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

Jamieson (Jim) Rowe jimrowe at optusnet.com.au
Thu Feb 6 23:15:21 EST 2014


Hi Ed,

Thanks for those suggestions -- much appreciated. I guess you're right -- if 
the case was magnetised, inverting the unit wouldn't make any difference. So 
presumably, the problem is due to a dry joint or similar problem inside, as 
you suggest, or perhaps the local magnetic field after all (I do have quite 
a bit of electrical wiring and electronic gear in my work room).

All the best,

Jim Rowe


-----Original Message----- 
From: Ed Palmer
Sent: Friday, February 07, 2014 1:54 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] How I got my FE-5680A to lock in Sydney, Australia

Hi Jim,

On 2/6/2014 3:32 PM, 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...

My first thought was to make a typical 'down-under' joke and suggest you
run the 5680A upside down, but you beat me to it! :)

> 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.

My very limited knowledge regarding mu-metal is that it is so
magnetically 'soft' that it can't be magnetized.  If it was somehow
magnetized, inverting the unit wouldn't make any difference, would it?

> Or should I just run the FE-5680A upside down permanently – the simple but 
> ‘crude’ answer?

I don't believe for a second (pun intended) that the earth's magnetic
field has any effect on the locking of your 5680A.  It's just not strong
enough.  The same applies to the C-field which can only nudge the
frequency one way or another by a small amount.

Since flipping the unit DID make a difference, my money would be on a
trivial, boring mechanical issue inside the unit.  Could be a bad solder
joint, broken wire, floating piece of debris, or something like that.
Worst case might be a broken glue joint somewhere in the physics
package.  That could be ugly.  I would definitely open it up and see if
anything falls out.

> 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?

The RS-232 commands affect the DDS - assuming your unit does have one.
It will have no effect on the locking, only on the output frequency.

A few years ago I bought a dead Datum SLCR Rb standard.  It's a cousin
to the LPRO.  I thought I'd learn some things by trying to fix it.  The
problem was intermittent.  I tore it apart and found that one of the
legs of the crystal had never been soldered!  Never overlook the obvious.

> I hope a much more experienced time nut can provide a few answers, please.

Maybe the blind leading the blind is a closer description.

Ed

> Jim Rowe

_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to 
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there. 



More information about the time-nuts mailing list