[time-nuts] Datum 4065A Cs Tube Response

[paul swed]

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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