On 10/02/2014 06:03 AM, Richard (Rick) Karlquist wrote:
On 10/1/2014 1:04 PM, Hal Murray wrote:
drkir...@kirkbymicrowave.co.uk said:
Anyway, later today (tomorrow ??) I will post a plot of frequency vs
time.
The question is though, how long is thing thing likely to take too cool?
I'd expect an exponential decay so you need to specify how close to
ambient
you want to get. I'd guess a ballpark of 10x the warm up rate.
You can probably measure it if you have the warmup graph. Turn it
off, wait
a while, turn it on, measure the freq, consult warmup graph.
When I was still with Agilent, I did some experiments with unpowered
10811's. Both the oven and oscillator were unpowered and I measured
the temperature by looking at the B mode resonance of the crystal.
I wanted to get rid of any linear frequency drift. As a rough
rule of thumb, 1 hour of cool down is pretty good for most purposes.
For extreme measurements, I would allow 10 hours. This reduced
any exponential tail to below the ability to measure temperature and/or
below the effects of the ambient. I had to put a box over it to
reduce the effects of air currents. If I did not do that, then 1 hour
was all I needed.
Just putting a card-board box around the oscillator does indeed make
short term deviation (breath, hand-waving, walking around and pushing
air) reduce significantly. What is needed to get anything decent out of
crap oscillators. Doesn't do as much for longer term shifts (AC,
day-variations etc)
Your cool-off numbers is about where I would guess for better ovens.
Naturally, a fan can speed the process up, but let it sit there for some
time without the fan to have less temperature gradients.
Cheers,
Magnus
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