AGC and rtl-sdr dongles

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mesocyclone
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Joined: Wed Jul 11, 2018 11:55 pm

AGC and rtl-sdr dongles

Post by mesocyclone » Thu Jul 12, 2018 12:04 am

First, my questions, then the background.

Questions:
  • what is the problem with hardware AGC?
  • why isn't software AGC used?
  • Does changing the gain cause a disruption, or can the gain be changed at any time in the C++ libraries?
Why I am asking...

I am interested in making an aviation receiver for a single frequency. The software I have looked at does not implement any AGC function (rtlsdr_airband and rtl_fm). ...airband has AVC (AGC on the audio) but does not adjust RF gain. Also, neither turns on the tuner AGC.

So, what is one to do? The dynamic range of the dongle is not as good as it needs to be without AGC. In my application, I would rather have intermod and blocking than no AGC, but the signal of interest may range from very weak to very strong.

My testing shows that at high gain, strong signals become unintelligible - no doubt due to clipping either in the analog circuitry, or exceeding the ADC range. At low gain, of course, weak signals disappear.

Thanks in advance.

rtlsdrblog
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Posts: 1788
Joined: Mon Nov 19, 2012 11:54 pm

Re: AGC and rtl-sdr dongles

Post by rtlsdrblog » Fri Jul 13, 2018 12:24 am

The AGC on RTL-SDR dongles was designed for DVB-T TV signals, which are wideband. So it doesn't tend to work well for the more narrowband signals that most SDR users are interested in.

If you have narrowband signal that you're focusing on, you could try to implement your own AGC in software. Not sure what you mean by disruption, but you can change the gain at anytime without restarting the dongle.

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