Rick, Adrian, I think there are a number of possibly material related areas
that could cause a varicap to go noisy but I have no experience of them
First a reversed biased diode makes quite a good particle detector :-)) and
remanent radioactive atoms in say the glass could cause localised
avalanches. This would seem unlikely because it would affect a whole batch
but possible. I also wonder what the effect of strain, crystaline
dislocations etc might do to the very small current flowing in a very high
impedance. These can lead to odd effects when they penetrate the junction..
It could just be an unfortunate sample. I suspect the level of noise current
is way below any specified meaured value.
Alan
G3NYK
----- Original Message -----
From: "Adrian" <rfn...@arcor.de>
To: <rich...@karlquist.com>; "Discussion of precise time and frequency
measurement" <time-nuts@febo.com>
Sent: Friday, October 19, 2012 9:10 PM
Subject: Re: [time-nuts] HP 10811 Tuning Diode
I didn't notice any damage, nor was there a visibly bad solder joint.
However, I couldn't reproduce the excess noise with a quick bench test at
30V / 100k neither.
DC-wise there are a few microvolts only. Same with the SMD diode.
Exept, when the original glass diode is exposed to light. With a LED
Maglite, I got up to 50 mV (across 100 kOhm) out of it!
But there is no light shining inside of a 10811, is there?
I had first suspected a 0.1 ceramic cap, but that didn't cure the phase
noise desease.
The diode in the original circuit appeared to be heat sensitive,
and the excess phase noise has gone away since I replaced the diode.
The noise was clearly there, and it was also there using a test adapter
and two lab power supplies.
Adrian
Rick Karlquist schrieb:
I wonder if the glass case got a crack in it or if the kovar seal
was failing. Maybe a failed solder joint (which gets fixed when you
install the new diode). I've never heard of a noisy varactor before
either.
Rick
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