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

Bob Camp kb8tq at n1k.org
Sun Aug 30 12:00:03 EDT 2015


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 <rfnuts at 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 <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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