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

[Matt Ettus]

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.

Matt