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
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