[time-nuts] FE-5680 frequency jump

Magnus Danielson magnus at rubidium.dyndns.org
Tue Apr 2 00:13:17 UTC 2013


On 04/02/2013 01:12 AM, Attila Kinali wrote:
> On Mon, 01 Apr 2013 23:57:48 +0200
> Magnus Danielson<magnus at 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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