[time-nuts] Antique Rubidium Standard Questions

Tue Apr 24 12:50:54 UTC 2012

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 forgot to ask, did you find a manual?

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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