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Re: [Discuss-gnuradio] Reclocking URSP and phase imbalance

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Reclocking URSP and phase imbalance
Date: Thu, 05 Jun 2008 11:04:53 -0700
User-agent: Thunderbird (X11/20080501)

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.


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