Maybe I'm confusing this with something else but I always thought 1N4007's
were 1000 V PIV, 1 A rectifiers.

I believe "any" diode has varactor characteristics to some extent but if you
need a varactor you'd be better off using one designed as a varactor.

Dr. G.



In article , JE wrote:
The common 1N4007 seem to work for HF but what is the max. frequency they
can be used as varactors?

And how about zener diodes?

JE

Dr. Grok wrote:
Maybe I'm confusing this with something else but I always thought 1N4007's
were 1000 V PIV, 1 A rectifiers.

I believe "any" diode has varactor characteristics to some extent but if you
need a varactor you'd be better off using one designed as a varactor.

Dr. G.



In article , JE wrote:

The common 1N4007 seem to work for HF but what is the max. frequency they
can be used as varactors?

And how about zener diodes?

JE
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.

Any diode will work as a Zener, the zener voltage is where the reverse breakdown
occurs. In a true Zener, the breakdown voltage is stable over a large range
of current without the diode self destructing. Using a common diode as a zener
requires limiting the reverse current to a small value, but if the current isn't
large enough the zener voltage won't be stable.

And yes the 1n4007 is a 1000 vpiv rectifier diode. It also makes a good
rf switching diode thanks to it's PIN like structure. It's also quite cheap.

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

Dr. G,
You are correct, but many circuits have used the 1N4000 series diodes as
varactors with acceptable results. Any diode is a varactor (even transistor
junctions), but those called varactors are just optimized for and
measured/spec'ed as variable caps. If the cap value works in the circuit
and losses/noise/whatever-else aren't a problem, then no problem. I chalk
it up to: Some of us don't have the luxury of either well stocked basements
or pocket books, or perhaps the willingness to run out to the local parts
store (if you have one) and pick something out just to throw some neat
little thingy together.

Don't (some) zeners generate noise???...or is that only near the breakdown
reigion?

73, Steve, K.9/D;C'I


"Dr. Grok" wrote in message
...
Maybe I'm confusing this with something else but I always thought 1N4007's
were 1000 V PIV, 1 A rectifiers.

I believe "any" diode has varactor characteristics to some extent but if
you
need a varactor you'd be better off using one designed as a varactor.

Dr. G.



In article , JE
wrote:
The common 1N4007 seem to work for HF but what is the max. frequency they
can be used as varactors?

And how about zener diodes?

JE

Steve Nosko wrote:
. . .
Don't (some) zeners generate noise???...or is that only near the breakdown
reigion?

Some are very noisy. The noisiest I've seen have been ones in the 12 -
15 volt range when biased at considerably less than a mA. I've used one,
followed by a 50 ohm amplifier "pill" IC, as a broadband noise source to
see filter responses with a spectrum analyzer. The noise is easily
visible well up into the UHF region.

But all zeners generate some noise, so you have to use appropriate
filtering in sensitive applications. In my experience, though, band gap
references can be even noisier than a typical zener.

Roy Lewallen, W7EL