[Discuss-gnuradio] Deterministic TX phase on a pair of RFX400

[Peter Volgyesi]

Hi,

In our application we need to emit pure tones on two RFX400
daughtercards sitting on the same USRP board. More importantly, the
phase of the two RF carriers need to be in sync or should have a
deterministic difference.
I put together a simple experiment (GnuRadio 3.1 release branch r8117,
USRP rev 4.5, FLEX400 10-11-05) which sends a pair of constant signals
to the USRP in the baseband. The RF transmit outputs are directly
connected to a scope (5GS/s, 50 ohm imp.). My problem is that the
phase relationship between the two sines is random and changing every
time I run the python script. I suspected that the DUC(s) in the CODEC
chip(s)  cause this, so I was trying to "turn them off" by modifying
db_flexrf.py (self.set_lo_offset(0)) and using carriers where the
RFX400 board can tune to without the help of the DUC. I verified with
two LFTX boards that the CODECs "generate" constant outputs.
Still, the RF waves have random phases, which I cannot explain (since
the PLLs on the daughtercards are "driven" by the same clock coming
from the USRP).
Since I have very limited RF background, I need you to explain this
and/or suggest a solution for the initial problem.

Thank you.

--
peter