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

Bill Byrom time at radio.sent.com
Wed Aug 19 23:02:48 EDT 2015

I'm impressed with your great documentation ... but I wish to differ with some of your circuit analysis. I don't have an HP5065A and can only see the portion of the schematic you copied. 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 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. 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. In the first 900 seconds I would guess that the
C-coil current sense voltage is rising at about 40% of the rate of the
20 V supply, then during the 1,000 second event the 20 V supply dropped
maybe 30 seconds before the C-coil current sense voltage dropped, and
the current sense voltage dropped about 80% of the power supply drop.
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 temperature coefficient of the resistors may be much more important
than anything else, especially for an old product. I also wonder if
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.

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. It looks to me
like the noisy jumps in these are not related to each other. 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 also disagree with your estimate of the CR5 zener current. By your
report of the C-coil current and R10 || R11 sense resistance (691.6 ohms
if those stated resistance values were perfect), the current sense
voltage is 3.1266 V. So the voltage at the base of Q6A is also 3.1266 V.
I assume that Q6A/Q6B are a matched dual transistor in the same case, so
they are at very close to the same temperature. This means that the
current through R8 is (8.786 - 3.1266 V)/1333 = 4.2456 mA. The current
through R7 is (20.089 - 8.786 V)/925 = 12.2195 mA. So if there is
nothing drawing significant current in the wire extending left from C4,
the CR5 diode current must be (12.2195 - 4.2456 mA) = 7.9739 mA. That's
pretty close to the zero temperature coefficient current for the 1N938
you describe. So I do not recommend changing the diode current.

So I recommend changing all of the old electrolytic capacitors in that
area of the schematic for good measure (C4, C5, C6, and C7). Then
check the warmup voltage as you did before from terminal 1 of the C-
coil to the ground end of the current sense resistors R10 || R11. Be
sure to not use a different ground, since there may be significant
current and ground drop through the ground trace or plane. Be sure to
have the covers in place so the airflow and thermal characteristics
are as HP designed.

Bill Byrom N5BB
On Wed, Aug 19, 2015, at 04:45 PM, Poul-Henning Kamp wrote:
> Tonight I hooked my HP34972A DAQ up to the HP5065A and struck gold
> right away:  The reference zener has a tempco of 20PPM.
> Possibly also of interest:  The C-field driver is not very good
> and noise on the +20V supply leaks straight through it.
> Full details and plots here:
> http://phk.freebsd.dk/hacks/hp5065a_temp/index.html
> --
> 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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