It's not really noise, the feedback loop is marginally stable. What you are seeing is how the stability changes due to minor differences in the parts used in the loop, and the thermal masses and insulation of the oven.
-Chuck Harris Ed Palmer wrote:
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 minutes. 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? Thanks, Ed
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