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

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).

"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

Asimov wrote:

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.

They also have a whale of a lot more capacitance than a conventional
diode. So I don't think it's a fair comparison.

Roy Lewallen, W7EL

....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