Hi Magnus,
Thanks for those suggestions also. So if I understand you right, I'd be
better off trying to tweak the oscillator tuning -- using the trimcap? Or
did you mean via the RS-232C 'offset adjustment' command?
Regards,
Jim Rowe
-----Original Message-----
From: Magnus Danielson
Sent: Friday, February 07, 2014 2:36 PM
To: [email protected]
Subject: Re: [time-nuts] How I got my FE-5680A to lock in Sydney, Australia
On 06/02/14 22:32, Jamieson (Jim) Rowe wrote:
Hi again folks,
You may (or may not) recall that a month or so ago, I asked for any
information that might be available regarding how to fix a ‘used’ FE-5680A
rubidium module from China (via ebay) which was tested by the supplier in
China as working OK, but would not seem to lock up to rubidium here in
Sydney. There wasn’t a great deal of info available, it seems, so I kept
on checking ideas myself – mostly with no luck. The module would never
lock, but kept cycling back and forth between about 9.999770MHz and
10.000036MHz – ‘searching’ for a lock, but never finding it.
Anyway, a couple of days ago I was reading more about the operation of
rubidium vapour oscillators, and noticed that the ‘filter cell’ is very
sensitive to magnetic fields – hence the mu-metal shielding case, and also
for the ‘C-tuning’ coil. And I wondered if the main reason why the
FE-5680A had apparently worked in China, but wouldn’t lock up in Sydney
(Australia) might be caused by the fact that Quangzhou (China) is in the
northern hemisphere while I’m ‘down under’ in the southern hemisphere –
where the earth’s field is presumably somewhat different, in terms of both
strength and direction.
So I decided to test this in a crude way, by inverting the FE-5680A and
seeing what happened. And – lo and behold – it locked up within 2.5
minutes, and stayed locked until I turned off the power and let it go cold
again. The next morning I applied power again, and within 3 minutes it
locked up again with no problems. And it’s been locked up now for over 48
hours...
So it seems that the different magnetic field here may have been the
problem – either that, or it may have received a ‘jolt’ in transit, which
prevented in from locking unless it was inverted.
But how do I tell which of these explanations is right, without ‘opening
her up’ again and looking for some kind of subtle physical fault?
Another idea: perhaps the mu-metal shield case had acquired a small dose
of magnetisation in transit (via a physical shock, or from a strong field
metal detector). I guess in this case that I would have to remove the two
halves of the case, and bake them in a furnace to demagnetise them again.
Or should I just run the FE-5680A upside down permanently – the simple but
‘crude’ answer?
I’m not sure if this FE-5680A has the ‘C-tuning’ gizmo fitted, or wired
up. Am I right in thinking that another approach might be to try varying
the tuning via the RS-232C serial port? Does this work via the C-tuning
coil anyway, or by tweaking the DDS?
I hope a much more experienced time nut can provide a few answers, please.
The magnetic shield can loose it's shielding capabilities.
C-field tuning doesn't really change the opportunity for locking, unless
your oscillator is on the edge of the locking range, so trimming the
oscillator might work. Putting the oscillator upside-down might be the
2G shift needed. This only tells you that you may consider trimming the
oscillator.
If you have a strong signal, it's usually the crystal oscillator that is
too far off the mark for being pulled in. Measure the frequency as it
tries to lock-in. If the sweeps is too high or too low then you need to
look at that oscillator.
Cheers,
Magnus
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