Hi Another counter argument against the “ratio mode” is that with zero current through the wiper, odd things can happen. There is *always* something else to consider in any real design.
Since they needed to calibrate the voltage swing, there aren’t a lot of options with the technology they were using at the time. Today there are a lot more choices of how to get this sort of job done. Bob > On Aug 30, 2015, at 8:05 AM, Adrian <rfn...@arcor.de> wrote: > > Bob, > > agreed. But in the 5065A C-field circuit, the pot is wired as a variable > resistor. > The Bourns data sheets specifies +/- 50 ppm / deg C max. > Together with the two series resistors, the effective TC of the pot > drops to about 20 ppm. > That is indeed just 2 times more than the effective temperature > coefficient of the reference diode (1N938). > > Btw. the 1N938 is running at about 7.5 mA +/- 1 mA, not at 12 mA as > mentioned earlier. > Some 3.5 to 5.5 mA are going through R8, R6, R12. > To get the correct diode current, this current has to be subtractrd from > the total current through R7 (Q5, Q6 base currents neglected). > > Adrian > > > Bob Camp schrieb: >> Hi >> >> Most potentiometers have very good TC when used in a “ratio mode”. >> You can quickly mess that up by putting resistance in series with >> the pot. It’s a bit surprising how may times you see this inherent stability >> messed up that way. >> >> Bob >> >>> On Aug 29, 2015, at 9:03 AM, Adrian <rfn...@arcor.de> wrote: >>> >>> Keep in mind that potentiometer R6 is one of the key components of the >>> C-field circuit. >>> Together with R8 and R12, it defines the reference voltage to Q6A/B. >>> The tempco of that wire-wound potentiometer can certainly not be neglected. >>> >>> I would suggest to check the C-field temperature dependency with an >>> external precision current source feeding the C-coil directly. >>> >>> Adrian >>> >>> Poul-Henning Kamp schrieb: >>>> 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 >>>> >>>> ... and found out that the current-sensing resistors R10||R11 have >>>> a very high temp-co. >>>> >>>> Getting closer... >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.