On 01/03/2019 11:34 AM, Brais Ares wrote:
Hi Marcus,
Theoretically, there'll be analog group delays that will add some
harder-to-predict latency, but they'll be much smaller than the
time interval represented by the timestamps.
Ok, we can consider those analog delays negligible. Sorry to insist
but, do I still need to compensate for the digital rx path delays
then? So far I'm assuming uhd::rx_metadata_t::time_spec value refers
to whenever the first sample enters the kernel radio buffer.
Timestamps occur as packets are formed to be sent over USB on the radio
side. You'll still need to measure and compensate for
digital group delay.
If you use a timed command to set the gain, then you can at least
get a bound on which samples are likely to represent the
post-gain-change.
But given that this is an analog process, there'd be almost no
way to get down-to-the-sample precise.
Well, I overlooked those commands all these years. UHD drivers never
ceases to amaze me.
However, are you sure this should work with *set_rx_gain* command? I
can't make it work.
I configured the streamer as follows:
/uhd::rx_streamer::sptr rx_stream = usrp->get_rx_stream(stream_args);
/
/uhd::stream_cmd_t
stream_cmd(uhd::stream_cmd_t::*STREAM_MODE_START_CONTINUOUS*);
/
/stream_cmd.num_samps = num_samples;
/
/stream_cmd.*stream_now = true*;
/
/rx_stream->issue_stream_cmd(stream_cmd);/
I set commands to increase the gain linearly with time (3 dB every one
second), rx_gain = 3 * t:
/const uhd::time_spec_t now = usrp->get_time_now();/
/for (int i = 0; i < 16; ++i) {/
/usrp->s*et_command_time*(now + uhd::time_spec_t(i * 1.0));/
/usrp->set_rx_gain(3*i);/
/}/
/usrp->*clear_command_time*();/
Then I start a capture of 20 seconds straight away, but there are no
changes in the received power signal whatsoever. The actual applied
gain during the whole capture is the last configured/commanded value
(g=48), as if the set_command_time instruction was completely ignored.
Unfortunately, I mis-spoke. Timed commands for things that are set on
the AD9361 front-end are not supported on the B2xx series.
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