Re: [Discuss-gnuradio] Unwanted relative phase change after programming a device

[Marcus D. Leech]

On 11/01/12 08:31 PM, Armando Rocha wrote:
> Hi
>
> Please note that I am referring to a differential phase. Even if the NCO
> phase is not continuous the differential phase should remain unchanged (I
> think the NCO is the same).
>
> Thanks for your contribution
>
>
>   
Ah, OK.  So you're dealing with two independant DDC chains here, which
operate off of two
  different NCOs, as far as I know, since in most cases the two chains
may be tuned to entirely
  different frequencies.   I think the NCOs are programmed separately,
from UHD, which means
  there will be a phase difference between them every time the (two)
NCOs are programmed.


Fixing this would require FPGA changes in the USRP1, which at this point
is utterly frozen as far as I know.
  You'd need some kind of "commit" logic that caused the two NCOs to be
re-started at exactly the same time
  after being programmed.  I don't *think* this can be done purely
within UHD, but maybe it can. I'm sure
  Josh or Matt could comment.

This is similar to what happens with the synthesized analog cards--they
end up with some random
  differential phase offset every time their frequencies are changed,
due to the way the frac-N synthesizers
  work.



-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org