[time-nuts] FE-5680 frequency jump

[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 at 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<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
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to 
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.