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

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

Roy ...

I've been playing around (ahem, excuse me, heuristically engineering) with
zener noise sources for a while using the same spectrum analyzer trick and
as yet I haven't been able to make the noise as "flat" across the passband
as I'd like. I've tried varying the bias, the voltage, and a few other
tricks, but as yet, no joy.

Can you shed some light on what you've found to make the noise power/voltage
fairly level across the band?

Jim




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

RST Engineering wrote:
Roy ...

I've been playing around (ahem, excuse me, heuristically engineering) with
zener noise sources for a while using the same spectrum analyzer trick and
as yet I haven't been able to make the noise as "flat" across the passband
as I'd like. I've tried varying the bias, the voltage, and a few other
tricks, but as yet, no joy.

Can you shed some light on what you've found to make the noise power/voltage
fairly level across the band?

It's not particularly flat over the whole band, and I haven't attempted
to make it be. My interest has been mostly in relatively narrow band
filters, or the shape of a main filter rolloff. The noise is adequately
flat over the bandwidths I've been interested in. It's been a while
since I've fooled with it, but as I recall, the shape of the noise
spectral distribution changed all over the map as I changed the diode
bias. I imagine that it changes with temperature and with individual
diodes, too. So any circuit used to flatten it would only work for a
particular diode, current, and probably temperature. The bottom line is
that it's probably a lousy way to try to generate a flat broadband noise
spectrum.

Roy Lewallen, W7EL

RST Engineering wrote...

Roy ...

I've been playing around (ahem, excuse me, heuristically engineering) with
zener noise sources for a while using the same spectrum analyzer trick and
as yet I haven't been able to make the noise as "flat" across the passband
as I'd like. I've tried varying the bias, the voltage, and a few other
tricks, but as yet, no joy.

Can you shed some light on what you've found to make the noise power/voltage
fairly level across the band?

Good zener noise sources are carefully bred and tested, they
do _not_ come naturally from garden-variety zener diodes. I
suggest you go read the hundred or so messages in the famous
zener oscillation thread a few years back here on s.e.d. In
this you'll learn of my substantial investigations into the
topic, some physics - and, very important, learn what a zener
microplasma is. You'll see my ASCII waveform plots of actual
bench measurements showing exactly what's going on. After all
this you may decide to avoid using zener diodes for calibrated
noise sources. Sorry about that!


--
Thanks,
- Win

RST Engineering wrote:
Roy ...

I've been playing around (ahem, excuse me, heuristically engineering) with
zener noise sources for a while using the same spectrum analyzer trick and
as yet I haven't been able to make the noise as "flat" across the passband
as I'd like. I've tried varying the bias, the voltage, and a few other
tricks, but as yet, no joy.

Can you shed some light on what you've found to make the noise power/voltage
fairly level across the band?

Jim




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



What I did for some project which needed equal amplitude
uncorrelated noise,is amplify the zener noise with a wide
band video opamp,with high pass and lowpass filtering.
Used the low pass as input for a zero cross detector,
delayed the zerocrossing 10 microseconds,and used that
for clock to a circulating bit in a shift register.
each parallel output of that register controlled a
sample/hold opamp,sampling the highpass signal.
Voila!! 8 audio frequency, non-correlated noise sources.
The zerocrossing clock was made this way,to avoid detectable
clock tones int the output.(2 to 20 microsec between
crossings)
the 10 microsecond delay was used to get a voltage at
the sample and hold opamp which was not correlated to the
zerocrossing.
If you need only one signal ,leave out the shift register,
and just use a 10 and a 1 microsec. oneshot for the s/h
opamp clock.
The application? A wind and engine noise generator for a
car simulator.

On Tue, 10 Jan 2006 14:30:01 -0800, "RST Engineering"
wrote:

Roy ...

I've been playing around (ahem, excuse me, heuristically engineering) with
zener noise sources for a while using the same spectrum analyzer trick and
as yet I haven't been able to make the noise as "flat" across the passband
as I'd like. I've tried varying the bias, the voltage, and a few other
tricks, but as yet, no joy.

Can you shed some light on what you've found to make the noise power/voltage
fairly level across the band?

Jim




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


These folks will sell you serious noise diodes...

http://www.noisecom.com/NC/default.htm

John

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