On 30/05/2023 11:02, Mena Ghebranious wrote:
Hi Marcus,
I took a closer look at the end of my transmission; what originally
appeared to be a lack of symmetry between the start and end delays is
actually a cutoff of 31 samples at the end of the transmission - in
other words, I'm missing the 31 samples at the end of the TX that I
put into the TX streamer.
Looking into the FPGA logic, I believe there is actually a bug in the
most recent implementation - the transmission strobe that controls the
TX output is based on the TX state machine in the radio TX core block,
who's timing does not take into account the group delay of the DUC
filter. Regardless of whether or not we are using ATR to control
GPIOs, the transmission gets cut off and the last set of samples do
not appear at the TX output (the number of samples missing is equal to
the group delay / latency of the filter for a given sample rate.)
As a temporary workaround, we could zero pad the end of our TX
waveforms, but some of the waveforms we want to run have tight PRFs
and this will heavily limit the rate at which we could run them.
I don't recall there *ever* being a time when the TX state machine
"knew" the state and depth of the DUC filters, which is why
nearly-everyone zero-pads their bursts. This has been a "thing"
with radio hardware at various times scales over the decades
for systems transmitting digital data.
I'm pretty sure that R&D would consider this behavior "design intent".
Partially because "it's always been done that way", and
partially because "fixing" it would be challenging (it would require
re-architecting parts of the FPGA chain considerably, I think).
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