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

[Jamieson (Jim) Rowe]

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: time-nuts at febo.com
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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