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Question about unwanted VFO pulling during transmit

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November 18th 08, 11:44 PM posted to rec.radio.amateur.homebrew

Doug White Doug White is offline

Doug White is offline

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First recorded activity by RadioBanter: Oct 2007

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Question about unwanted VFO pulling during transmit

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

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