Issue with AGC operation

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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

Issue with AGC operation

Postby DaveB » Fri Mar 13, 2015 8:44 pm

Using the latest HDSDR plug-in I've noticed some strange behaviour with the LNA GR threshold in relation to the Overall gain reduction. Basically with the gain reduction threshold reduced to turn off the LNA the (Total) Gain reduction figure increases. Examples in 5 MHz B/W in different parts of the band:

LNA Gain threshold (LNA on/off) Total gain reduction
1. 59 on 51
40 off 54 new total GR

2. 50 on 34
30 off 38 new total GR

Now I would have thought that the total GR would go down with the LNA turned off as a 24dB gain reduction has been manually induced. Unfortunately the HDSDR EXtIO doesn't show the GR figures for each stage like the SDR# one does so can't see what if happening to the Mixer and IF gains.

Can the SDRPlay team replicate this and comment please. It seems to me to be affecting the performance.

Thanks
David

TyTanYGraig
Posts: 14
Joined: Wed Jan 07, 2015 1:58 pm

Re: Issue with AGC operation

Postby TyTanYGraig » Sat Mar 14, 2015 5:42 pm

Hi David,
Can you say what band you were operating in? My own understanding is that the gain reduction figure is the total receiver gain reduction required by the AGC to achieve the necessary ADC set-point. i.e., it includes the gain reduction of the LNA. Ideally, this number would remain unchanged whether the LNA GR threshold was passed or not. However, with the LNA off, the overall receiver NF will increase and so the total voltage into the ADC (signal plus noise) for a given bandwidth will increase. This may force the AGC to turn the gain down a bit further to keep the ADC input voltage at the required level. This effect is likely to be more prevalent in the wider band settings and in bands where signals are pretty small, so the overall noise voltage integrated across the selected bandwidth is quite significant compared to the signal level. I see something similar to what you do, but in the bands that I am using the changes in total GR are only 1-2 dB (max) between when the LNA is on or off.

Cheers


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