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Re: [Discuss-gnuradio] Good example of AGC after RTL_SDR

From: Vanush Vaswani
Subject: Re: [Discuss-gnuradio] Good example of AGC after RTL_SDR
Date: Fri, 18 Jul 2014 08:41:16 +1000

Wouldn't it be easier to add some front-end filtering?

On Fri, Jul 18, 2014 at 1:32 AM,  <address@hidden> wrote:
> Thanks Tom ... you are right it is not surprising -- but what I am surprised
> at is how good a job the builtin AGC does for most cases and maximizes the
> value of the limited dynamic range. Also, your explanation of the AGC
> matches my observations so its out.
> This is what I'm thinking (out-loud  so don't laugh) ... I have a channel
> that should be free of most signals (through FCC allocation and observation
> ... lets call it dark_channel). I can put that through an xlat_fir and get
> the resulting power of the noise floor in that channel ( should be relative
> noise floor of the sampled spectrum, call it dark_channel_power). I assume I
> can also measure the peak coming out of the source (call this
> peak_source_power). I'm envisioning a block that would set a gain based on
> the limiting peak_source_power to less than say 80% of signal peak and
> limiting dark_channel_power to some acceptable upper limit (probably derived
> from experimentation). The gain value would have to be filtered obvious to
> some reasonable time-constant.
> Questions:
> -- Does this sound reasonable? Can anyone poke a hole in it? Is there a more
> conventional approach?
> -- What would be a good block would measure the power of the dark_channel
> and source signals?
> -- For the near-far paradox ... dark_channel_power sets my max gain for far
> signals and source_power sets my min_gain for near signals -- is this a
> reasonable approach
> -- Is there a good starting point for the block to convolve these two power
> measurements into a hardware gain setting? (remote, but had to ask)
> Jim
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