[time-nuts] Datum 4065A Cs Tube Response

J. L. Trantham jltran at att.net
Mon May 5 08:20:47 EDT 2014


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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