[time-nuts] OCXO DC Current Question

Ed Palmer 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.

Any thoughts?


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