[time-nuts] Datum 4065A Cs Tube Response

Ed Palmer ed_palmer at sasktel.net
Mon May 5 17:14:16 EDT 2014

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!


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.
> 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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