Hello Andreas,
The digital frequency offset is handled in the DUC block which does not run
in the same clock domain as the Radio block. The phase accumulator register
will only increment when samples are being processed. The simplest way to
cause the phase accumulator to continuously run (from the timing
perspective of the DAC) is to send samples continuously rather than in
bursts. This incurs a potentially significant downside of having to push
out the extra samples from the computer. Since the DUC does not have
knowledge of the timestamp data of each sample it would require invasive
changes to make the phase accumulator look as if it were continuously
running between bursts.
Regards,
Derek
On Mon, Jul 16, 2018 at 2:20 PM, Andreas Leuenberger via USRP-users <
usrp-users@lists.ettus.com> wrote:
> Hello all,
>
> I am using a USRP X310 with a UBX-160 daughterboard. The transmit signal
> on port TX/RX is attenuated and looped back to the receiver on port RX2.
>
> Every millisecond I am sending a short rectangular pulse (about 10 us long)
> using the 'time_spec' attribute of the transmit meta-data
> ('start_of_burst' and
> ''end_of_burst are true).
>
> The frequencies of the transmit and receive LO's are set to the same
> frequency. Also the digital transmit and receive frequencies offsets are
> set
> to the same value.
>
> I receive continuously and monitor the phase difference between the
> transmitted
> and received pulses.
> If the digital frequency offset is zero, the phase difference of
> consecutive
> pulses stays constant, as expected.
> If the digital frequency offset is different from zero, the phase
> difference
> of consecutive pulses jumps.
>
> What causes this phase jumps?
> Maybe the digital oscillator of the transmitter is running only while
> sending. If yes,
> is there a way to force the transmit digital oscillator to run
> continuously?
>
> I am using the following software versions:
> linux; GNU C++ version 5.4.0 20160609; Boost_105800;
> UHD_4.0.0.rfnoc-devel-788-g1f8463cc
>
> Thanks in advance,
> andreas
>
>
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