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Re: [Discuss-gnuradio] Clock Sync Problems + Timestamp question

From: Tim Pearce
Subject: Re: [Discuss-gnuradio] Clock Sync Problems + Timestamp question
Date: Tue, 20 Oct 2009 19:00:18 +0100

Ah! The 10MHz was coming out the back of the siggen so would have been locked to the same reference.

Thanks for your help Doug/Matt -- next time I'll stop and think through my test setup!



On Tue, Oct 20, 2009 at 6:16 PM, Matt Ettus <address@hidden> wrote:
Tim Pearce wrote:

   There is no frequency which can be received by both the BasicRX and
   the DBSRX.  What frequency are you putting in?  What frequency are
   you telling the USRP2s to tune to?



Sorry I probably confused things by mentioning the BasicRX

DBSRX is the only receiver we have 2 of at the moment so I was testing with a 2 GHz Sine wave, tuning the USRP2 to 2GHz and setting decim to 50.

The problem was seen on both usrp2's after enabling the 10MHz ref lock.

When you lock the reference of the USRP2, it will be exactly on 2 GHz. So if you put in a 2 GHz sine wave which is also locked to the same reference or locked to something very close, it will also be at exactly 2 GHz.  Thus, when it is downconverted to baseband, it will be at DC, and will be removed by the DC offset correction.  Everything is functioning exactly as it should.

Before enabling the ref lock the sine wave appeared on the oscilloscope as expected.

That is because without the ref lock you will have some frequency error, and the signal won't be on exactly 2 GHz.

I tried one USRP2 with the BasicRX just to check the same problem was appearing -- a 25MHz and 150MHz signal were tested (same decim and tuning to the frequency the sig gen is generating on). Both frequencies looked correct on the scope sink until the program was run with the clock locked to the reference in. - the symptom was slightly different (appearing as a DC signal rather than noise) which is why I mentioned.

The BasicRX has no DC offset correction, so you will see a DC level, which is exactly what you are asking for.

I think that what you really want is to tune the USRP2 and the sig gen to different frequencies.


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