Hi Mark-Jan,
let me quickly reply in-text:
> - Is this LO filter automatically selected, and where should I look for the
rules ?
yes, and: uhd/host/lib/usrp/dboard/db_ubx.cpp; look for the set_{tx,rx}_freq
methods.
- Is there some way to control this LO filter selection via UHD, similar to a
tune request?
That's currently not exposed, no. You'd have to go and modify the aforementioned methods
to use some settings you specified through another function, and then do the frequency
setting respecting that.
- The IQ mixer ADL5371 documentation warns that the LO's 3rd harmonic
needs to be well suppressed.
yep
Is there benefit to be gained
by using an external, more filtered LO instead of the on-board UBX-160 LO plus
its filters ?
Yes, but you'd need to bring a rather nice LO synthesizer to the table to begin with,
modify the hardware –as far as I remember the board layout (currently don't have a UBX in
my hands)– and would need to somehow hook that LO synthesizer up to UHD so that you can
still tune with it.
So yeah, you can *always* make a nicer oscillator, the question is just how much money and
time you bring ;)
As a first step, but I'm assuming you're already doing that: try the `mode_n=integer`
tuning argument, and see whether it reduces the spurs you see in your spectrum.
Best regards,
Marcus
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