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Re: [Discuss-gnuradio] rssi questions

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] rssi questions
Date: Tue, 7 Aug 2007 17:32:46 -0400

On 8/7/07, Johnathan Corgan <address@hidden> wrote:
> It's a "digital RSSI" value, meaning it's based on the output of the ADC
> and not the true RF power at the antenna.  Furthermore, it's really a
> single pole low pass averaging filter on the absolute value of the ADC
> value, with no way to adjust the time constant.
> You would have to use this value on the host, which has knowledge of the
> daughterboard in use, gain settings, ADC configuration, etc., to
> estimate the Rx power.
> > Should this value be an expression in dB of the received power?
> That would be nice, but would require actually measuring this in the RF
> front end using some sort of logarithmic amplifier or square law
> detector.  This has it's own disadvantages as it diverts power from the
> receive chain (and of course adds cost, power, complexity, and board space.)

I haven't taken a look at the daughterboards, but do they all use
different amplifiers and different numbers of stages?  Are the
amplifiers setup in a linear or logarithmic mode?  Do they change
significantly between the boards, or could a generic AGC module be

Since the FPGA would control the values written to some AGC DAC, it
could be possible to calculate a pretty close estimate to the actual
RF RSSI without extra components.  Is the gain just set once
currently?  Does the FX2 write to the DAC, or is it within the FPGA?


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