[time-nuts] Datum 4065A Cs Tube Response

Ed Palmer ed_palmer at sasktel.net
Mon May 5 21:40:55 UTC 2014

Hi Tom,

Yes, that's the right chip, but the 'L' at the end means it's the DIP 
package.  The PLCC package has an 'M' instead of the 'L'.  I also found 
a lot of sellers on a far east site who have the DIP package (probably 
used chips pulled from sockets), but none who have the PLCC package.  
The price is also a lot less than $65, but of course, you're taking a 
risk that you might be buying a dead chip.


On 5/5/2014 3:18 PM, Tom Knox wrote:
> Hi
> Is this the right chip:
> http://www.oemstrade.com/search/STEL-1173%252FCL/
> At $65 it is pricy but a bargain if it fixes your clock.
> Thomas Knox
>> Date: Mon, 5 May 2014 15:14:16 -0600
>> From: ed_palmer at sasktel.net
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] Datum 4065A Cs Tube Response
>> I know very little about today's DDS chips, but I think that emulating
>> the STEL-1173 would be a challenge.  It provides 48 bit frequency
>> resolution.  I counted 13 different frequencies that are used to monitor
>> the signal to make sure that it's on frequency. Based on the manual,
>> they were shifting it many times per second. I'm not surprised the chip
>> died - they worked it to death!
>> Ed
>> On 5/5/2014 6:49 AM, paul swed wrote:
>>> I will agree with Joe. I have a CS tube thats darn near impossible to read
>>> the beam current and yet it still locks. That truly amazes me. I seem to
>>> recall other comments ages ago about that chip failing. There should be a
>>> way to emulate it these days with all of the DDS chips and such that are
>>> available.
>>> Good luck.
>>> Regards
>>> Paul.
>>> WB8TSL
>>> On Mon, May 5, 2014 at 8:20 AM, J. L. Trantham <jltran at att.net> 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.
>>>> 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.
>>>> 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.
>>>> I'd recommend continuing with the repair.
>>>> Good luck.
>>>> Joe
>>>> -----Original Message-----
>>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>>>> Behalf Of Ed Palmer
>>>> Sent: Monday, May 05, 2014 12:46 AM
>>>> To: Discussion of precise time and frequency measurement
>>>> Subject: [time-nuts] Datum 4065A Cs Tube Response
>>>> I'm playing with my first Cs standard.  It's a Datum 4065A which appears to
>>>> have a dead STEL-1173 synthesizer.  Before I put too much effort into
>>>> replacing that, I thought I'd check the tube and see if it has any life
>>>> left.  I've attached a chart showing the response of the central peak.
>>>> My methodology was similar to TVB's as shown here:
>>>> http://www.leapsecond.com/pages/cspeak except that I measured the tube
>>>> output directly with a digital voltmeter.  The system is reporting wildly
>>>> varying levels for the beam current so I didn't want to use any of it's
>>>> circuitry.
>>>> Does this look like a usable tube?  Healthy or on it's last legs? What
>>>> response levels are typical for a Datum 7504A tube?  I see that these
>>>> levels
>>>> are somewhat lower than those shown on leapsecond for the 5061A tube, but
>>>> that could just be the specifics of the measurement.
>>>> Thanks,
>>>> Ed

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