[hpsdr] Image rejection

[David Woodhead]

I hope this is not out of place here but I had little luck on the flex reflector...
 
I'm evaluating a homebrew SDR based on the SDR1K and not sure if the performance I'm seeing is normal.

I am finding that the image rejection is not consistent from band to band but the software only supports one set of correction values. I can optimize on each band with a different set of values. Is this normal? I susspect it is as the QSD balance (gain and phase) is unlikely to be consistent across the bands. It would be nice to be able to store seperate correction values for each band.

My offsets are:
............... 60m....40m....20m.......15m.......10m
phase.......-.89.....-1.11...-2.08.......0.02.......-5.44
gain......... 6.8.....7.05.....5.14.......6.61.......19.33

How much image rejection should I expect from this design? I'm adusting it by tuning the sig gen to the image frequency and then nulling the received image signal. I can acheive ~70dB null across 6KHz. However, this is not optimum for the observed image on the panadapter screen when tuned to the desired signal - the displayed image is only ~ 60dB down (unless I optimize for it). Are my results consistent with others measurements?

Also,

I have a noise spike in the panadapter (corresponding to DC). Its level bounces - it peaks at -80dBm to -115dBm depending on the preamp gain setting. I susspect this might be AC hum getting into the IQ audio. Is this normal or might I have a ground loop or something?

David
KM5TZ