Post by Chuck McManisHi 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.
Yup, that's exactly what I already do with the AMSAT UK Funcube Dongle
Pro+. If the RF channel I want is outside the front end's passband, I
calculate a front end tuner frequency to park it at a nominal baseband
frequency, typically +/- Fs/4 so I can avoid DC artifacts. The Funcube
has a fixed 192 kHz sample rate, so I typically choose +/- 48 kHz.
Then I ask the front end to go there. Although the API lets me specify a
tuner frequency in 1 Hz steps, the fractional-N synthesizer can't
actually give them to me; the actual tuning step for that hardware and
choice of TCXO frequency is (IIRC) 1000/2048 Hz or some small integer
multiple thereof depending on the synthesizer's frequency-dependent
output divisor.
The Funcube's API doesn't tell me its actual exact frequency, so I
extracted the synthesizer programming formulas from its firmware and
execute them myself. Then I take the difference between the actual and
requested tuner frequency and fold it into my software oscillator (the
one running near +/-48 kHz).
As a fallback, I found that if I always request multiples of 125 Hz,
I'll get them exactly. (I can use smaller steps in the lower frequency
ranges, but it was easier to just pick 125 Hz as a worst case.)
Once the front end is close to the desired frequency I can make minor
tuning changes entirely in software, leaving the front end tuning alone.
I'd like to do the same thing(s) with the HackRF. I.e., if I can't get
the exact frequency for any requested frequency, I'd like to find the
smallest tuning step that will always give me exactly the frequency I
request.
Post by Chuck McManisThe 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.
Yup, I do exactly the same thing. I use multicast RTP to convey my IQ
data from the SDR front end to my software receiver, so I can simply run
multiple copies of the latter as long as they're tuned closely enough to
each other. I used it recently to receive APRS position reports from a
balloon that transmitted on both 144.39 and 144.34 MHz. Worked great.
Phil