[time-nuts] Antique Rubidium Standard Questions

[Magnus Danielson]

On 04/25/2012 01:12 AM, Ed Palmer wrote:
> Magnus,
>
> On 4/24/2012 10:49 AM, Magnus Danielson wrote:
>> 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.
>
> I knew I'd forget something. I checked the temperature of the lamp and
> it's around 100C. But I forgot to check the cell / cavity. It's at 55C.
> Neither temp is specified in the manual but the cell temperature seemed
> low so I raised it to ~70C. Bad idea. The higher it went, the lower the
> maximum error amplitude went. From a reading of 40 (50 is full scale),
> it dropped to 20.

OK. Fair enough. You have at least tried it now.

>> 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.
>
> I checked this and the frequency vs. dial setting for the C-field is
> definitely not linear. Half of the change is concentrated in the last
> 10% of the dial.

OK, then you got the right transition.

>> 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'll play with the lamp temperature. The lamp frequency is within spec.
> It runs at the unusually low frequency of 20 - 25 MHz.
>
> Another thing has me puzzled. When you take off the end cap of the
> physics package there's a hole that goes through two shields, two
> circuit boards, and the wall of the cavity. The hole through the cavity
> wall isn't threaded so there wasn't a screw of some kind. You can't see
> the lamp so it wasn't for lamp inspection. The only other thing I can
> think of is a probe and since it goes through those other parts, it must
> be for use during manufacturing. I built a probe by shorting the shield
> to center conductor on a piece of tiny coax and made some measurements.
> The SRD is driven at ~69.742 MHz and the 98th harmonic is the Rb
> frequency of 6.834 GHz. The 92nd harmonic (6.416 GHz) was the strongest
> signal I measured. The 93rd (6.485 GHz) and 95th (6.625 GHz) harmonics
> were also strong. There was no trace of the 98th harmonic. Shouldn't the
> strongest signal be the Rb frequency? I tried moving the probe around,
> but although it changed the levels, it didn't show any different
> frequencies.

Sounds like the cavity tuning is off the mark. That could be it, since 
it too would get the same form of lowering the inbalance that I spoke of.

> I just realized that I haven't mentioned that there's a partial manual
> for this thing online. It's missing a few schematics, but is otherwise
> complete. The URL is http://sundry.i2phd.com/ServiceManual_304b.pdf .

Great. It would be fun if you could have the full manual scanned.

Cheers,
Magnus