Re: [Discuss-gnuradio] weird interaction between start of burst and oscillator phase

[Matt Ettus]

On 10/25/2010 11:20 PM, Arun Pillai wrote:
> Hi folks,
>
> I am seeing a very bizarre problem. I am using the USRP2 to transmit a
> stream of packets interleaved with silence. Each USRP packet obviously
> spans multiple Ethernet packets. When I send each USRP packet as part of
> its own burst (i.e. start of burst = 1 on the first ethernet packet
> corresponding to a USRP packet, end of burst = 1 on the last ethernet
> packet corresponding to that USRP packet, immediate = 1 on all packets,
> and repeated for each USRP packet), the channel that I see at the
> receiver for each packet has a very different phase even after
> accounting for CFO. However, if I send all the USRP packets along with
> complex zeros for intervening silence as one burst, I see that the
> channel phase is as expected i.e. the change in channel phase from
> packet to packet corresponds to exactly the CFO.
>
> Any idea why this could be happening?
>
> Thanks,
> Arun.


See line 61 of dsp_core_tx.v, which is the NCO.  During a burst, "run" 
is asserted, so the NCO continues to turn.  When run is deasserted (i.e. 
between bursts), the NCO is reset to 0.

Alternative ways of handling this would be to either just freeze the NCO 
when run is deasserted, or allow it to run all of the time.  There are 
various reasons why you might want one mode or another.

If this behavior is not desired, you can easily change those lines to 
have the NCO run all the time.

Matt