[time-nuts] PICDIV stability (was: Crystal oscillator for a begginer)

[Attila Kinali]

On Wed, 5 Jan 2022 19:42:27 -0800
Keelan Lightfoot <keelanlightfoot at gmail.com> wrote:

> When I read that it uses a only a PIC, I was skeptical, but then when I read that jitter is in the range of 1 pico second, I was intrigued

It is exactly because it is a PIC that the jitter is so low.
These tiny PIC are very simple devices, there is only a single clock, no internal
oscillators or house-keeping mechanisms that would interfere. That means their
behaviour is almost completely determined by their logic and the code they are
running, which does not change.

The two biggest outside influences on the PICDIV are supply voltage and temperature.
The former is "easy" to control by just having a good power supply that is
stable (e.g. LTC6655 or use a Jung SuperRegulator that uses an LM329/LM399 as reference).

Temperature is a bit harder, as you need either an oven or peltier or something
like that to keep the temperature stable. Or you can decide that just keeping
the PICDIV in a box with large thermal mass (let's say 2kg of old screws) would
stabilize temperature enough for the temperature related drift to be so low
that you don't care anymore. I haven't tested the PIC specifically, but for
a lot of electronics a delay variation of 2-5ps/°C seems to be quite common.

Oh.. and if anyone is going to build a DIY oven for some instruments, a 55x35x30cm
styrofoam box with 2cm wall thickness, suspended in air has a thermal resistance of
approximately 3K/W. But beware that proper seating of the lid is quite critical as
that alone can drop the thermal resistance by a factor of 3 (guess how I know).
If you want actively steer temperature inside the box, I suggest putting a fan inside
to have quick equalization inside the box. Otherwise the position of the temperature
sensor relative to the heater determines how much delay the control loop sees
(can be easily several minutes) and thus make it unstable.

			Attila Kinali
-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering