[time-nuts] HP5065A C-field current is temperature sensitive
timeok at timeok.it
timeok at timeok.it
Sat Aug 22 09:39:48 EDT 2015
i have done some short time measurements (30 Min.) on the current generator, in particular on the reference input and feedback input of the differential amplifier Q6A and .Q6B (base pin to GND).
On the reference input Q6A I have a stability equivalent to more than +/-1nA , In the Q6B I have a slow current changing about some hundred nA.
We can say the Reference voltage, R8,R6,R12 are stable over time. The problem can be only on R11,R10 or the Voltage to Current converter gain.
An Idea is to try to disconnect R10. This resistance is factory selected to set the correct current range, but is not an High stability like the R11.
At the moment I cannot disconnect the HP5065A under test but will be the next operation I will do.
On Fri 21/08/15 22:22 , "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
> I had this morning, and put it into action right away:
> I disconnected the A15R8 resistor from the tempco afflicted A15CR5 zener
> and hooked it up to my Fluke 732A's 10V output (which is speced to 10mA).
> I had expected to see a vast improvement in C-field drive stability, but
> while the improvement is measurable, it is a lot less than I had expected.
> Having eliminted that one, there are two influences left:
> * Noise from the +20 V supply leaks through
> The data evidently shows this to be the case, but the correlation does
> not explain all the C-field current variability.
> * Tempco in the constant current driver circuitry
> I'm increasingly leaning this way, because even though A15R8 is fed
> from the rock-stable Fluke, the voltage at the A15Q6A base still
> varies quite a lot with temperature, but the short term jitter is
> much better.
> I don't have enough data yet to conclude if the MVAR floor is improved,
> but it looks quite promising so far: The curve has very confidently
> dipped below 1e-13 at 5000 seconds. It used to flatten at 3e-13.
> Next experiment is going to be driving the C-field solenoid directly
> from the Fluke 732A through a suitable low tempco current-limiting
> series resistor.
> Anybody want to bet what *that* will do to the MVAR floor ?
> I'll write this up on my web-pages once I have more data.
> 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
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