Re: [Discuss-gnuradio] unknown phase-relation between rx and tx

[Eric Blossom]

On Sun, Jun 22, 2008 at 05:21:41AM -0700, wolfgang buesser wrote:
> 
> Sorry,

> I may be completely off track, but I dont' see any clock-source on
> the schematic of my LF-RX/TX (not basic RX/TX).  As far as I
> understand they only process the signals as baseband without any
> frequency conversion.  Yes, I have to use 2 daughter-boards (one
> LF-RX and one LF-TX) to complete the path, but if they both transfer
> the signals to the same AD9862 without any conversion I expect the
> phase-correlation to be an issue of the interpolator/decimator
> inside the AD9862 or potentially the FPGA-code.
>
> Thank's
> Wolfgang

Wolfgang, 

The standard configuration uses the built-in digital upconverter in
the AD9962 on the transmit path.  One problem with that is that
although we have control of the frequency tuning word, there's no way
to control or reset the absolute phase of the the NCO in the DUC.

On the downconversion side, we do have control over the absolute
phase, and thus can ensure coherence.

>From usrp_standard.h:

  /*!
   * \brief Set the digital down converter phase register.
   *
   * \param channel     which ddc channel [0, 3]
   * \param phase       32-bit integer phase value.
   */
  bool set_ddc_phase(int channel, int phase);


Once upon a time, we did handle the digital upconversion in the FPGA,
but it's been years since that code was used or tested.  We use the
one in the AD9862 to save resource in the FPGA.  If you only need a
single tx and rx path, there's most likely enough room in the FPGA and
you could try to resurrect that code (fpga and host).

Eric