Hi Andrew and group.
The FUNcube dongle has the DC spike in the center of it's passband. That is the frequency of it's oscillator.
The software you use when running the dongle, in my case SDR sharp, has a couple of tick boxes for "Correct IQ" and "Swap I & Q"
Correct IQ should reduce the DC spike to a minimum, while Swap I&Q allows you to swap audio channels in the software rather than changing the left and right audio by moving cables etc.
A frequent but understandable mistake is to set the dongle's oscillator to the frequency that you wish to monitor. Performance wise this is the worse thing you can do....As I discovered when I first listened to AO-85.
AO-85 transmits on 145.980 (ish) If you set the dongle to that frequency the DC spike and the phase noise around it will degrade receive performance. You will get much better performance on the dongle and other QSD type SDRs (like the softrock) if you set the oscillator at least 10kHz away from the frequency you are interested in. Here I have the dongle running at 145.955 and reception of AO-85 telemetry greatly improved.
73
David G0MRF
Has anyone managed to reduce the DC offset on the FCD ?
It appears as a spike right where I don't want it
Andrew