[time-nuts] OCXO DC Current Question
ed_palmer at sasktel.net
Mon Feb 18 14:40:09 EST 2013
I know that when making AC measurements on various OCXOs of the same
type, you have to expect wide variations in the results. e.g. TVB's
Allan Deviation measurements on a selection of 10811A oscillators at
http://www.leapsecond.com/pages/z3801a-osc . But what about DC current
measurements? How much variability should you expect?
I recently bought 4 MTI 260 oscillators with thoughts of doing some
3-cornered hat experiments. I thought I'd use the best 3 of 4. One
test I always do on an OCXO is to measure the DC current drain as it
warms up. Nothing radical - I have an HP 6622A GPIB-equipped linear
power supply. I just do GPIB queries as fast as I can and log the
results. I get about 6 readings per second. More than enough for my needs.
This time, I was surprised by the results of this test. The attached
picture shows why. I've offset the traces horizontally and vertically
for clarity so I deleted the axes. The horizontal lines are 200 ma
apart, but the position of each trace is arbitrary. All four
oscillators start at a current-limited value of ~1 Amp and have a
steady-state current drain of ~230 mA. The length of the graph is ~20
Although the family resemblance is obvious, I was surprised by the
different noise levels. I let one of the noisy units run for a day to
see if it would settle down, but there was no improvement. Are these
results reasonable, or do I have one oscillator with a good oven (blue
trace), one marginal (purple), and two rather poor ones (red and
green)? I'm thinking that the noise on the oven could affect the Allan
Deviation due to either or both of the thermal inconsistencies or
varying load on the power supply.
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