wrote in message
...

If you have log before ADC I think you need a superhet conversion rather
than direct conversion to baseband, for the reasons I mentioned earlier.
If
you just ADC'ed the lot there'd be some chance of sorting the confusion
out
in software, though it'd take a brave heart to try it.

So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB
range from
it, you'll find a fair bit of noise from your average sound card - well
the
first 3 bits will be noisy), one with the I and the other with the Q (from
the
DC output) and do it that way ?

I still think you stand a fair chance of problems, when essentially you're
doing your signal strength measurement in baseband and sweeping at rates in
the same frequency range. The aquisition time for the signal strength
measurement needs to be much shorter than the sweep, or the signal strength
will change while you're attmpting to measure it.

I guess it's a bit like the problem of designing audio-derived AGC in direct
conversion receivers. It takes too many cycles to measure the audio volume
at low frequencies to get a correction signal to control gain. Similar
problem. There might be a way of disentangling it in software, I don't know
the mathematics but I can intuitively imagine that it could work.

But I'm no expert on this, just a clown with soldering iron...

Hans G0UPL
http://www.hanssummers.com