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

[Magnus Danielson]

Dear Poul-Henning,

I have been suspecting this very mechanism to exist in the HP5065 among 
others. I have not been overly impressed by the stability by which the 
current is produced.

It would be interesting to see to what degree the surrounding 
temperature as well as the mains supply voltage does. The mains supply 
voltage will creep in two ways, one way for the raw rectified voltage 
and second on the burning off the difference voltage causing the main 
voltage to power to temperature variation.

It would be interesting to see how good damping the main voltage to 
various internal volages there really is.

Cheers,
Magnus

On 08/18/2015 10:05 PM, Poul-Henning Kamp wrote:
> I have been measuring the C-field current of my HP5065 for a couple
> of days using my HP3458A.
>
> To be more precise, I have measured the voltage across the two
> parallel current sensing resistors A15R10 and A15R11.
>
> I have not set up a precise temperature measurement for this
> experiment, but eyeballing room monitoring, I find a close correlation
> of approx 0.4 millivolt/C.
>
> I also find that the phase-offset relative to a GPSDO correlates
> significantely with the voltage measured.
>
> If we assume that R10||R11 are not the cause of this, it corresponds
> to 0.5 microamp/C sensitivity for the C-field current, and thus
> C-field.
>
> The total adjustment range of the C-field pot is 2e-9 which changes
> the C-field current from 2.5 to 6 mA (ref: pg 8-48).
>
> That means the C-field sensitivity is 5.7e-7/Ampere.
>
> My 0.5 microamp/C therefore corresponds to 3e-13/C
>
> ...which is pretty much the MVAR floor in my measurements.
>
> It seems plausible that a better C-field driver could improve the
> stability.
>
> Pretty much all the components in circuit could be causing this.
>
> The ultimate reference for the C-field current is A15CR5, a 1N938
> temperature compensated zener which is also the reference for the
> +20V supply.
>
> In the HP5065 the 1N938 is driven at approx 12mA, where later
> datasheets indicates that 7.5mA is optimal for TempCo purposes.
>
> Once the present measurement run is over, I'll do a run where I
> measure the A15CR5 voltage directly, and if I can arrange it, also
> with a spot temperature measurement.
>
> If A15CR5 is the culprit, the next obvious step is to change A15R7
> to 1500 Ohm, and see if that improves stability.
>
> Another obvious experiment is to drive the C-field with a very
> stable external supply, and see what that does for the MVAR.
>
>