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

Bob Camp kb8tq at n1k.org
Sat Aug 29 12:56:22 EDT 2015


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.
> 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...
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