Hi Bob,
On 2/18/2013 2:42 PM, Bob Camp wrote:
Hi
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 results.
Thanks Bob,
Ed
Bob
On Feb 18, 2013, at 2:40 PM, Ed Palmer <ed_pal...@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?
Thanks,
Ed
