Last night I performed additonal tests on the Softrock to determine the impact of third LO harmonic response in the medium wave broadcast band. Broadcast signals were stronger than the previous night and the situation got ugly in a hurry after sunset. This receiver will need external filtering to be useful on 630 meters.
Here is a recording of the 630 meter WSPR segment, first without and then with an external bandpass filter.
That just about says it all, but here are some images that tell the story. I will be trying a prototype for a filter designed specifically for this application and will post about that when finished.
Here we see the entire available band when using a sound card sampling at 192 kHz. This is with no external filtering. Note the many spikes in the center pane. Many of these are broacast stations that are not really in this band.
Full band view with external band pass filter. Note how many of the signals disappear.
Zoomed in view of 630 meter band without external filtering. Note the broadcast signal right in the middle of it.
This shows the 630 meter band with an external bandpass filter. The unwanted signal is gone.
This is the WSJT-X waterfall. I had the external filter in for most of this. I switched it out breifly at around 2335z. Notice all the junk that shows up, wiping out the WSPR signal.