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

[Matt Ettus]

Peter Volgyesi wrote:
> 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).
>   


The RFX400s have PLL chips on them.  The PLL chips on the 2 boards are 
fed the same reference clock since it comes from the motherboard.  
However, they both divide the clock down by the "R Divider" value, which 
is typically 8.   The RFX400 has a second divide-by-2 as well for a 
total of 16.  Thus there would be an 8-way ambiguity, since the dividers 
could be at different phases.  Thus the relative phases between 2 boards 
can only be one of 16 discrete values.  This value will stay constant as 
long as you don't retune to a different frequency or allow the pll 
enable pin to go low.

If you set your RF frequency to a multiple of 64 MHz, however, there 
will be no ambiguity.

Matt