[time-nuts] HP5065A C-field current is temperature sensitive

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Aug 20 04:55:00 EDT 2015

In message <1440039768.738221.360877705.34D2F4EC at webmail.messagingengine.com>, 
Bill Byrom writes:

>I don't have an HP5065A and can only see the portion of the schematic
>you copied.

The manual is on K04BB if you want the full monty.

>Here is
>what I can surmise from your measurements (assuming you don't have
>any ground loops or measuring instrument or test lead thermoelectric issues):

The measurement setup isn't optimal, I can only do one measurement
every five seconds on each point and the HP34972A is only a 6½ digit
instrument, but otherwise the setup is solid.

>The change of the 20 V power supply when you lock is roughly 400
>uV (20 ppm).
>The change of the C-coil current isn't really repeatable. I see a big
>500 nA (111 ppm) drop at 1,000 seconds, but no such clear change earlier
>in the test.

It didn't actually lock until 1000 seconds, so one difference is that
the logic "continous operation" logic didn't reach final state and
its lamp didn't turn on in the previous attempts.

>But the actual voltage changes you are measuring seem to be
>roughly correlated between the 20 V power supply and C-coil current
>sense resistor changes.

As they should be, because both are derived from the same A15CR5 zener.

>I'm guessing about these values, based on converting your current
>numbers into voltage based on a perfect R10 || R11 parallel combination
>of 691.6 ohms. I see that R10 has an * asterisk, and I wonder what is
>shown for that note. R10 might be a selected value at manufacturer, or
>it might have a specific temperature coefficient.

The asterix means "selected".  I have not been able to figure out
what criteria it is selected for.

>The temperature coefficient of the resistors may be much more important
>than anything else, especially for an old product. 

All the important ones are wire-wound, probably for exactly that reason.

>those old electrolytic capacitors (C4, C6, and C7 for example) are still
>OK, or whether they are showing any changing leakage currents. You might
>want to change them with new capacitors just in case.

Good point.

>So I'm not convinced that the time curve is showing a correlation based
>on the 20 V power supply affecting the C-coil current.

No, the main correlation is via the common zener, but the step at 1000s
is *not* present in the zener voltage, which means that the current
generator has really bad supply sensitivity.

>It's possible that measuring system errors (such as where you connected the
>measurement system ground) might cause some of these changes. So you
>might want to check your setup and the instrument and test lead
>computed accuracy.

I think the way I've done it is OK.  The 34972A has floating inputs
and I measure from a local GND for all six points.  I'm not seing
any noise-artifacts.

>I also disagree with your estimate of the CR5 zener current. [...]
>the CR5 diode current must be (12.2195 - 4.2456 mA) = 7.9739 mA.

Good point.

>pretty close to the zero temperature coefficient current for the 1N938
>you describe. So I do not recommend changing the diode current.

The 7.5 mA optimum is from a much later data-sheet, and may be for
a particular "high performance" variant of the 1N938, so there is
no guarantee that there even is a zero-tempco current for the one
in my HP5065.

Either way, fixing the zeners tempco is only half of the solution,
it looks to me like the "real" solution involves an entirely new
C-field current driver.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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