[time-nuts] Antique Rubidium Standard Questions

[Ed Palmer]

Hi Joe,

On this unit (not sure about others), the 2nd harmonic is used to 
unambiguously determine that it is locked.  But the 2nd harmonic has no 
part in the acquisition or maintenance of the lock.  That is done by the 
fundamental.  I can tune through resonance and see the standard curve 
like fig. 5-7 in the HP 5065A manual (see attachment) except mine 
doesn't quite make it to the full-scale saturation level.  I then tune 
to the resonance point and flip a switch to close the loop.  It doesn't 
turn on the nice green light because that's done by the 2nd harmonic.  
But it also doesn't drift like an OCXO.  Take a look at the second 
attachment for an ~10 hour data run.  The relatively poor performance 
below 1000 seconds is due to my measurement setup.  I was looking for 
high Tau performance, not low Tau.

Ed


On 4/23/2012 7:58 PM, J. L. Trantham wrote:
> I don't understand how it can 'lock' without a 2nd Harmonic Signal.
>
> Joe
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Ed Palmer
> Sent: Monday, April 23, 2012 12:54 AM
> To: Time-Nuts
> Subject: [time-nuts] Antique Rubidium Standard Questions
>
> I'm playing with a Tracor 304-B Rubidium Standard from 1969.  I'm using
> it as a learning exercise to find out more about the guts of a Rubidium
> standard and how it works.
>
> This thing is a beast!  Rack-mount, 3U high, 39 pounds (~18 kilos), 9
> plug in circuit boards.  The OCXO is an oddball frequency that is
> multiplied directly to 6.8 GHz.  There's no synthesizer in that chain.
> A synthesizer is used to convert the oddball frequency to a 5 MHz output.
>
> It's sort of working.  The error signal isn't up to spec, but it's
> strong enough to give a stable lock although there's no trace of a
> second harmonic signal.  Allan Deviation is in the Xe-12 range from 1K
> to 10K seconds.  The OCXO has a not-yet-resolved issue that is probably
> degrading the results.
>
> The lock frequency suggests that the Rubidium cell has drifted down by
> ~30ppt over the 40+ years since it was built.  Is that reasonable?
> That's much more drift than the specification states, but I doubt if the
> spec was intended to be valid for 40 years!
>
> Could the drift be at least partially responsible for the lack of second
> harmonic?  A message on the list (
> http://www.febo.com/pipermail/time-nuts/2006-April/020562.html ) said
> that you could peak the second harmonic by adjusting the cavity tuning.
> If the cell and the cavity are out of sync would that kill the second
> harmonic?  How close to they have to be?  If this thing has a cavity
> tuning adjustment I haven't found it.
>
> FYI, I checked my counter (Racal 1992 referenced to an Efratom FRK-H
> Rubidium) against my Z3801A and Tbolt.  Both measure 10.000 000 000 MHz.
> so I'm confident that my numbers are good.
>
> Thanks,
>
> Ed
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