[time-nuts] How I got my FE-5680A to lock in Sydney, Australia

Jamieson (Jim) Rowe jimrowe at optusnet.com.au
Thu Feb 6 16:32:43 EST 2014

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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