Actually, I'd be tempted to rig a phasing type conversion to baseband from
the 300 MHz or whatever the first IF is....

At best it might get 40-50 dB of suppression of the other sideband, but it
would remove most of the undesired spurs and something like an AD8307 log
detector could be used to convert to a DC level.

After that, feed it into an A/D converter and into a PC using the printer
port.

Given a dual channel A/D converter, I'd also sample the tuning voltage for
the VCO and feed that into the PC as well.

All of this would move the programming from a possibly complex DSP problem
to a relatively simple Parallel port input and then number crunching to a
display.

There would be spur responses 40-50 dB down, but generally speaking, it
should give useful information.

===========

Granted, one problem is how fast the data transfer is through the parallel
port, that would limit how many samples/second the system could process.

There is the second issue, about maximum sweep speed Vs the IF bandwidth,
and the maximum useful bandwidth would be a function of the Phase shift
detector system. Still, I think this approach has some possibilities.

Almost forgot... WIN XP and WIN2000 make getting to the parallel port for
something like this MUCH more difficult then it was under WIN 98.

You could use RS232 and perhaps two COM ports to allow getting the AD8307
and the VCO sweep voltage in parallel, so to speak. Once again, it would be
a limit on how many samples per second the systme could provide.

If you want to be more complex, you could feed OUT commands to a
synthesizer to tune the VCO and this would allow really slow sweeps and very
precise accuracy, but it complicates the design.

I have to admit, I've been looking at the synthesizer design John Miles,
KE5FX, did which tunes from 1 GHz to 2 GHz with very fine tuning steps and
all osrts of ideas for various test gear built around it keep dancing in my
mind. I suppose I should see if I can build one and get some idea as to
the cost and so on. But it'd make a wonderful starting point for a
digitally tuned spectrum analyzer !!!

Admitted, the close in phase noise could limit dynamic range 'way below
the range pssible with an AD8307 detector, but even so the frequency
accuracy of an analyzer would be fantastic.

=========

Overall, Avery is probably correct that the result may be a lot of work an
less than perfect performance, but as a homebrewer it does seem like a fun
way to get something fairly useful and learn a lot as the project goes on.




Jim Pennell
N6BIU