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Re: [Discuss-gnuradio] An interesting application for my new USRP... som

From: Larry Doolittle
Subject: Re: [Discuss-gnuradio] An interesting application for my new USRP... some input requested
Date: Thu, 26 May 2005 12:28:49 -0700
User-agent: Mutt/1.5.9i

John -

On Thu, May 26, 2005 at 03:03:12PM -0400, John Ackermann N8UR wrote:
> I was thinking of just programming the four DACs (through two 
> Basic RX boards) to each look at a, say, 10kHz wide chunk of spectrum 
> around the nominal frequency, and then do a deep FFT of each of those 
> streams.

If you did the digital downconversion in the FPGA, you could do
the same thing with as single ADC input.  I'm not sure the existing
firmware supports that, since the ADC has it's own DDC.  <tappety-tap>
Well, you can hook up a single ADC to the input of each rx_chain, and
each rx_chain has its own phase accumulator and (optional) CORDIC
unit, so I don't see why that approach wouldn't work.  No matter
what, you need to end up with four complex (eight real) low frequency
data streams.

I think the FFT-based analysis is less than ideal.  You really want
to track the phase, count cycles, and measure the phase difference
over a defined time.  delta(phase)/delta(time) is the average frequency.
Your phase error could be as small as a few milliradians, and you said
the integration time is at least 10 minutes: your measurement error could
approach a micro-Hertz, if your reference oscillator were perfect.

> I was hoping that with the USRP I'd be able to measure all four bands 
> simulateously.

That's the best approach.

     - Larry

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