[time-nuts] HP5065A C-field current is temperature sensitive
rfnuts at arcor.de
Sun Aug 30 08:05:14 EDT 2015
agreed. But in the 5065A C-field circuit, the pot is wired as a variable
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
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).
Bob Camp schrieb:
> 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.
>> On Aug 29, 2015, at 9:03 AM, Adrian <rfnuts at 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.
>> 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":
>>> ... and found out that the current-sensing resistors R10||R11 have
>>> a very high temp-co.
>>> Getting closer...
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