Reclocking URSP and phase imbalance

GNU Radio
1

To the USRP experts out there:

I reclocked my USRP from 64 to 65.536 MHz and have recently discovered
that the I & Q samples are no longer orthogonal, rather they seem to be
only 70 degrees or so apart. I would guess this is due to some hard
coded values in the CIC or HB filters? Is there a USRP command to
correct for this phase imbalance? Thanks!

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2

Gregory W Heckler wrote:

To the USRP experts out there:

I reclocked my USRP from 64 to 65.536 MHz and have recently discovered
that the I & Q samples are no longer orthogonal, rather they seem to
be only 70 degrees or so apart. I would guess this is due to some hard
coded values in the CIC or HB filters? Is there a USRP command to
correct for this phase imbalance? Thanks!

That’s an odd problem. Sounds like a timing issue. Keep in mind that
the ADCs are only spec’ed to 64 MHz and the FPGA image is compiled to
meet timing at 64 MHz. You might need to recompile it with the stricter
timing numbers. All of the DSP stuff in the FPGA should not care about
the absolute frequency as long as it meets timing.

You also need to make sure that the tuning word sent to the DBSRX is
based on a 65.536 MHz reference frequency, not 64.

Matt

3

On Thu, Jun 05, 2008 at 11:04:53AM -0700, Matt E. wrote:

That’s an odd problem. Sounds like a timing issue. Keep in mind that the
ADCs are only spec’ed to 64 MHz and the FPGA image is compiled to meet
timing at 64 MHz. You might need to recompile it with the stricter timing
numbers. All of the DSP stuff in the FPGA should not care about the
absolute frequency as long as it meets timing.

You also need to make sure that the tuning word sent to the DBSRX is based
on a 65.536 MHz reference frequency, not 64.

Gregory,

You can handle that last part by modfiying this line in usrp_basic.h
and recompiling everything:

/*!

  • \brief return frequency of master oscillator on USRP
    */
    long fpga_master_clock_freq () const { return 64000000; }

Eric

4

Gregory W Heckler wrote:

I reclocked my USRP from 64 to 65.536 MHz

Greg,

Just out of curiosity, why did you do this? Related to GPS?

Chris