Scott,
On 08/10/2014 08:54 AM, Scott Newell wrote:
I'm curious if an OCXO has an SC or AT cut crystal in it. There's no
EFC, so I can't measure sensitivity. I did measure the frequency change
from cold startup, thinking that an SC cut would stabilize quicker than
an AT. I don't have a good AT OCXO to compare against, but I do have a
Morion MV89A (should be an SC, right?).
Graphs:
http://n5tnl.com/time/mv89a/mv89a.png
http://n5tnl.com/time/mv89a/unknown.png
Thoughts?
The two units are the same size and shape. The Morion ran without the
outer can.
Sure looks like a SC cut oscillator. We did this exercise many years
ago, and if you dig the archives you can find the numbers for AT, BT and
SC cut oscillators. AT cut was in the -145 ppm range as I remember while
SC is -20-25 ppm off.
Good that you made the cold-start measurement!
Extra points for measuring currents. Interesting to see the shut-off
ringing.
War-story: I was testing one oven oscillator which looked nice on paper.
Measuring it I saw a bump in the ADEV at 7 s, and looking at the
frequency & phase plots I saw the wobble. Hooking up a DMM I can barely
see the wobble. As I power-cycled the oven, I see the same wobble
already from power-up. Turns out that the oven controller has a very
high Q value and is ringing and that the ringing never really dies out.
The vendor had transfered a design to a new board, so the oven was
sitting on a ceramic board rather than a FR-4 board inside the can. FR-4
has much more thermal mass so it kept the loop nice and stable, but not
the re-tuned oscillator.
Measurement note: my 5370A is sick, and my 5345A doesn't do GPIB. So I
shot video with the counter (1 s gate time, using a tbolt as the 10 MHz
ref) and meter (measuring current) in the frame. From there, I used vlc
to spit out still images at about 1 Hz, then imagemagick to cut out the
sections with the seven segment displays. ssocr
(<http://www.unix-ag.uni-kl.de/~auerswal/ssocr/>) did the real magic of
converting the seven segment displays in the images to text. Had to do
some hand cleanup on a few data points, but it was a lot less
frustrating than typing in thousands of data points by hand. I'm sure
it'd work better with a more careful layout and lighting.
Messy, but it works. Hope you find a smoother measuring path soon.
Sure would like if TimeLab could log current and temperature too.
Cheers,
Magnus
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