Ken Scharf wrote:

All diodes exhibit varactor and zener traits but not all are stable
as such. All diodes when reverse biased exhibit a value of capacitance
across them. In most cases it's usually quite small, less than 5 to 10pf.
Increase the reverse bias and the capacitance goes down. At very high frequencies
the capacitance change is enough to make a useful tuning diode. If you need
a varactor to work at medium to high frequencies the tuning effect won't be
very useful and you should use a true varactor diode. Such diodes have
either larger areas, or thinner substrates to increase th capacitance.
. . .

Zener diodes have much more capacitance than this, with the amount of C
being greater as the zener voltage gets lower. It's been a long time
since I've looked at this, but as I recall you can get well over 100 pF
from something like a 5 V zener. For the same reason, reverse biased E-B
junctions can give quite a bit of C. Of course, the limited breakdown
voltage limits your tuning range. Higher power zeners have higher C yet.

I've used zeners for varicaps many times in HF rigs, to offset a VFO
when switching bands, and for RIT. Haven't tried one as the main tuning
capacitor, but I haven't tried a regular varicap, either.

Diodes specified for varicap use have more predictable
capacitance-vs-voltage characteristics, and you can get a variety of
different characteristics. They might have lower noise, too, but I've
never used one in an application where that was critical, so don't know
if that's the case. But for a lot of one-off projects, zeners work fine
as varicaps.

As for using something else as zeners, emitter-base junctions work well.
You don't get much variety, though -- most break down at around 5-6
volts. I've got a power supply I built over 30 years ago which uses a
couple of series transistor E-B junctions as the voltage reference. It's
still my main bench supply.

Roy Lewallen, W7EL

It was just that they are dirt cheap and available.
I'll pony up for real ones.


JE

On Mon, 09 Jan 2006 23:38:29 -0500, JE wrote:

It was just that they are dirt cheap and available.
I'll pony up for real ones.

JE

I'd go with try it first.

Also base collector junction of many transistors is fairly decent
varactor. Low power transistors tend to be low capacitance
and the amount of capacitance increases with device dies size
(power level).

Allison

Zeners are also often used as noise sources, which might be a good reason NOT
to use them for the tuning, especially in a receiver.

Dr. G.


In article , Roy Lewallen
wrote:
Ken Scharf wrote:

All diodes exhibit varactor and zener traits but not all are stable
as such. All diodes when reverse biased exhibit a value of capacitance
across them. In most cases it's usually quite small, less than 5 to 10pf.
Increase the reverse bias and the capacitance goes down. At very high
frequencies
the capacitance change is enough to make a useful tuning diode. If you need
a varactor to work at medium to high frequencies the tuning effect won't be
very useful and you should use a true varactor diode. Such diodes have
either larger areas, or thinner substrates to increase th capacitance.
. . .

Zener diodes have much more capacitance than this, with the amount of C
being greater as the zener voltage gets lower. It's been a long time
since I've looked at this, but as I recall you can get well over 100 pF
from something like a 5 V zener. For the same reason, reverse biased E-B
junctions can give quite a bit of C. Of course, the limited breakdown
voltage limits your tuning range. Higher power zeners have higher C yet.

I've used zeners for varicaps many times in HF rigs, to offset a VFO
when switching bands, and for RIT. Haven't tried one as the main tuning
capacitor, but I haven't tried a regular varicap, either.

Diodes specified for varicap use have more predictable
capacitance-vs-voltage characteristics, and you can get a variety of
different characteristics. They might have lower noise, too, but I've
never used one in an application where that was critical, so don't know
if that's the case. But for a lot of one-off projects, zeners work fine
as varicaps.

As for using something else as zeners, emitter-base junctions work well.
You don't get much variety, though -- most break down at around 5-6
volts. I've got a power supply I built over 30 years ago which uses a
couple of series transistor E-B junctions as the voltage reference. It's
still my main bench supply.

Roy Lewallen, W7EL

On Wed, 11 Jan 2006 00:50:49 GMT, (Dr. Grok) wrote:

Zeners are also often used as noise sources, which might be a good reason NOT
to use them for the tuning, especially in a receiver.

In the breakdown region yes, in the revserse bias region no. I've
tried some .4w 22V Zeners and they are decent varactors save for
you need to keep far enough from the 22V and -.5V or they exhibit
their other charactersistics.

Allison


Dr. G.


In article , Roy Lewallen
wrote:
Ken Scharf wrote:

All diodes exhibit varactor and zener traits but not all are stable
as such. All diodes when reverse biased exhibit a value of capacitance
across them. In most cases it's usually quite small, less than 5 to 10pf.
Increase the reverse bias and the capacitance goes down. At very high
frequencies
the capacitance change is enough to make a useful tuning diode. If you need
a varactor to work at medium to high frequencies the tuning effect won't be
very useful and you should use a true varactor diode. Such diodes have
either larger areas, or thinner substrates to increase th capacitance.
. . .

Zener diodes have much more capacitance than this, with the amount of C
being greater as the zener voltage gets lower. It's been a long time
since I've looked at this, but as I recall you can get well over 100 pF
from something like a 5 V zener. For the same reason, reverse biased E-B
junctions can give quite a bit of C. Of course, the limited breakdown
voltage limits your tuning range. Higher power zeners have higher C yet.

I've used zeners for varicaps many times in HF rigs, to offset a VFO
when switching bands, and for RIT. Haven't tried one as the main tuning
capacitor, but I haven't tried a regular varicap, either.

Diodes specified for varicap use have more predictable
capacitance-vs-voltage characteristics, and you can get a variety of
different characteristics. They might have lower noise, too, but I've
never used one in an application where that was critical, so don't know
if that's the case. But for a lot of one-off projects, zeners work fine
as varicaps.

As for using something else as zeners, emitter-base junctions work well.
You don't get much variety, though -- most break down at around 5-6
volts. I've got a power supply I built over 30 years ago which uses a
couple of series transistor E-B junctions as the voltage reference. It's
still my main bench supply.

Roy Lewallen, W7EL

Dr. Grok wrote:
Zeners are also often used as noise sources, which might be a good reason NOT
to use them for the tuning, especially in a receiver.

You mean that a zener diode biased at less than breakdown voltage
generates more noise than a diode specified for varactor use biased at
less than breakdown? I assume this noise would be in the form of time
jitter of the capacitance, since the leakage current would be very
small. Have you seen this in some specification, or is it from experience?

Roy Lewallen, W7EL