G'morning mike from NE USA. You beat me to it! You are fast. That response is much like the HPS40, its not too bad.
I agree fully to 1 MHz, its at its best. Between 1-4MHz, is 'ok', and above 4 MHz, you need to read between the lines. If I saw anything above 10 MHz, I better open the windows and make sure my glue containers are shut tight
. Given its 40 MHz clock and sampling rate, I limit its use to 4MHz and below.
Since the PIC MCU uses its internal oscillator, I think jitter is heavily part of the low internal clock stability, but above 4 MHz or so, the sampling rate increasingly plays havoc on waveform stability since its can't trigger at the right levels consistently. Square waves are stable at 1~3 MHz, to suggest its able to lock in a bit better on step responses than sine.