[time-nuts] Datum 4065A Cs Tube Response

[Ed Palmer]

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.

> 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.

> 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.

> 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! :)

Thanks for all the tips!

Ed

> 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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