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

From: John Ackermann N8UR
Subject: Re: [Discuss-gnuradio] An interesting application for my new USRP... some input requested
Date: Tue, 24 May 2005 17:19:39 -0400
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

Larry Doolittle wrote:

[ snip ]
In principle, you could dedicate a second channel to recording the
GPS band, and extract near perfect frequency calibration from that.
Unfortunately for your time-frame, GPS reception in itself is a
serious project that has not been fully solved by the GNU Radio
community (i.e., Krzysztof).

2. If I clock the USRP from a highly stable external reference, is there any other source of possible frequency error in the system (since we're at HF, I shouldn't need to use a downconverter)? In other words, will the measured FFT frequency have any error source other than errors in the external reference?

Not in the long term.  Any acquisition hardware adds phase noise,
but that mostly averages away for the measurement you want to make.

Thanks for the response, Larry. You're right that phase noise should average away over these time intervals. I could also record a pilot frequency (ie, put 10MHz from my frequency standard) as part of the stream -- I did that at audio for the beat-note method I used in the past.

Actually, over these time intervals GPS wouldn't be as accurate as my local references -- assuming you're doing normal GPS time extraction, you need to get out to a few thousand seconds of averaging before GPS reaches its best accuracy.

I snipped your earlier comment about the data stream, but are you suggesting just capturing a single wide data stream instead of four discrete ones? I was thinking that going with the four narrow channels would reduce the data transfer requirements, but if that's not an issue, what the heck...

Thanks again,


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