I connected the PA3CNO frequency display and huff & puff to my BITX20 using the circuit shown here.
Well, almost as shown here. I had made a mistake with the added LED/cap/resistor circuit and I was also using a locking switch instead of a momentary switch. The effect of this was that when the switch was in the lock position the Huff & Puff circuit was constantly re-sampling and not ever getting the chance to compensate. Not that it could have changed anything with the LED/cap/resistor circuit incorrect.
Once I corrected these errors the VFO is much more stable. I can hear the Huff & Puff working when listening to a solid carrier from the DDS generator and see the movement of the VFO back and forth a very small amount on Hamscope.
Now I need to start on the front panel layout.
I also need to work out a microphone and speaker arrangement, I have a couple of spare speaker/mics and I think one of these would work well for pedestrian mobile operation.