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

On Sat, 07 Jan 2006 17:26:23 -0500, 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?

I'd expect that all to depend on manufacturer (and, even, date code.)

HNY es 73
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK

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
I suspect any si diode will work to some extent as a varicap or zener.
Not all diodes will be stable as zeners, the 1N506x series will probably
work better as a zeners than the 1n400x series. BTW the 1n5061 will work
as an 850v zener! Also the 1n4007 makes a good poor man's PIN diode for
switching rf.

On Sat, 07 Jan 2006 17:26:23 -0500, JE wrote:

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

I've used them at low VHF, the limiting factor there is minimum
capacitance my be too high for some VHF uses.

And how about zener diodes?

As varicaps? Some are ok some very poor. You require a breakdown
voltage well above the applied bias.

Allison

Maybe 2N4401 will work, I built some simple 1 transistor FM transmitters
that modulate the base.

If not, who sells dual varactors in small quantities?

JE

On Sat, 07 Jan 2006 17:26:23 -0500, JE wrote:

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

I've used them at low VHF, the limiting factor there is minimum
capacitance my be too high for some VHF uses.

And how about zener diodes?

As varicaps? Some are ok some very poor. You require a breakdown
voltage well above the applied bias.

Allison

Since the 1N4007 isn't a dual, why do you require a dual varactor? I'd
expect diodes specifically made to be varactors would have less
variation from part to part with respect to capacitance than would some
other random diode.

Have you tried Mouser (as Professor suggests) or DigiKey, or some of
the surplus places like MPJA?

I've heard that 1N4007s commonly make decent PIN diodes. If that's the
case, if they really are made as a PIN structure, I would think they'd
make lousy varactors. Wide-tuning-range varactors are commonly
"hyper-abrupt" junctions, NOT PINs.

Cheers,
Tom

Why not just use a varactor... as a varactor... LOL
They are available in small quanities at www.mouser.com searchword
"varactor"...

Professor
www.telstar-electronics.com

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