[time-nuts] Datum 4065A Cs Tube Response

Magnus Danielson magnus at rubidium.dyndns.org
Mon May 5 17:30:44 EDT 2014


Ed,

On 05/05/2014 10:56 PM, Ed Palmer wrote:
> Hi Joe,
>
> I didn't realize that the info in the 5061A manual was somewhat
> 'generic'.  Thanks for the pointer.
>
> On 5/5/2014 6:20 AM, J. L. Trantham wrote:
>> Ed,
>>
>> If I have the math correct, and you are measuring the voltage to ground
>> through a 10 MegOhm input impedance DMM, you have about 7.5 nA beam
>> current
>> which seems a bit low compared to what I remember of the HP 5061A.
>> However,
>> you still have a definable 'peak' with a 'peak to valley' voltage of
>> about
>> 60 mV or a 'useful signal current' of about 6 nA.  If your unit's
>> circuitry
>> can properly amplify that and keep it a clean signal, it should work.
>> However, I would recommend setting the OCXO precisely on frequency with a
>> GPSDO before trying to close the loop and 'locking' the signal to the CS
>> tube.  It will dramatically lower the work load of getting everything
>> adjusted properly, particularly in a setting of low beam current.
>
> Easier said than done.  This oscillator has no manual frequency
> adjustment and if there's an electrical one, I haven't found it yet. But
> the oscillator's response seems very vague as though it's just kind of
> wandering around so I think there's something not right there.  It's
> likely related to the bad synthesizer chip.

The 4065 use the STEL DDS to jump around in an interesting patten to 
measure the Zeeman position of the main lobe and of the first side-lobes 
to steer the C-field in a separate control-loop. This means that the 
C-field is completely under CPU control, and no manual trimming. I think 
you can disable this and steer it manual over the serial interface, but 
I don't recall the details.

The 6 side-lobes separate with heavy dependence on the C-field, so 
servo-loop on their position relative the main lobe works really well.
With the C-field servo-controlled like this, the C-field shift of the 
main lobe, which is a much weaker dependence and also one of the major 
aspects in selecting Cesiums in the first place, a more accurate 
Si-realization becomes possible, as well as with much better repeat in 
realization as the arbitrary C-field shift can be almost completely 
canceled.

As the Zeeman steering and centering is done, locking to Rabi feature is 
done in a similar sense, by measuring multiple points. In fact, this is 
where it spends most of it's time and the for some of the TDM slots jump 
around for the Zeeman measuring points every once in a while.

All that is described in a patent, which then describes this being 
driven by the 1802 CPU, but that was later replaced by the 6800-series.
Precision time with whopping 8-bitters. It gives the same warm and fuzzy 
feeling as rubidium and cesiums with 741s in PSUs and control-loops :)

So, this core looks pretty brainless with not as much fun trimmers as 
the 5061A for instance, but it has quite better brains, it's just that 
the faded STEL 1173 drivers makes it blind.

The STEL 1173 DDS functionality isn't very hard to copy good enough, but 
it is still some work do be done. I don't remember where the datasheet 
went for it.

>> Somehow, the value of 40 nA sticks in my mind from the 5061A.  The 5061A
>> manual says end of life of the HP CS tube is a peak beam current of 8
>> nA or
>> less.  However, I have units with less current and they still lock.
>> The HP
>> manual also says to measure the voltage at the output of the tube with
>> a 100
>> MegOhm or higher input impedance DMM.  If yours is less, that may
>> artificially lower your values.
>
> I reconfigured my test setup to use a 100 Mohm meter and converted the
> results into nanoamps.  The 10 Mohm meter was loading down the tube a
> bit.  The new graph looks somewhat better.

Great.

>> EOL of the tube is a multifactor issue, including Signal to Noise
>> ratio and
>> the 'useful signal current' to 'background current' ratio.  The
>> 'background
>> current' is what you see with no RF signal applied to the tube.  Have you
>> measured that?  A ratio of 1 is EOL per the HP manual.  If yours is about
>> 4.5 nA, as suggested by the 'off peak' values shown, or less, you
>> still have
>> a useful signal and, hopefully, a useful tube.
>
> The ratio of signal level to background level looks quite good.  The
> background current is only 1.5 na for a ratio of 6:1.  I checked that
> measurement twice because it looked suspiciously good.

Great.

>> I'd recommend continuing with the repair.
>
> I will, but it could be world's slowest repair as I try to find one of
> those synthesizer chips.  I've seen places that claim to have them, but
> only in the 48 pin DIP package rather than the 44 pin PLCC.  I REALLY
> don't want to do that conversion!
>
> In any case, I have many other toys to amuse me! :)

While you wait, don't waste that tube.

> Thanks for all the tips!

I wish I had a 44 pin PLCC variant of the STEL 1173 lying around to help 
you out.

Cheers,
Magnus



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