Thank you for this information, Bob! I have to reconsider the frequency
jumps of my GPSDOs...
Volker
Am 05.04.2013 20:59, schrieb Bob Quenelle:
I found the cause of the 4 mHz frequency jump. I have an LPRO-101,
an FE-5680, power supplies and a Motorola M12T GPS board in a surplus
case. When I put the case away to work on another project I piled the
hockey puck antenna and lead in the case and it happened to land on
the FE-5680. I noticed the antenna stuck firmly to the FE-5680 case
when I got the project back out. I found I could get a 6 mHz (0.6
ppb) shift comparing the frequency with residual magnetism from the
magnet stuck on the FE-5680 case to the frequency after demagnetizing
the FE-5680 case. Unintentional C field adjustment. Dope slap, live
and learn.
Bob
-----Original Message----- From: Magnus Danielson
Sent: Monday, April 01, 2013 5:13 PM
To: time-nuts@febo.com
Subject: Re: [time-nuts] FE-5680 frequency jump
On 04/02/2013 01:12 AM, Attila Kinali wrote:
On Mon, 01 Apr 2013 23:57:48 +0200
Magnus Danielson<mag...@rubidium.dyndns.org> wrote:
On 04/01/2013 10:06 PM, Bob Quenelle wrote:
I’ve been running an FE-5680 for maybe a total of 50 hours over the
last
several months. I found that an offset setting of 180 made it
track GPS and
(previously-set) LPRO-101 10 MHz signals. Even with power cycling,
after
about 1/2 hour, with an offset setting of 180 the FE-5680 was
stable. The
last time I turned on the FE-5680, it drifted with a setting of 180
and
needed a new setting of –415 to track the other signals. That’s a
change of
595 counts and with a resolution of 6.8 uHz per count, a frequency
change of
4 mHz (0.004 Hz) and 0.4 ppb. Operation at the new setting is
stable for
now. The lock signal indicates lock and the power supply voltage
is still
15V. I haven’t checked lamp voltage or VCXO voltage as that requires
opening the case.
How long have it been turned on since last power-up?
Let it sit for a day at least.
I've found that it is easy to be in too much hurry to judge the
situation and trim things efter power-up. The crystal oscillator just
doesn't get the time to settle in.
That might be indeed the case. Figure 3 in [1] gives "quite high"
frequency aging differences after switch on and long run time.
Attila Kinali
[1]
http://www.pi5.uni-stuttgart.de/common/show_file.php/lectures/100/blaetter/The%20Rubidium%20Clock%20and%20Basic%20Research.pdf
You are confusing the VCXOs frequency drift with that of the rubidiums
(which is the result of the FLL locking of the VCXO to the rubidium
"resonance").
If the VCXO still has a fair distance to drift, then false locking can
occur while compating the initially quite vigorous drift rate. The only
real way to handle that is to sit and wait for it to settle down. Only
after that may trimming of the oscillator be done to zeroize the
integrator state.
A small commercial rubidium doesn't need very long to "get a feel" if it
is in good condition or not, but sitting on your hands and let it warm
up gives you a fair idea of just how skewed situation it is. That's also
true for caesium clocks.
So, sit on your hands and let it settle. Better yet, leave on while you
do other things. Just recall to put enought cooling on it!
Cheers,
Magnus
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