Question about unwanted VFO pulling during transmit
 

Kendal Goodson wrote:

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

Ah, good detective work. I would have never seen that coming!

-Bill

Question about unwanted VFO pulling during transmit
 

On Nov 18, 5:21*pm, Kendal Goodson wrote:
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.

Ahah! The SA612's input impedance will change by a lot when it is
keyed up vs keyed down.

Two ways to get around this:

1. Additional buffering between the VFO and SA612

2. Swamp the impedance change from power up to power down by adding
some parallel resistance. Should be DC-isolated from the SA612's input
because otherwise you will upset the balance.

Solution #1 requires some additional parts and current drain. I really
like the two-transistor W7EL buffer.

Solution #2 is just a capacitor and a resistor, but also assumes that
the VFO has enough drive that it won't object to being swamped.

Tim N3QE

Question about unwanted VFO pulling during transmit
 

On Nov 18, 5:21*pm, Kendal Goodson wrote:
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- Hide quoted text -

- Show quoted text -

Are you sure its the VFO and not the SA612?

Jimmie

Question about unwanted VFO pulling during transmit
 

Tim Shoppa wrote:
On Nov 18, 5:21 pm, Kendal Goodson wrote:
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.

Ahah! The SA612's input impedance will change by a lot when it is
keyed up vs keyed down.

Two ways to get around this:

1. Additional buffering between the VFO and SA612

2. Swamp the impedance change from power up to power down by adding
some parallel resistance. Should be DC-isolated from the SA612's input
because otherwise you will upset the balance.

Solution #1 requires some additional parts and current drain. I really
like the two-transistor W7EL buffer.

Solution #2 is just a capacitor and a resistor, but also assumes that
the VFO has enough drive that it won't object to being swamped.

Tim N3QE

That sounds like it's the problem, and those are the solutions I'd
suggest -- Tim just got there first.

Do check the supply voltages to the VFO while you're at it, though --
changing VFO (and buffer stage) supply voltages will also pull the VFO
frequency.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html