broadcast satellite receiving systems

[airborncigar]

1. Does anyone have circuitry for the Microwave freqency changing circuits in a typical Universal lnb?
2Ditto for the channel selection system?

 

[betwixt]

I do not have schematics but the LNB has very little t do with channel selection. All the LNB does is down-convert it's input frequency by around 10GHz so it is low enough to carry along a coaxial cable. The channel selection is done by tuning across the down-converted spectrum and in the case of digital broadcasts, by de-multiplexing as well.

Brian.

 

[airborncigar]

I do not have schematics but the LNB has very little t do with channel selection. All the LNB does is down-convert it's input frequency by around 10GHz so it is low enough to carry along a coaxial cable. The channel selection is done by tuning across the down-converted spectrum and in the case of digital broadcasts, by de-multiplexing as well.

Brian.

Thankyou Brian.
One of my current pastimes is attempting to repair faulty STB's .It's hampered by being unable to find sufficiently detailed info on circuitry.
This particular problem is loss of low band signals,which could be due either to the failure of the Lo band local oscillator ,or the failure of of the lo band tuning system . The first is in the LNB but it has to be enabled from the STB which means an enabling signal must be generated in the STB and passed up the cable to the LNB. The second is in the STB and must be enabled by the Channel selection system I think hence the reason I would like more detailed information. A/BCigar

 

[davenn]

Band selection is done by sending a 22kHz signal up to the LNB from the STB, Horiz or Vert selection done by changing the DC voltage
So if it isnt changing from one band to the other check that the STB is producing 22kHz when required.
there is no tuning /changing of freq within the 2 bands that is done entirely by the STB all the 22kHz does is to select/deselect one of the 2 LO's in the LNB
if the fault is in the LNB just buy a new one, they are dirt cheap

Dave

 

[BigBoss]

LNBs are not repaired, buy new one.. 15$ ..more or less..

 

[betwixt]

I agree with BigBoss, unless you have special SMD handling tools, capable of working on Teflon substrates, I would avoid repairing LNBs. It can be done and I've done it several times but it isn't easy and the HEMT transistors are expensive and difficult to get hold of.

The STB only tunes one band, as Davenn states, the LNB LO is switched when it sees the 22KHz tone to decide which part of the satellite band is down shifted to match the receiver, the frequency display takes into account whether the tone is switched on or not.

From a recent experience here: beware that chaining receivers using the "IF out" or "loop" facility which basically gives a DC blocked copy of the LNB input socket may not isolate the tone signal. I spent ages working out why a spectrum analysis of a satellite downlink was showing wrong frequencies until I realized a receiver down the chain was still managing to switch the LNB to the wrong half of the band!

Brian.

 

[davenn]

From a recent experience here: beware that chaining receivers using the "IF out" or "loop" facility which basically gives a DC blocked copy of the LNB input socket may not isolate the tone signal. I spent ages working out why a spectrum analysis of a satellite downlink was showing wrong frequencies until I realized a receiver down the chain was still managing to switch the LNB to the wrong half of the band!
Brian.

hahaha good one Brian, thats a nasty trap, wonder how long it took to realise that not to be repeated ...huh
(dont answer )

cheers
Dave

 

[betwixt]

I will answer - it took long enough for me to decide the LNB was faulty and order a new one! Arrgghhh!!!

Brian.

 

[davenn]

I will answer - it took long enough for me to decide the LNB was faulty and order a new one! Arrgghhh!!!
Brian.

ohhh dear
yeah and after 40 years in electronics, I still solder cables onto plugs and forget to slide the plug housing on first

Dave