Hello,

I have a general question regarding tuning diodes:
Does the oscillation voltage alter their capacitance, or is capacitance a
slow changing value - like resistance and PIN diodes?

The reason I ask is that I am spicing (simulation) a VCO I wish to build
(wide range 140-240MHz) and get an oscillation voltage of 20 volts peak
(maybe the Q is too high). This high oscillation voltage would easily swamp
the tuning voltage, 3-15 volts, and forward bias the diode itself.

If anyone wishs to reply directly they must remove the "xxx" from the return
address or write to:
james dot fenech at nec dot com dot au

Thanks,
James.

The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are
used for frequency multiplication -- that capacitance change "squeezes" the
pulse to generate lots of harmonics.

So your oscillation voltage will certainly affect the tuning, and the
forward bias effects will be most unfortunate.

I have modelled varactor diodes by dinking with the area parameter of the
SPICE diode model, but I'm not a pro in that regard and don't know the
"right" way to do it. SPICE is also not the tool for determining the
behavior of nonlinear high-Q circuits (I believe the best tool still
involves FR-4 and lots of cussing).

"James Fenech" wrote in message
...
Hello,

I have a general question regarding tuning diodes:
Does the oscillation voltage alter their capacitance, or is capacitance a
slow changing value - like resistance and PIN diodes?

The reason I ask is that I am spicing (simulation) a VCO I wish to build
(wide range 140-240MHz) and get an oscillation voltage of 20 volts peak
(maybe the Q is too high). This high oscillation voltage would easily
swamp
the tuning voltage, 3-15 volts, and forward bias the diode itself.

If anyone wishs to reply directly they must remove the "xxx" from the
return
address or write to:
james dot fenech at nec dot com dot au

Thanks,
James.

On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott"
wrote:

The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are
used for frequency multiplication -- that capacitance change "squeezes" the
pulse to generate lots of harmonics.

Rubbish. Varactors are not used for freqency mulitplication. They are
freequenty used in VXCOs for modulating (or otherwise varying) the
oscillator output frequency by means of applying a DC bias voltage
which alters the capacitance of the PN junction.
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

Paul Burridge wrote...

Tim Wescott wrote:

The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors
are used for frequency multiplication -- that capacitance change
"squeezes" the pulse to generate lots of harmonics.

Rubbish. Varactors are not used for freqency mulitplication. They are
freequenty used in VXCOs for modulating (or otherwise varying) the
oscillator output frequency by means of applying a DC bias voltage
which alters the capacitance of the PN junction.

Your own rubbish, Paul. In truth of fact, varactors are nicely used
for frequency multiplication, just as Tim stated. They also work well
for VCO FM modulation, but users must keep in mind their instantaneous
capacitance-change property and be careful to bias them in appropriate
higher-voltage bias regions, or risk unintended distortion. When used
as multipliers, low-voltage bias with large signals enhances the effect.

Thanks,
- Win

whill_at_picovolt-dot-com

On 4 Jan 2004 19:27:25 -0800, Winfield Hill
wrote:


Your own rubbish, Paul. In truth of fact, varactors are nicely used
for frequency multiplication, just as Tim stated.

You're right. I can't believe I posted that load of nonsense. Maybe
it's a case of "late onset mad cow disease." :-|
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

Paul Burridge wrote...

You're right. I can't believe I posted that load of nonsense.
Maybe it's a case of "late onset mad cow disease." :-|

OK, you are forgiven, my son. Go and sin no more.

Thanks,
- Win

whill_at_picovolt-dot-com

Paul Burridge wrote...

You're right. I can't believe I posted that load of nonsense.
Maybe it's a case of "late onset mad cow disease." :-|

OK, you are forgiven, my son. Go and sin no more.

Thanks,
- Win

whill_at_picovolt-dot-com

On 4 Jan 2004 19:27:25 -0800, Winfield Hill
wrote:


Your own rubbish, Paul. In truth of fact, varactors are nicely used
for frequency multiplication, just as Tim stated.

You're right. I can't believe I posted that load of nonsense. Maybe
it's a case of "late onset mad cow disease." :-|
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Mon, 05 Jan 2004 02:40:07 +0000, Paul Burridge
wrote:


Rubbish. Varactors are not used for freqency mulitplication.

If you are determined to be arrogant, it helps to occasionally be
right. Varactors are still used for frequency multiplication. They
were also commonly used as parametric amplifiers and mixers before
other semiconductors got fast enough to work at microwave frequencies.
Varactors were even used as paramps in the front ends of opamps.

Google "varactor frequency multiplier" and "parametric amplifier" and
maybe learn something.

John

On Mon, 05 Jan 2004 02:40:07 +0000,
said...
On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott"
wrote:

The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are
used for frequency multiplication -- that capacitance change "squeezes" the
pulse to generate lots of harmonics.

Rubbish. Varactors are not used for freqency mulitplication. They are
freequenty used in VXCOs for modulating (or otherwise varying) the
oscillator output frequency by means of applying a DC bias voltage
which alters the capacitance of the PN junction.

Dude, please.
--
Best Regards,
Mike