Free message board, discussion forum, online community for techies in South Africa

programmers, message board, freelance developers, South Africa
techies.co.za Forum Index

a free bulletin board, forum, online community for techies based in South Africa

Log in Register FAQ Memberlist Search techies.co.za Forum Index

techies.co.za Forum Index -> Home Entertainment ->  Satellite

Satellite

How a LNB works

Post new topic     Reply to topic
View previous topic :: View next topic  
Author Message
ShaneW



Joined: 16 Jul 2008
Posts: 37
Location: East London, RSA

PostPosted: Fri 08 Aug 2008 16h51    Post subject: How a LNB works Reply with quote

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.


Back to top
View user's profile Send private message
cclarity



Joined: 20 Jul 2007
Posts: 243
Location: JHB

PostPosted: Mon 11 Aug 2008 21h05    Post subject: Reply with quote

Very nice, can you tell us about the SATCR LNB?


Back to top
View user's profile Send private message
ShaneW



Joined: 16 Jul 2008
Posts: 37
Location: East London, RSA

PostPosted: Tue 12 Aug 2008 23h06    Post subject: Reply with quote

Smile thats your job


Back to top
View user's profile Send private message
red_dragon_za



Joined: 07 Feb 2007
Posts: 105
Location: Cape Town, South Africa

PostPosted: Wed 13 Aug 2008 12h53    Post subject: Reply with quote

But can you tell me why the rest of the World uses 10.600 for the upper LOF and we in SA have to change our receivers to 10.700.



_________________
Modified Radix DT100, Delsat DSR5500, Compro S-350,Dreambox 500, Multichoice 660, Dishes at 15/22W, 68.5E & 7E
Back to top
View user's profile Send private message
ShaneW



Joined: 16 Jul 2008
Posts: 37
Location: East London, RSA

PostPosted: Fri 15 Aug 2008 10h57    Post subject: Reply with quote

One reason : MULTICHOICE !!!!

AFAIK they decided we should use 10.7 here... the crazy thing is they don't even use the High Band


Back to top
View user's profile Send private message
red_dragon_za



Joined: 07 Feb 2007
Posts: 105
Location: Cape Town, South Africa

PostPosted: Fri 15 Aug 2008 16h58    Post subject: Reply with quote

They did use 12.517G when they first started broadcasting using PAS4 along with SENTECH analogue. When PAS 7 was launched they migrated there and PAS 4 moved from 68.5 to 72E. They did a swap out on the LNBS.



_________________
Modified Radix DT100, Delsat DSR5500, Compro S-350,Dreambox 500, Multichoice 660, Dishes at 15/22W, 68.5E & 7E
Back to top
View user's profile Send private message
cclarity



Joined: 20 Jul 2007
Posts: 243
Location: JHB

PostPosted: Fri 15 Aug 2008 21h12    Post subject: Reply with quote

If my memery serves me correctly (not often) Sentec where broadcasting first and one of there frequencies was very high, with a 10600 LNB it was out of the spectrum.


Back to top
View user's profile Send private message
red_dragon_za



Joined: 07 Feb 2007
Posts: 105
Location: Cape Town, South Africa

PostPosted: Sat 16 Aug 2008 09h44    Post subject: Reply with quote

If you checkout the TELETEXT on SABC 2 in the technical section it gives the freq as 12.5 and 12.7, it also gives the WEGENAR freqs for the sound channels. Who says that SENTECH is not up to date??????????



_________________
Modified Radix DT100, Delsat DSR5500, Compro S-350,Dreambox 500, Multichoice 660, Dishes at 15/22W, 68.5E & 7E
Back to top
View user's profile Send private message
pieteras.meyer



Joined: 16 Aug 2008
Posts: 6
Location: Krugersdorp

PostPosted: Tue 19 Aug 2008 22h17    Post subject: Reply with quote

Hi what is the best LNB to use with a SkyStar HD2 DVB-S2 card.

BR


Back to top
View user's profile Send private message
cclarity



Joined: 20 Jul 2007
Posts: 243
Location: JHB

PostPosted: Tue 19 Aug 2008 22h29    Post subject: Reply with quote

How many inputs does it have and what sat are you pointing at?


Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    techies.co.za Forum Index -> Satellite All times are GMT + 2 Hours
Page 1 of 1
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2002 phpBB Group
Theme created by Vjacheslav Trushkin

Privacy Policy