[time-nuts] poor-man's oven

Attila Kinali attila at kinali.ch
Mon Jun 5 05:15:33 EDT 2017

On Sun, 4 Jun 2017 22:01:45 -0700
Chris Albertson <albertson.chris at gmail.com> wrote:

> Voltage is proportional to the product of resistance and absolute
> temperature.   As an experiment place a voltage across a high value
> resister like say one 1M  raise the volts until you are near the limit of
> the resister and connect it via a coupler cadaster to an audio amplifier.
> You will hear white noise in the speakers.

Yes, Johnson noise increases by about 20% (or about 0.8dB)
when going from 25°C to 90°C. But Johnson noise is usually
not the first limiting noise one runs into when designing
a crystal oscillator. I would rather use a simpler,
"high temperature" oven and invest the time saved into a
desgining better oscillator structure.

> There is a similar effect in semi conductors.  The best example of this is
> visual noise in digital camera, when you tern the gain way up (set the ISO
> high) you can see it in the photo.
> All of this is proportion; to absolute.
> As I remember we ran the "oven" at -20C There was a valve used to flash out
> the air inside with inert welding gas to reduce ice.

If the -20°C was used on the camera, then this is not to reduce Johnson
noise, but the dark current noise. While the former is due to thermal
vibration of the atom lattice, the later is due to spontaneous forming
of electron-hole pairs in the p-n junction region. Thus also their
temperature characteristics differ: While Johnson noise is linear
in temperature, dark current noise is (almost) exponential in temperature.
Going down from room temperature to 0°C is something like a factor 100
(IIRC, from memory, could be wrong) in noise for a CCD. While for
a resistor, the difference is almost negligible.

			Attila Kinali
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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