Erratic RF Gain control

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hfsdpk11
Posts: 15
Joined: Mon May 01, 2017 5:00 pm

Erratic RF Gain control

Postby hfsdpk11 » Sun May 28, 2017 8:14 pm

Is it just me, or do others find the RF Gain (Gain Reduction) control on the Main window to be inconsistent? With IF AGC enabled in Main-SETT-INPUT, moving the RF gain slider results in non-linear, and sometimes reversing, changes in Gain. E.g. at mid-range, it might report 40 dB. Gain, then 35 dB. at the next higher gain step, and 50 dB. at the next higher step. At first, I thought this might be because the IF AGC is stepping to keep the ADC input level within the acceptable window, but I get the same kind of behaviour when the IF AGC is disabled. With IF AGC = off, I'd expect the gain to vary montonically with the slider position, but it doesn't - it behaves in the same fashion as with IF AGC = on. Also, if I set RF Gain to max and IF AGC to min (20 dB.) in the hope of maximising noise figure, re-opening the Main-SETT-INPUT window to optimise IF Gain causes the IF AGC gain to peg out at max (59 dB.) and I have to re-set it. There are other similar quirks I've noticed with IF AGC-RF Gain interaction, but these are the most fundamental.

It seems that the logic controlling the RF-IF Gain values isn't working right. Can anyone else corroborate, or give advice please? What I'd expect is: (1) with IF AGC = on, that Gain should increase to some maximum value with increasing RF Gain until the ADC input reaches max then the IF AGC kicks in to keep overall gain constant and within the ADC limit, or (2) with IF AGC = off, Gain increases monotonically regardless of whether ADC input goes over limit and the only indicator of overload is the warning message in the Main window.

Thanks.

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Paul
Posts: 33
Joined: Fri Feb 17, 2017 9:52 am

Re: Erratic RF Gain control

Postby Paul » Mon May 29, 2017 6:29 am

From my very strictly non-technical aspect - yes, it does seem to be rather random, not behaving in a "linear" fashion - each step in either direction being inconsistent.
There may of course be an explanation that is beyond my very limited knowledge.
I have simply settled for whatever setting gives the best audio.

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W2DLC
Posts: 40
Joined: Tue Apr 04, 2017 1:51 pm

Re: Erratic RF Gain control

Postby W2DLC » Wed Jun 07, 2017 6:16 pm

I think you are going to figure it out after you play with it for a while. Maybe you have figured it out already. There are several variables that influence the RF gain control and the AGCs (yes, there is more than one). Look for them and experiment. One major factor is the level of RF noise at the antenna, but there are other things as well. The LNA affects the AGC too.

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MTG
Posts: 7
Joined: Sun Feb 12, 2017 6:07 pm

Re: Erratic RF Gain control

Postby MTG » Thu Jun 08, 2017 9:32 am

So, it would be useful to have a guide of how to adjust the various controls to optimise for minimum noise both with and without AGC(s?). Or failing, that a list of controls that can be adjusted, what they do and the order they should be changed - I think I have just said the same thing twice:-)

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W2DLC
Posts: 40
Joined: Tue Apr 04, 2017 1:51 pm

Re: Erratic RF Gain control

Postby W2DLC » Fri Jun 09, 2017 5:47 pm

You are working on a guide? That's great!

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hfsdpk11
Posts: 15
Joined: Mon May 01, 2017 5:00 pm

Re: Erratic RF Gain control

Postby hfsdpk11 » Sat Jun 10, 2017 1:23 am

Actually, I've developed an SDRUno user guide, albeit incomplete, that's available from my Dropbox:

https://www.dropbox.com/sh/nz5t2x9ge8td ... g3-pa?dl=0

I've posted this link in the "new users" forum as well. It's in LibreOffice Calc .ods format.

As mentioned, it is incomplete, but near enough as. I've published it for the general benefit of the community and in hopes of getting feedback as to clarification/explanation for those few functions that are highlighted as questionable or just plain not undertsood.

Back to my original post, I'm finding the "RF Gain" reported on the Main window to be inconsistent relative to increasing or decreasing adjustment of the gain slider when IF AGC is enabled. It's not just the value reported on the Main window, the gain is actually changing by huge (20 dB. or more) jumps for a single step change of the slider. I can see the gain change by the wholesale jump in displayed levels. This doesn't seem to be an issue with HDSDR (which I don't particularly like, but anyway), so I'm curious as to whether others are seeing this kind of behaviour in SDRUno and if this might be a software bug or if somethings gone West in my RSP.

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