[time-nuts] HP 10811 Tuning Diode

[Alan Melia]

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" <rfnuts at arcor.de>
To: <richard at karlquist.com>; "Discussion of precise time and frequency 
measurement" <time-nuts at 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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