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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