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

[Arun Pillai]

Thanks, Matt. That's probably it - looking at the code from 
tx_control.v, it seems like just twiddling the start/end of burst bits 
alone is not enough either, the host needs to keep sending, otherwise 
the FPGA thinks it is an underrun. So, it seems like the correct 
solution would be to change the NCO code.

One other question: Where is the UHD code in terms of maturity? Will 
future FPGA/firmware fixes happen on the UHD code, or will the existing 
code also be maintained? I will have to change a bunch of my host side 
code to use UHD, wondering if I should invest in that effort to avail of 
more robust firmware/FPGA code. (I don't care about portability or for 
the code to work on anything other than RFX daughterboards).

Rahul


On 10/26/2010 2:42 AM, Matt Ettus wrote:
> 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