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

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

...the famous zener oscillation thread a few years back
...my ASCII waveform plots of actual bench measurements
showing exactly what's going on.
Winfield Hill

http://groups.google.com/group/sci.e...lation&f wc=1

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

I think that whether noisy or not, the noise generation mechanism
kicks in, only when the zeners are reverse conducting via the zener
breakdown process, so if you keep the voltage well below the zener
knee, say, half of that, they are not particularly noisy compared to
other diodes. So, yes, only near the breakdown region, especially
just below the rated voltage (low reverse current).

JeffM wrote...

...the famous zener oscillation thread a few years back
...my ASCII waveform plots of actual bench measurements
showing exactly what's going on.
Winfield Hill

http://groups.google.com/group/sci.e...lation&f wc=1

That's one thread, perhaps the first in a series. That thread
doesn't have the waveforms I was referring to (although there
are some waveforms in posts 51 and 66). Tony, Bill, Roy and I,
and some others here wasted masses of time on this subject over
a period of a few months, eight and a half years ago. We took
bench measurements, did calculations, found the scientific
literature (it was a subject that occupied physicists in the
late 50s, see posts 72-76), and we did plenty of speculation.


--
Thanks,
- Win

....impossible to tell! nobody ever characterized them for high
frequency use. the usual effects of the varactor are pernicious! they
are crucial for low phase noise of the VCO using them. the only truly
effective way to check is build one and measure the performance with a
spectrum analyzer or a phase noise meter, both expensive items. my
recommendation is to build one and try to listen to its signal on a
receiver (stable one!), if the note is clean and not ragged it's OK. if
not the VCO is too noisy for use. don't be afraid to check more than
one diode in the same circuit. experience has shown that some units
perform better than others.
Saandy 4Z5KS


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

Winfield Hill wrote...

JeffM wrote...

Winfield Hill wrote...

...the famous zener oscillation thread a few years back
...my ASCII waveform plots of actual bench measurements
showing exactly what's going on.

http://groups.google.com/group/sci.e...lation&f wc=1

That's one thread, perhaps the first in a series. That thread
doesn't have the waveforms I was referring to (although there
are some waveforms in posts 51 and 66). Tony, Bill, Roy and I,
and some others here wasted masses of time on this subject over
a period of a few months, eight and a half years ago. We took
bench measurements, did calculations, found the scientific
literature (it was a subject that occupied physicists in the
late 50s, see posts 72-76), and we did plenty of speculation.

All of which led Roy McCammon to remark (post 90), "I'd have to
say that it is the best thread this year." He said that Aug 5th,
after 3 weeks of posts, and yet the followup threads in Aug and
Sept on the same topic were just as long, and perhaps even more
interesting. Ah, those were that days!


--
Thanks,
- Win

"John Popelish" bravely wrote to "All" (10 Jan 06 22:14:12)
--- on the heady topic of " 1N4007 varactors"

JP From: John Popelish
JP Xref: core-easynews rec.radio.amateur.homebrew:90859

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

JP I think that whether noisy or not, the noise generation mechanism
JP kicks in, only when the zeners are reverse conducting via the zener
JP breakdown process, so if you keep the voltage well below the zener
JP knee, say, half of that, they are not particularly noisy compared to
JP other diodes. So, yes, only near the breakdown region, especially
JP just below the rated voltage (low reverse current).


I never tested a zener when used as varactor but I think these have a
much greater reverse saturation current (even far below breakdown
threshold) and it is this that might cause comparatively more noise
than a conventional diode with a tiny leakage current. Well, at least
that is what the junction noise equations would seem to indicate.

A*s*i*m*o*v

I never tested a zener when used as varactor but I think these have a
much greater reverse saturation current (even far below breakdown
threshold) and it is this that might cause comparatively more noise
than a conventional diode with a tiny leakage current. Well, at least
that is what the junction noise equations would seem to indicate.
==============================
Zener diodes are often used as wide band 'noise generators'for use in an
impedance bridge used in conjunction with a receiver.

Frank KN6WH / GM0CSZ

OK, then. A zener makes a poor noise source according to what I'm reading.
Noise.com used to sell off-spec diodes by the onesies for we poor peons to
play with, but for whatever reason that doesn't seem to be the case any
more.

Given that a zener (at whatever current) is a poor noise source, what is a
good source of electronic broadband noise from low HF through high UHF --
say, 5 to 500 MHz.? (No smart remarks about spark gaps.)

Jim



"Winfield Hill" wrote in message
...


We took
bench measurements, did calculations, found the scientific
literature (it was a subject that occupied physicists in the
late 50s, see posts 72-76), and we did plenty of speculation.