I’ve had some questions about the FCD and spikes on the passband.
Out of the box, like all zero IF direct conversion analogue mixer SDRs, incuding the FCD and the Softrock, there are two tweaks that immediately come to mind. Firstly there’s the DC offset which you’ll find in the middle of the SDR’s passband, and secondly there’s the image rejection. By adjusting these settings, you can null out the DC offset and image signals.
The pictures shown here are deliberately shown without any DC or image corrections so that you can compare your own unit. The precise amount of DC offset and image rejection will vary from unit to unit, and is also frequency and gain specific.
Below you’ll see marker 1 on the right as the main signal, in the middle is the DC offset marker 2, and marker 3 on the left is the signal image. Ignore the fourth marker!
Using Spectravue the DC offset and image rejection are corrected in the SoundCard In Setup menu item, but it’ll vary depending on the SDR software you’re using.
It is highly recommended that you use the “Defaults” button in the FCD Controller app each time the FCD’s plugged in: this sets the FCD’s gains and filters to sensible values.
When you have strong signals in the receiver’s pass band you will see the noise floor rise. This can be alleviated to some degree by reducing the mixer gain. This increases the dynamic range from 80dB to 90+dB at the expense of some sensitivity as you can see in the picture below. This shows a strong signal on the input. The rasied noise around the signal is as a result of the FCD’s LO phase noise. To the left hand side, we still have about a -103dB noise floor, with a -11dB input signal.
At this point we are now close to the physical abilities of the FCD. Firstly the LNA is close to saturation and secondly, we are very close to the ADC’s dynamic range.
One further point: the FCD is primarily designed to be sensitive enough to receive satellites directly. However it is also an exceptionally wideband receiver, and although there is fixed and programmable front end filtering, you may also find that if you are in a high RF environment, and particularly at VHF, a low loss band pass filter will help if you find the noise floor rises when you plug in your antenna. Typically, quality masthead preamps often have these installed anyway.