Well, since I have Tom’s oscilloscope, I started probing around, measuring the voltages and waveforms of pretty much every location I could find. And, low and behold, I didn’t really find anything wrong. I began to question my earlier assessment that anything was wrong at all. So, I hooked the gadget up again, wired on the antenna, and recorded a minute of audio at 96khz, 24 bit on my Macbook using Audacity. I then took my spectrum analysis code that I wrote, hacked out some bits, made it read I/Q signals, and fed in the audio. Here’s what I got:

Two strong morse signals, and part of a third!

A couple of things leap to mind here: it’s pretty clear that the sound card on my laptop can’t actually do 96Khz sampling. Audacity offers this as an option, but I suspect that somewhere along the audio chain, something is interpolating to give us 96Khz signal wihtout any actual data. The other thing is that we see a symmetry: identical signals above or below the center line. I suspect this means that I’m only getting one of the I/Q channels, not both. Were they properly phased, one of those would cancel out, and we’d just get the version of it above or below. So, I’m not out of the woods, but I am at least on the right track. I suspect some painstaking checking of solder joints might yield results.

It’s clear I also need to get some real interactive software running. Oh well, I feel like relaxing today a bit away from the radio, but maybe I’ll get back to it tomorrow. Once I get this 40m one debugged, I’ll try to take what I’ve learned and get going on my 30m one.

Addendum: I uncovered a couple of bugs while mucking around a little bit more, and now have the following image:

I checked the RMS amplitude of the I and Q signals separately, and it appeared that the Q is significantly lower in amplitude. I tried a quick patch to normalize it to the other amplitude, but it appears to not be cancelling out the opposite sideband much better. I’ll have to think about this some more.

Addendum: The basic software appears to work. I tried using it on a recording I fetched from the web, and it worked perfectly for that. Must be something wrong with my softrock still. I suspect something in the op amp circuit. I’ll check it all out tomorrow.