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

"Highland Ham" bravely wrote to "All" (11 Jan 06 09:47:40)
--- on the heady topic of " 1N4007 varactors"

HH From: Highland Ham
HH Xref: core-easynews rec.radio.amateur.homebrew:90890

HH Zener diodes are often used as wide band 'noise generators'for use in
HH an impedance bridge used in conjunction with a receiver.

I understand to generate the noise that the zener diodes are operated
within their normal breakdown current range. However, as a varactor it
isn't desirable to have a breakdown current flow. The only tiny
current that flows is a saturation current which is related to
setting up the P-N junction boundaries within the semiconductor.

M*i*k*e

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

Roy Lewallen wrote:
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.

Okay, if we are discussing strange and high capacitance varactors,
what would you think of using a PIN photo diode as a varactor? They
have more chip area per dollar and per package size and capacitance
than many rectifier diodes, and that are made to have low leakage
current and often have low inductance packaging.
For example:
http://rocky.digikey.com/WebLib/Osra...pw34fa_fas.pdf
looks like a nice 15 to 70 pF capacitor.

I guess you would have to keep them in the dark, unless you wanted to
tune them (biased by a high resistance to a fixed voltage) with a
variable intensity light beam.