Strange AGC problem

[captain jack]

Following my Penta 85 installation on my balcony with a very short cable run to the receiver, I have been having problems with strong satellites, in that the signal reading was reading way below what it should have been. At the same time the weaker satellites have been stronger than they ever have been, including extremes of the arc (up 68.5E!), so this rules out bad alignment... So, it looks like the strong satellites, combined with a short cable run is overloading the tuner.

To test the theory, I have tuned in to 19.2E NHK World transponder, which is breaking up(!) and the signal (SNR) jumps in and out of lock.

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However, after I put a wet cloth over the LNB....

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....the signal jumps to 43% and the picture is stable!

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I've never had this problem before with any receiver and this only started happening recently when I put the dish up with a short cable run. I have another bog standard receiver, which I have tried on the same transponder as above and this one does NOT exhibit this problem. I've tried two different Black Ultra LNBs and I don't really want to change it to something "deafer" as my weaker satellites (5E et al) will suffer.

It seems like the AGC in the Uno doesn't seem to be functioning correctly as it doesn't seem to hold back to strong signal that's coming into it... Any ideas? Could it be a firmware issue or is my tuner/receiver faulty?

I am using VIX 2.3 with the latest kernel 3.1.1.

[Maxwell]

you need to fit an attenuator, the very short cable and high effiency lnb and dish is indeed overloading the tuner

[captain jack]

Won't the attenuator affect weaker signals, which I need to remain as strong as they are now? Also, it's strange that a different receiver does not have this issue on the same dish/LNB/cable run.

[Maxwell]

The difference between the 2 receivers is the tuner sensitivity obviously one is more sensitive than the other, fitting an attenuator will only marginally affect your weaker signals but should bring the stronger sats down enough for the tuner to handle them, I would try a variable attenuator to determine whats best and can be had for less than a tenner

[captain jack]

Yes, you could be right. I've seen those variable ones and may be worth a shot. I have a "deafer" LNB lying around (some Titanium or Technomate 0.1dB), so I will try that one first and see what it does with strong/weak signals and then invest in the attenuator.

Other suggestions I've had elsewhere is to get a passive 2-way switch in line with the current run... Or I think I could even put in an old DiSEqC switch there, which should introduce some attenuation.

[Maxwell]

the reason for those suggestions is to attenuate the signal, you have already proved it is needed by your experiment with the wet cloth, you could also use a longer cable for the same effect but tbh for the price of a variable attenuator is it really worth the time and effort

[captain jack]

Yes, I certainly didn't have this problem when I had a 35m run in my previous place but the weaker satellites were a few SNR % points down.

[Maxwell]

SNR is not that important and any SNR measurement on a receiver is unreliable at best, what is important is quality of signal so ideally you should be looking for BER to be as low as possible ie 0 regardless of SNR readings.
Contact the forum sponsor for a variable attenuator and save yourself the headaches

[captain jack]

I am comparing SNR on the very same receiver, so it's fair test in this case. And isn't SNR (Signal to-Noise Ratio) the same thing as "quality"? I'd prefer dB readings that my old cloned DM800HD had but seems like Uno's hardware doesn't report readings in dB.

[Maxwell]

No SNR is not the same as quality as you have discovered, the SNR can go down (which is what happened when you put the wet cloth on the lnb) and your previously unwatchable picture becomes stable due to the BER dropping. The issue you have is that the signal is that strong that it gets to the point that the tuner can no longer differentiate between wanted data and noise so your BER will be very high making the processor unable to make head nor tail of the information it is trying to decode

[captain jack]

OK, I technically understand what you are saying. But surely the receiver's/tuner's AGC should kick in and rein back the signal, right? It's very odd issue to have and I would almost consider it a design flaw if prevalent on all Vu+'s especially on a decent receiver like the Uno. I shouldn't be adding attenuators here at this receiver's price. BTW, most "strong" satellites channels work fine but the SNR is in the 60's and 70's on Astra 1/2 and Hot Bird so, in most cases, it's not really a problem.

Has anyone else had these issues on their Vu+'s? If you are in the UK and point a 1.2m dish at 28.2E, based on your explanations, the receiver will go loopy, right?