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RE: [Discuss-gnuradio] Changing external reference frequency with USRP2.

From: Ian Holland
Subject: RE: [Discuss-gnuradio] Changing external reference frequency with USRP2...
Date: Wed, 18 Aug 2010 11:43:32 +1000

Hi Matt

Thanks so much for your help. I tried your latest suggestion, and this gets my 
frequency offset between Tx and Rx down to a mere 1 Hz. This is much better and 
should make my testing considerably simpler.



-----Original Message-----
From: Matt Ettus [mailto:address@hidden
Sent: Wednesday, 18 August 2010 1:09 AM
To: Ian Holland
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] Changing external reference frequency with 

On 08/16/2010 09:22 PM, Ian Holland wrote:
> Please disregard my last. I must have got something wrong in my
> testing. It now compiles, but it seems I need to use txrx_xcvr.bin
> instead of txrx.bin with the latest git trunk. Please correct me if
> this is wrong (note I have XCVR2450 as my daughterboard).

This is correct.  Due to the size of the code, the xcvr was split out to
its own file.  Also, you are right about the prescaler.

> Nonetheless, I still seem to get a time varying frequency offset
> between a transmitted and received BPSK waveform, when using the same
> local oscillator of 36 MHz at each end. In fact, about every million
> samples, this frequency offset disappears, then comes back getting
> larger, then smaller and disappears again about 1 million samples
> later.
> Is this expected when using a reference different to 10 MHz? When I
> have used two USRP2s both locked to a 10 MHz reference, I never saw
> this problem.

No, you should not see that.  It sounds like it is not locked, and I
think the reason is loop bandwidth.  The original setup is for a 10 MHz
compare frequency, and you are using a 1 MHz compare frequency.  This
will mess up the loop dynamics by dividing the loop bandwidth by 10.

The greatest common divisor of 100 MHz and 36 MHz is 4 MHz, so I would
use that for the compare frequency.  Then also increase the charge pump
current to the maximum.  That should bring you closer to the original
loop bandwidth.


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