This is a waterfall made in gnuradio with a simple audio source->waterfall block at 12ksps. The filtering I'm talking about is therefore all on the Elecraft KX2.
At the top of the waterfall the filter is pretty wide. But there's some unrelated signal (in red) in the top right. I want to change the filter to remove that.
I press "FIL" on the KX2, and reduce the bandwidth. You can see that the yellow portion becomes narrower for a short bit. When it goes below about 1.8KHz filter BW, it seems to create a band stop filter around 1.5KHz.
That doesn't seem right. I just want to make the filter narrower, not notch out the middle of the band.
At the bottom of the waterfall I reset the filter bandwidth to about maximum.
Is this a bug in the Elecraft KX2 DSP, or am I misunderstanding what I'm seeing?
If you're curious, this is me listening to 14.074 JS8, but there's some (I think FT4) high up in the band. It's not necessarily actually interfering with me being able to decode the signals below it, but I do want to understand the Elecraft filters.
So tl;dr question: Why the notch filter when the filter BW goes below 1.8KHz?
Edit: second screenshot shows that this notch stays about the same, as the BW continues getting narrower.
To be clear: This is not I/Q signalling, it's the upper sideband only. The X axis is correct, in that it starts from 0. I see the same in wsjtx/js8call.