Question about unwanted VFO pulling during transmit
 

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?

Thanks to all,
Kenn
KA5KXW

Question about unwanted VFO pulling during transmit
 

Howdy,

If the supply voltage to the oscillator changes, even though the
tuning voltage is stable, it may chirp or pull.

If the change is slow look for thermally induced changes like
You may not be able to feel this change with a finger. Applying
a bit more heat to suspected parts might indicate the culprit.

Also look for changes in the voltage on large capacitors. It
wouldn't be unusual for bias voltages to change when the oscillator
starts up. An extreme case of this is squegging.


73,
Grumpy

Question about unwanted VFO pulling during transmit
 

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

Question about unwanted VFO pulling during transmit
 

hI:

Try shielding your VFO
Good Luck

Heriberto
LU6DBU
escribió en el mensaje
...
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?

Thanks to all,
Kenn
KA5KXW

Question about unwanted VFO pulling during transmit
 

On Nov 18, 10:21*am, Grumpy The Mule wrote:
Howdy,

If the supply voltage to the oscillator changes, even though the
tuning voltage is stable, it may chirp or pull.

If the change is slow look for thermally induced changes like
You may not be able to feel this change with a finger. Applying
a bit more heat to suspected parts might indicate the culprit. *

Also look for changes in the voltage on large capacitors. It
wouldn't be unusual for bias voltages to change when the oscillator
starts up. *An extreme case of this is squegging.

73,
Grumpy

Thanks Grumpy. I'll check supply voltage next. It's not a thermal
problem. The output is nice and stable until key down when it pulls.
It's also frequency stable during keydown, just on a different
frequency. The oscillator isn't keyed, so I don't think it's an
oscillator startup issue.

73,
Kenn

Question about unwanted VFO pulling during transmit
 

Kendal Goodson wrote:

Thanks Grumpy. I'll check supply voltage next. It's not a thermal
problem. The output is nice and stable until key down when it pulls.
It's also frequency stable during keydown, just on a different
frequency. The oscillator isn't keyed, so I don't think it's an
oscillator startup issue.

73,
Kenn

I'm missing something. You said that the 'driver' etc were disconnected
and the osc is not being keyed.

So what is actually being keyed?

-Bill

Question about unwanted VFO pulling during transmit
 

On Nov 18, 1:19*pm, exray wrote:
Kendal Goodson wrote:
Thanks Grumpy. *I'll check supply voltage next. It's not a thermal
problem. *The output is nice and stable until key down when it pulls.
It's also frequency stable during keydown, just on a different
frequency. *The oscillator isn't keyed, so I don't think it's an
oscillator startup issue.

73,
Kenn

I'm missing something. *You said that the 'driver' etc were disconnected
and the osc is not being keyed.

So what is actually being keyed?

-Bill

I see I've done a poor job of writing up the problem statement. The
VFO at ~3.050 feeds a SA612 whose internal oscillator runs at 4.0 Mhz
and mixes the two to come up with ~7.050Mhz. Output from the mixer
goes through a tuned bandpass filter to a class A driver and then to a
class C final. When I found I had a problem with the VFO staying on
frequency during transmit, I disconnected the driver stage and
replaced it with a resistor termination. The problem stayed
consistent. The SA612 and the buffer/driver stage are both keyed.

-Kenn

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

Question about unwanted VFO pulling during transmit
 

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

Question about unwanted VFO pulling during transmit
 

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