[Hackrf-dev] Precise synthesizer tuning

[Phil Karn]

I would like to determine the *exact* frequency of the HackRF when set
to a given frequency. Like nearly all modern tuners they use
"fractional-N" synthesis. They can only tune to frequency steps that are
often not a simple fraction of 1 Hz, and this step size often varies
with operating band.

I'd like to have either of two things, ideally both:

A formula that gives the exact frequency to which the HackRF tunes when
set to frequency 'f'.

A frequency where the HackRF synthesizer is exactly correct (assuming an
accurate timing reference) and the minimum frequency step that will keep
the synthesizer exactly on frequency throughout the HackRF's coverage
range. (This step size may vary with band as various dividers are
changed, but using the largest step size across the full coverage is the
easiest way.)

If I know the actual frequency of the synthesizer I can easily correct
for any offset in the software oscillator I use to downconvert the low
digital IF to DC.

I could probably figure this out by digging into the firmware and the
synthesizer data sheets, but I wanted to first see if someone could save
me the trouble.

Thanks!

Phil Karn, KA9Q