[time-nuts] Antique Rubidium Standard Questions

Ed Palmer ed_palmer at sasktel.net
Tue Apr 24 13:29:32 UTC 2012


Joe,

On 4/24/2012 6:50 AM, J. L. Trantham wrote:
> Ed,
>
> If it is, indeed, stable at exactly 5.000 000 000 MHz, it is probably
> locked.  That being said, I would wonder if there is an indicator issue and
> perhaps a control logic issue.
>
> On the HP units, the AC Amplifier, that receives the signal from the Rb
> assembly, has been the source of some problems on my units.  Also, the AC
> Amplifier sends a signal that indicates both a 137 Hz fundamental frequency
> being present and a 274 Hz 2nd harmonic signal being present to the control
> logic assembly as two of the 'required' signals to get a 'lock' indication.
> You might want to take the AC Amplifier out, put it on the bench and feed it
> a low level fundamental frequency, track that through the assembly then
> repeat with a 2nd harmonic signal and look at it's output.  Same thing for
> the logic assembly to make sure that some of the 'required' signals are
> indeed present rather than 'failed on'.

I did that.  There were some bad solder joints in the amp / filter 
circuit but otherwise, it was good.  To test the entire signal chain, I 
replaced the Rb lamp with an LED that was modulated at the fundamental 
(155 Hz in this case) and then at the 2nd harmonic (310 Hz) and the 
signals appeared & disappeared as expected.

> I forgot to ask, did you find a manual?

That was one of the reasons I bought this thing.  It included the 
original manual.  There's also a partial manual online so I was able to 
familiarize myself with the unit before I made my bid.

Ed

> Joe
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Ed Palmer
> Sent: Tuesday, April 24, 2012 12:55 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Antique Rubidium Standard Questions
>
>
> Hi Joe,
>
> On 4/23/2012 9:45 PM, J. L. Trantham wrote:
>> Ed,
>>
>> I am not familiar with the Tracor units, only the 5061A and B as well
>> as the 5065A.  These units use the 2nd Harmonic as an integral part of
>> the feedback loop.
>>
>> Without the 2nd Harmonic, is there another way to 'unambiguously
>> determine that it is locked', other than comparing it to a 'known',
>> 'locked' signal?
> Strictly speaking, the answer is probably 'No'.  After all, why would
> they include the 2nd harmonic circuitry if they didn't need it?  There
> should be 2nd harmonic and I hope to find some somewhere.  Remember that
> this unit is being brought back from the dead as a learning exercise so
> a few 'minor' issues aren't a show-stopper.  The unit has been running
> for most of the day.  I flipped the switch to open the loop.  The
> frequency went from 5 MHz to 5MHz +0.045 Hz while the error meter went
> from 0 to -25 on a scale of 50.  Close the loop and the frequency
> returned to 5.000 000 000 MHz and the error meter went back to zero.
> That certainly sounds like locking behaviour to me.
>
>> I guess another way to ask the question is do you think you happen to
>> have a particularly good OCXO?
> It's a 40 year old AT-crystal that hasn't had nearly enough recent run
> time to work the kinks out.  I would be astonished to find that it's
> that good.  But I realized that I've never looked at the oscillator by
> itself so I did a quick test.  I measured an aging rate in the range of
> 0.2 ppm / day.  If I cancel out all the aging, the results start to look
> like the earlier attachment.  But not when it's unlocked.
>
> Ed
>
>
>> 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 10:17 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Antique Rubidium Standard Questions
>>
>>
>> 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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