[time-nuts] Datum 4065A Cs Tube Response

[Brooke Clarke]

Hi Ed:

Some links to the related patents are on my 4060 & 4065 web page:
http://www.prc68.com/I/FTS4060.shtml#Pat

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html

Magnus Danielson wrote:
> 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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