Hi Tom,
Thanks for your comment, which seems to confirm that the earth's magnetic
field (plus the fields associated with my other gear might well be
associated with the problem. Perhaps I'd be better off getting hold of a
soft steel box to put the complete FE-5680A into, along with its mu-metal
shield case...
Best regards,
Jim Rowe
-----Original Message-----
From: Tom Harris
Sent: Friday, February 07, 2014 2:31 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] How I got my FE-5680A to lock in Sydney, Australia
I got into trouble once when we got a centrifuge from the US (I'm in
Australia) and it didn't work and I suggested to the Yanks that they should
have made it revolve in the opposite direction for working S on the
equator. The idiots tried this by rewiring the motor and the basket (the
large heavy casting that holds stuff to be centrifuged) promply unscrewed
itself and nearly came off the shaft. 30Kg of steel about 500mm in diameter
spinning at 5000rpm careening round the lab would not be much fun. I never
thought that there could actually be a real reason why things stuff up
across the equator.
Actually the vertical component of the magnetic field here has the opposite
sense, which is why compasses made for Europe are unusable in Australia,
the card just tries to align itself at 45 deg to the horizontal and refuses
to work as it fouls the internals of the mount, so this might make sense.
Tom Harris <[email protected]>
On 7 February 2014 08:32, Jamieson (Jim) Rowe
<[email protected]>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.
Jim Rowe
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