[time-nuts] OCXO DC Current Question
ed_palmer at sasktel.net
Mon Feb 18 17:32:06 EST 2013
On 2/18/2013 2:42 PM, Bob Camp wrote:
> Like any control loop, gain, bandwidth and noise are all related. In a DOCXO you have two control loops and they do interact. That said, there's nothing grossly wrong with the four OCXO's. The noisy parts have a bit more gain in the controller. The quiet parts have a bit less gain. The easy way to see this is the ringing after the current changes.
Yes, when I zoom in on the ringing after the first step change, they all
show a period of ~9 seconds. The blue trace (the quiet one) has a nice
smooth damped sine wave while the other ones have varying amounts of
noise superimposed on the sine wave. The amplitude of the ringing on
the noisy ones is greater than the amplitude of the quiet one. In one
case, more than twice as big.
> Without ADEV numbers there's no way to know which one is good (or better) and which one is bad (or worse). The noisy parts may be responding to the ambient temperature rumble (thus correcting for it) and the quiet ones may be ignoring it (allowing it to hit the crystal).
I hadn't thought of that possibility. Thanks for that!
> It's also possible that the noisy ones have an electrical issue in the loop that generates the noise.
I was thinking of maybe a dead filter capacitor - hence the noise.
> If all the ovens started from the same temperature, there is a variance in the oven set points. Some take longer to warm up than others. You don't mention if they started the same, so that may or may not be significant.
They all started at the same temperature. Any apparent difference in
warmup time is mostly due to the arbitrary offset of the traces. The
offset was just so you could see all four traces clearly. However, isn't
it also likely that each oscillator has had it's oven tweaked to match
that particular crystal?
> To further complicate things, in a double oven, you can have a noisy outer oven that gets suppressed by the inner oven. Are your specific 260's double ovens? No way to be sure without tearing one open. The second "step" in the current plot could be an inner oven cutting back.
I wondered about that second step. The period of the ringing on the
second step is about 1 sec. longer than the ringing on the first step so
it's apparently a different circuit. The 260 series datasheet does NOT
say anything about it being a double oven. They talk about customizing
the unit to suit the customer, but that seems a bit extreme.
By the way, I purchased these oscillators from our favorite auction
site. If anyone's interested, the 260 series includes both AT and SC
crystals. Since these start out ~150 Hz low, they appear to be SC cut.
The output is a 5 MHz sine wave @ ~+7 dBm into 50 ohms. They work fine
with a 12V power supply. They have no mechanical frequency adjustment
(unless it's under a suspicious spot of solder on the case) but my 4
were all adjustable to 5 MHz via EFC. They were apparently removed from
Z3811 GPS receivers. They each have a sticker on the side that says
"Z3811-80010". There doesn't seem to be much info available on the
Z3811. I have no relationship to the vendor - just a customer.
So it sounds like the proper thing to do is file this information and
carry on. After making some Allan Deviation measurements, review
everything to see how, or if, oven 'noise' correlates to Allan Deviation
> On Feb 18, 2013, at 2:40 PM, Ed Palmer <ed_palmer at sasktel.net> 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?
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