On Nov 18, 4:44 pm, (Doug White) wrote:
In article , wrote:



Hello all,

Can someone explain to me the mechanism of how a VFO "pulls" during
transmit? I have a transmitter circuit I've breadboarded, ugly
construction style, and the VFO will pull up ~2kHz with the key down.
The transmitter circuit is borrowed almost verbatim from the K1SWL
SW40+ transceiver.

I've checked the following:

1. dropped off the driver and final amp circuit with no change in
behavior
2. the tuning voltage to the varactor in the tank circuit does not
change during keydown.

I know there could be several answers here. What should I be looking
for?

It could also be a loading/buffering issue. If the load the VFO sees
changes when you key on the rest of the system, that will also cause
pulling. You need lots of isolation between the two pieces. A gain
stage with good front-to-back isolation followed by an attenuator will
provide plenty.

Doug White
I've located the problem. I thought it would be something pretty
subtle and it turned out to be just that. One of the pins on the
SA612 did not go into the socket properly. It bent under and was just
almost touching the socket contact. I guess it was capacitively
coupling to the VFO output instead of getting a good feed. I've
straightened it and I'm up and running.

Thanks for all of the suggestions.

72,
Kenn
KA5KXW