circuit in the LNB is a filter circuit, which removes all the unwanted signals, including any ‘out of band’ noise, it then passes the wanted frequencies to an amplifier which amplifies the small signal from the satellite. It then goes to the mixer circuit. How an LNB down-converts to a lower frequency is by the process of mixing. It uses a LO (Local oscillator), which is just a circuit that gives a specific frequency out, this signal has no Data or program content on it, it is just a ‘pure’ carrier. This LO signal, is then added to the signal that is coming from the satellite.
When you add 2 signals together you get a few results:
When we mix the LO FREQ with the SAT FREQ we would typically get the frequencies LO+SAT, LO-SAT, SAT-LO, LOxSAT, LO and SAT. Out of all of these signals that are produced we only want 1 of them, so the wanted freq would be filtered out and sent down the coax cable to our Decoder, sometimes we use the LO-SAT and sometimes the SAT-LO, just depending on what will get us to L-B and.
Here is an example:
Hope Channel on C-Band is transmitted at 4070 MHZ and the Local oscillator of a C-Band LNB is 5150 MHz, when these 2 signals are mixed inside the LNB, there are a few results but the one we want in the L-Band range ( LO – SAT ) 5150Mhz – 4070 MHz = 1080 MHz is filtered out and given out the decoder.
Most Decoders do this calculation for you so that you only have to insert the Satellite frequency into the decoder, provided that the LO frequency programmed into the decoder is correct. This does sometimes become a problem, in SA for instance we use KU-Band LNBs with a High LO of 10.7 GHz and most of the rest of the world uses 10.6 GHz, so most imported decoders are set to 10.6 GHz LO as standard. This is How it would work:
Say for instance we had a channel on 12 GHz, we would program this frequency into our decoder. So the decoder does its calculation 12 GHz-10.6 GHz(SAT-LO) = 1.4 GHz so it will look at 1.4 GHz ( 1400 MHz) for the signal. BUT our LNB is actually a South African type and its LO freq is 10.7 GHz. So what is happening in the LNB is 12 GHz – 10.7 GHz (SAT-LO) = 1.3 GHz. The signal is there, but it is not where the decoder assumes it to be and will give us a ‘no signal’ error. This is why it is important to give the correct LNB LO frequency details to your decoder.
A LNB also requires some power to turn it on and power its circuits. This is given by the decoder along the coax cable. So, going up to the dish we have the DC supply voltage and coming down we have the RF signal.
To save space on the satellite and allow more channels to be added, they use 2 polarities, Horizontal and Vertical. Circular polarization can also be used but I won’t attempt to explain this as I don’t fully understand how they do it yet. Polarization is basically the way the antenna is mounted, either parallel to the ground (Horizontal – Most TV antennas) or Perpendicular to the ground (Vertical – Like your car’s FM aerial). When a receive Aerial/Antenna is placed in the opposite polarity to the transmitter, a large amount of signal (If not all) is not received, so therefore they can put channels on horizontal and vertical on the satellite, without them interfering too much. This is what you are adjusting when you turn the ‘skew’ of your LNB.
Inside your LNB, you have 2 small aerials that sit inside, at 90 degrees to each other. The one will be Vertical and the other Horizontal, and they are initialized by either a 13V (Vertical) or 18/19V (Horizontal) DC supply coming from your decoder. Your decoder automatically switches between Vertical and Horizontal, depending on the polarization of the required channel.
On KU Band to get more channels onto the satellite, they expand the bandwidth and add more channels/transponders. They now introduce 2 LO’s into the LNB, A Low and a High, the high LO is initialized by a 22kHz tone on the cable. The Frequency of the Low Band would typically be 10.7 GHz – 11.7 GHz and the High Band 11.7 GHz – 12.85 GHz, produced by a LO of 9.75 GHz or 10.7 GHz respectively.
There are 4 states that an LNB could be in, provided that it as a dual band unit: Horizontal - High Band, Horizontal – Low Band, Vertical - High Band and Vertical - Low Band.
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Dish PointingHow to align a dish yourself:Yes it is most definitely possible to do a self install, without any fancy test gear, just get the basics right!
You simply need a good sense of direction, a compass, a school protractor and a builders level, and some spanners that fits the nuts!. Of course you can always buy one of those caravaners kits because there is a simple pointing device in there, and then follow those directions! and a lot of patience is needed!
Make sure the pole mount that you screw to the wall is Vertical in all directions, as this make the pointing exercise much easier.
Mount the antenna on the pole
SKEW angle setting:(a) Set the ELEVATION angle (up and down) in such a way as to make sure the LNB is Horizontal to the ground (the part point into the antenna).Temporarily lock the antenna in that position just so that it does not move on you.
(b) Now looking into the antenna, adjust the LNB skew angle, down from the Horizontal, rotation clockwise, to approximately the 16H00 position.
(c) Use a simple school protractor, set 0 degree to horizontal, and then turn the LNB clockwise to angle calculated.
(d) lock the LNB in place tight enough so that it does not move on you, and mark the position. If you get this right now then later skew angle adjustments may not be needed at all!.
SET the ELEVATION:(a) to the calculated value, use the scale on the side of the antenna bracket, as this scale should already compensate for the offset angle.
(b) the pole mount is vertical, this angle will be almost perfectly correct
With the above two set you have now eliminated two of the 3 variables and only azimuth is now needed to be set!
SET the AZIMUTH:(a) connect up the the cables, switch on the PVR, choose Mnet 101.
(b) IF this is a new PVR, then I suggest that you connect it up at home. let it scan and activate the card BEFORE you leave for your holiday home.
(c) Use the compass to find the basic direction, point the LNB arm of the antenna in that direction. Mark the position on the flange so that at least you know where that direction is.
(d) Now rotate the antenna SLOWLY in one direction until you get a signal, ( that is if you did not have a single immediately). Move until the antenna is obviously completely off position.
(e) if no signal move in opposite direction, till you get a signal, set on maximum ( should be more than 80%), and mark the position.
(f) Now deliberately move off this point in one direction till the signal drops to "zero", mark the position.
(g) Move in opposite direction till you go through maximum and drop off again to "zero", Mark that position.
(h) Set the azimuth half way between to two marks, and that should be close to the original maximum position.
(i) tighten bolts so that antenna will not move but beware of over tightening!
PEAK the elevation(a) Now do the same as you did azimuth wise, vertically.
(b) MARK the position, loosen the clamps slightly, enough to allow antenna to just move, Move antenna UP till signal drops off to "zero", mark.
(c) Go DOWN till you get zero and mark. Divide by two and that should be where you set it originally!
(d) tighten elevation clamps
Peak the skew angle as previously described, tighten clamps enough to hold BUT beware of over tightening
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(DSD820 aka DSTV DualView Decoder).
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