Poor man’s panadapter

Bruce KC1FSZ has been experimenting with a panadapter for his µBITx that is likely to be of interest to constructors, especially to anyone thinking about one of those fancy ICOM 7300s.

The goal was to build a crude panadapter into his BITX rig without resorting to external PCs, SDR dongles, etc.  He used a few cheats in his scratch-build BITX that may make this a bit more tricky on a stock unit.  However,  Bruce is sure that someone in the group can figure out how to expand this into the mainstream.

It’s a bit like a Sweeperino.  Bruce has built his firmware to quickly sweep the VFO across the band of interest (40m phone in Bruce’s case) and then to look at the AF spectrum at each point and display the result in a simplistic spectrum display. Obviously, this interrupts the receive for a brief instant when it is happening, but he has also set things up with an extra MOSFET so that the audio output can be suppressed during the sweep to hide the annoying sounds. This mod may already be in your rig if you’ve added an AGC.

The more sophisticated way to go is to do some DSP fiddling of the overlapping 3 kHz segments of the spectrum that are sampled during the sweep. At the moment Bruce sweeps in 500 Hz steps so there is overlap in the samples of the audio. This needs some more experimentation to get the optimal sweep speed, the cleanest way to stitch things together, averaging, etc. but even something basic is good enough to show the activity on the band.

Bruce also uses a cheap 128×64 OLED display, which makes these simple graphics possible.  Be aware that these displays can be noisy.

The picture at the top of this news items shows what the initial display looks like. The caret at the very bottom is pointing to where the VFO is currently set. You know it’s working because you can always see the signature 7199 BITX birdie!

Bruce has produced a video with the sweep slowed and the audio on so you can hear how it works.  It sounds like a strange SSTV mode, but it’s actually the entire 40m phone band being analyzed for activity.