[Hackrf-dev] Determining exact tuning frequency

[Chuck McManis]

Hi Phil,

What I've done is that I center tune into the neighborhood of where I want
to tune, and then use either a DDC or a soft mixer function (in GRC or
another module) to tune exactly where I want. If you're trying to homodyne
to the precise frequency then I agree you'll not always be successful.

The other advantage of doing the tuning after you've got your pass band is
that you can run multiple receivers off the same pass band data if they are
all within range. One of my examples I show people is an FM radio with 5
different tuners. The HackRF is tuned to the center of the FM band and then
I can move the 5 tuners anywhere within that band, demodulate them, and mix
and match the audio. Nothing like having a bit of Classic Rock 98.5 in the
background when listening to KFRC (KCBS FM) 106.9.

--Chuck


On Fri, Jun 22, 2018 at 10:22 PM Phil Karn <karn at ka9q.net> wrote:

> Since the HackRF One (like most SDRs) uses fractional-N synthesizers, it
> can't always give you the exact tuner frequency you ask for. I would
> like to write a function to determine the actual frequency from the one
> I ask for so I can compensate for the difference in a later point in my
> processing chain. (I'm using a GPSDO clock so these small frequency
> offsets are actually quite noticeable.)
>
> I'm looking at the spec sheets and firmware for the various PLL
> constants but I'd like to know if anyone has already done this. Thanks.
>
> Phil
>
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