On 2022-02-09 18:36, Ian Buckley wrote:
Quick thoughts. Since you are RTT here, the Tx and Rx RF sections of AD9361 are
driven by different RF fractional-n synthesizers, they might lock run-to-run
with different phase relationships to their shared common reference. If you see
clustering at certain (similarly spaced) time bins in your result that might be
a hint towards this being involved.
I hadn't thought about that, but, yes, that *could* be a source of the
variability. I'm not sure whether the AD9361 supports integer-N
tuning mode, but for sure UHD doesn't expose it in the driver...
It occurred to me that as RF synthesizers end up being used almost
exclusively in DSP-based systems, there's little reason for
fractional-N tuning, as far as I can tell--it was invented to give
very-small step sizes when you're using a larger reference clock
into the PLL. But with these RF chains being backed a lot more by
flexible DSP architectures, where you can effectively achieve
sub-1Hz tuning step size, the synthesizers don't really need the very
fine step size. But that's just me waxing philosophical...
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