[time-nuts] Antique Rubidium Standard Questions

Tue Apr 24 13:32:08 UTC 2012

Great thought on using the led. That goes in the bagOtricks.
Regards
Paul

On Tue, Apr 24, 2012 at 9:29 AM, Ed Palmer <ed_palmer at sasktel.net> wrote:

> 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@**febo.com<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@**febo.com<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@**febo.com<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<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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