Ahh the small board -- good question as to why is it so dang small -- I think one of Tony's original goals for the gizmo was more of a demonstration vehicle than an experimenter vehicle. I for one wish it had more working room, but I will say the experience has vastly improved my soldering and rework in small places.
The central hump you're seeing around 7.056 is normal. The gunk you're seeing around this is a consequence of the software architecture. What is happening is that the hardware takes the swath of RF centered and 7.056 and downconverts it such that 7.056 is essentially at DC. Also down there near DC is noise and 60 hz (and its harmonics) hum. When one tries to tune down near 7.056, the local software oscillator will be down in the 100's of hz's. The output of this will be Software LO +/- 60 Hz (and harmonics). Things will always be a bit messy near the center frequency -- quieter less hummy sound cards can help, but I don't believe there's any practical way to get rid of all the gunk in the middle.
If you want to check out the hardware sans computer, and have a signal generator and a scope you can put in a -30 dbm signal at 7.050 or so into the antenna. Look at the audio output on the I and Q channels -- it should be about 6 khz (7.056 - 7.060) and on the order of about 0.8 Vpp or so (don't take my nums as precision numbers, they are all from memory). The thing to look at is if both the I and Q channels are comparable in magnitude -- if one is half the other something is not quite right in the hardware. You can also take a look at the phase relationship -- it should be about 90 degrees between teh two channels. While you've got the scope out check the clocking signals to the FST3126. If you've got a generator and no scope, you might be able to compare the two channels with a good RMS voltmeter.
One last thing to check -- make sure you're using a sound card with a stereo input. Using a mono mic in input would give the symptom of not being able to null the image.
Hope this helps.
Regards,
Bill (kd5tfd)
