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

[timeok at timeok.it]

I remember you my post:

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.

I summarize my suspect:

1)  R10//R11 stability
2)  voltage to current gain

the Reference and associated resistive divider R6,R7, R12 t are quite  stable. 
To do some tests I have wired a uA723 as V to A  Converter using the internal op-amp and the pass transistor. I have tested the internal reference is not so stable as the 1N938. The best way can be use the original reference and resistor setting.
next step can be : remove Q6 ,Q7, R9 and replacing it directly with the 723 or other similar device to solve the gain problem and replacing with an high stability resistor the existent R10//R11.
To be test.

Luciano 


On Fri 28/08/15 10:06 , "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:

> Using the Fluke 732A as voltage reference improved HP5065A stability,
> but not very much, so I looked closer at the constant-current
> generator and the C-field pots "calibration":
> 
> http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html [1]
> 
> ... and found out that the current-sensing resistors R10||R11 have
> a very high temp-co.
> 
> Getting closer...
> 
> --
> 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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