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

timeok at timeok.it timeok at timeok.it
Wed Aug 19 16:01:01 EDT 2015

Hi Poul-Henning Kamp,

What you write is very interesting. 
A series of updates starting from the capacitors to the conversion to Super Model (from Corby) with the application of the band pass filter on the lamp greatly improve the initial performance . I am sure that the 20V.regulator circuit  and the C Field regulator, can be significantly improved.
An old uA723 can be a good solution for regulator and reference for the C Field current generator. 
I think I will work at it , it would be interesting we can exchange more information. |


On Tue 18/08/15 22:05 , Poul-Henning Kamp <phk at phk.freebsd.dk> 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.
> --
> 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.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts [1]
> and follow the instructions there.
> Links:
> ------
> [1]
> http://webmail.timeok.it/parse.php?redirect=https://www.febo.com/cgi-bin/ma
> ilman/listinfo/time-nuts
Message sent via Atmail Open - http://atmail.org/

More information about the time-nuts mailing list