[time-nuts] Antique Rubidium Standard Questions

Tue Apr 24 15:54:21 UTC 2012

I actually think thats what time-nuts is about.
I know I have done things with RBs and a CS that I never would have thought
I would or could do.
Heck its broke and junk and it feels great when you get the thing working
again.
Whats amazing is that when these were designed they were so cutting edge
and yet even today the engineering seems useful.
Regards
Paul

On Tue, Apr 24, 2012 at 10:40 AM, Ed Palmer <ed_palmer at sasktel.net> wrote:

> Joe,
>
> Thanks for your thoughts and ideas.  I expected that this would be an
> interesting project and it hasn't disappointed me.  I'm currently reading
> an article from the Bell System Technical Journal on microwave cavities so
> the learning continues.  Hopefully, it will never end.
>
> Ed
>
>
> On 4/24/2012 8:10 AM, J. L. Trantham wrote:
>
>> Ed,
>>
>> Sounds like you are on the right track.
>>
>> These things are fairly simple and fun to play with.
>>
>> The Rb assembly on the 5065A can be opened at either end.  One end has the
>> Rb lamp and is mounted by 3 screws, against the 'end' of the 'cavity'.
>> Inside that is a 'filter' assembly and beyond that, at the other end of
>> the
>> assembly, is the 'detection' assembly.  The 'detection' assembly is
>> attached
>> with threads as the assembly 'screws' into the far end of the 'cavity'.
>> Opening the far (other) end of the assembly allows one to loosen a screw
>> that then loosens a 'wedge' that 'secures' the 'detection' assembly in
>> place.  Then, the 'detection' assembly can be simply 'unscrewed' from the
>> 'cavity'.
>>
>> I have never seen anything about 'tuning' the cavity in any of the 5065A
>> manuals but, by the way this part of the assembly is constructed, I
>> suspect
>> that, when tested on a bench, the 'detection' assembly can be moved in or
>> out to 'fine tune' the cavity, then 'secured' in place by screwing down
>> the
>> 'wedge'.
>>
>> I suspect that there are others on the list that can speak to this much
>> better than I can.
>>
>> I don't know how the Tracor Rb assembly is constructed but it might be
>> similar and allow a means of 'tuning' the cavity.
>>
>> Good luck.
>>
>> 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 8:30 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Antique Rubidium Standard Questions
>>
>>
>> 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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