RadioBanter - View Single Post - Question about unwanted VFO pulling during transmit

Bob[_18_] · November 18th 08, 04:58 PM posted to rec.radio.amateur.homebrew

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
behaviour
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?

There could be many culprits:

Check the supply voltage to the oscillator. Does it change when the key's
down? If so, try better supply regulation to the oscillator.

Does anything get warm either in the oscillator or nearby? If so, you've got
thermal problems - changing the layout may help, or correcting the thermal
drift with temperature-compensating components (I used to use thermistors to
change the bias on varicap diodes to do this).

Does the loading on the output of the oscillator change at key down? If so,
you need a better buffer stage. I usually use FET buffers for very light
loading (high input impedance).

If all else fails, you're going to have to either rebuild the oscillator, or
consider some kind of stabiliser. I sometimes use PLL circuits (you can get
1 kHz steps on any band up to 30 MHz with just three cheap 74HC chips) or a
fast "huff-and-puff" stabiliser for stable tuning with very small step size -
see http://downloads.hanssummers.com/qexnov98.pdf - which is astonishingly
stable and very cheap to build.

HTH

Bob