[USRP-users] Sequence Errors on USRP N200

Jahshan Bhatti jabhatti91 at gmail.com
Sun Jun 24 20:06:45 EDT 2012


On Sun, Jun 24, 2012 at 6:08 PM, Josh Blum <josh at ettus.com> wrote:
>
> On tx, the timespec is converted to an absolute tick count. When the
> USRP gets a packet with a timestamp, it waits waits until that time in
> the packet matches its current time. All subsequent data is
> backpressured the timestamp'd packet in the front.
>
> Since N200 has a 100 MHz clock, the timespec can specify a clock cycle,
> which is on the order of 10 nanoseconds.
>
> -josh
>
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Hmm. I've issued a stream command with a certain timestamp and as soon
as the rx data comes in, I send the data (with appropriate buffering).
The first packet of my tx burst has a timestamp that is some delay
later (10 ms, 100 ms, or 1000 ms). Yet I don't seem to get the delay
I'm expecting (which varies with different delays). As a test, I'm
delaying the GPS signal and feeding the authentic and delayed signal
into two timing receievers and monitoring the PPS. Is there some delay
inside the USRP that isn't accounted for?

~Jahshan




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