[time-nuts] Antique Rubidium Standard Questions

[Magnus Danielson]

On 04/24/2012 07:55 AM, Ed Palmer wrote:
> 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.

What I was speculating was either of:

1) Locking onto the wrong hyperfine line. You would be off in frequency, 
but the base response may be sufficient to integrate into a locked state 
and maintain lock. Since the wrong hyperfine line will have smaller 
amplitude, the response on both base and second harmonics will be much 
lower, so having second harmonic response of close to zero but still 
locking behaviour isn't all to hard to believe.

2) Similar to the above reasoning on the locking while lacking (strong) 
second harmonic, you can have other deficiencies which give similar 
behaviour on the main harmonic. For instance, if you have incorrect 
alignment of lamp temperature and isotopic filtering cell temperature, 
then the filtering will be miss-aligned so both D1 and D2 lines of the 
Rb-87 lamp will make it to the Rb-87 resonance cell, and then the 
state-inbalance which the single line optical pumping is intended to 
achieve is much less effective, so then the absorption effect which is 
detected will have much less response, and hence there will be a weak 
base harmonic and seemingly no second harmonic.

One way to separate between these two errors is vary the C-field. If you 
are locked to get base feature, then you move a little, but at the 
square of the of the C-field current. If you are locked to the side 
features, then you move dominantly linearly with the C-field current. 
One needs to measure at least three points to make any real conclusions.

Another way is to speculate on the temperature settings may have aged, 
so both measuring them and compare to the values they should have. You 
can also try to trim them one at a time and see if you can see if you 
start to receive second harmonics and then trim to stronger signal.

>> 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.

Maybe it needs to settle a little.

Cheers,
Magnus