Mark,
I have some good news. I decided to copy your test by powering up an
FRK that's been sitting in a box for over a year. Once it locked, I
measured the frequency and was surprised to find that it was about .06
Hz (6e-9) low. I added about 890 ohms between the two external
frequency adjustment leads and got an error of about 7e-11. So your
unit might be in much better shape than I thought. Sorry if I misled you
on that.
I'm also looking at the Allan Deviation. It's only been running for a
few hours, but one thing that I noticed is that the frequency is
drifting as the unit warms up and works the kinks out. This drift
degrades the AlDev numbers by looking like aging or a high noise floor.
Try using Hadamard Deviation instead of Allan Deviation. Hadamard is
insensitive to aging. So far, my AlDev values aren't much different
than yours but as I run the graph out beyond 800 sec., the numbers do
get a little better. Also, remember that most Rb standards aren't
optimized for low AlDev values since most applications don't need that.
Ed
On 5/28/2013 4:10 PM, Ed Palmer wrote:
The frequency offset of 3e-9 is still very high. Since the 9390 will
discipline the FRK, the C-field adjustment of the FRK doesn't really
matter. Before you open up the FRK again, see if the C-field can
change the frequency. If it can, and if the range is similar to the
specified range, then the C-field is working and there's no point in
opening the FRK.
As for your AlDev numbers, in no particular order.....
Have you measured the noise floor of your measurement system?
Check the power supply for hum and noise.
Is the crystal control voltage stable or is it noisy?
A weak Rb lamp might give you a noisy signal, even though it's
locked. Is the lamp voltage noisy?
Check for a bad cable or some other bad connection. If possible, use
only double-shielded cables.
Running down problems like this can drive you crazy. The levels are
so low that it could be almost anything from the computer you're
running Timelab on to your neighbor's wireless phone!
A good OCXO often takes a month or more to stabilize after you pick it
up in an auction. Rb standards aren't usually that leisurely about
settling down but I'd just let it run and see what happens. This might
be a good time to put it back in the 9390 and see what happens. Does
the 9390 have commands that will show you how well the FRK is performing?
You also might have a unit that's working properly but is a little
noisy. The specs don't cover something like this. You're well
outside the time for 'short term stability' but haven't yet got to the
'aging' spec. If you're using the 5370 by itself, you can't measure
the short term stability at all.
Ed
On 5/28/2013 11:37 AM, Mark C. Stephens wrote:
Ed and Bob,
The FRK has settled down to 0.03Hz out after 2 days undisturbed (back
in its black box).
Allan deviation @ 800 seconds is 4.88E-12 which I am not terribly
happy about.
-mark
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