[time-nuts] Heated crystal? & Rb tube corrosion (FE-5680A)

[Bob Camp]

Hi

The amount of Rb in the cell is quite small. Cell leakage would stop the
unit from working. There's pretty much no way that the cell can / would
cause corrosion and the unit keep working.

The crystal is heated to reduce the pull range of the VCXO. Since it's
locked to the Rb, improving it's temperature performance does not have a
significant impact on the unit's performance. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Peter Bell
Sent: Thursday, December 01, 2011 4:42 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Heated crystal? & Rb tube corrosion (FE-5680A)

Hi, Steve

Both the lamp and the absorption cell are made out of glass (or
possibly quartz - see attached image).  The unit mounted to the
crystal is a PTC thermistor wired directly across one of the power
supplies.  I suspect it's just there to heat up the xtal to a
temperature close to it's knee point - absolute accuracy is not that
important since the xtal is being run in a VCXO config and is locked
to the Rb cell.

None of the units I've looked at appear to have any corrosion on them
- the discoloration you can see on that photo is just a side effect of
that fact that the lamp housing (intentionally) runs very hot - about
100c.  The only negative effect I've seen from this is that the fiber
washers that hold the lamp housing in position get somewhat cooked and
the used ones are easily damaged if you tighten up the screws too
much.

Regards,

Pete

On Thu, Dec 1, 2011 at 1:05 PM, Steve . <iteration69 at gmail.com> wrote:
> I've been paying particular attention to the discussions involving the
> FE-5680A frequency standards of recent attention. I do not have a FE-5680A
> yet, but rather I am studying what is shared from the others prior to
> buying. At the very least I want to know what I'm up against should I get
a
> DOA module.
>
> It appears that these units use a heated crystal.(..i sure hope it's
heater
> and not an acoustic resonator). Has anyone performed sub 1degree c drift
> testing against a known stable source? What are the performance gains by
> using tighter temperature control? Also It appears that quite a few of
> these have corroded Rb tube interfaces. My guess is the corrosion is a
> tale-tale sign of small amounts of rb gas leakage in combination with the
> raised temperatures of the tube oven?  If this is the case I suppose a
> visual check of the tube interface for corrosion would yield a fair
> approximation of the tube condition?
>
> Lastly, is the Rb tube a quartz tube or is it a metal(silver
> lined?)canister sealed with polymer tape?
>
>
> Thanks,
> Steve
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