# [time-nuts] HP5065A C-field mods and optical unit mods

timeok at timeok.it timeok at timeok.it
Wed Nov 22 03:58:49 EST 2017

```   Charles,
Imagine having a source and fixed resistors with TC equal to zero.
The current variation of the coil varies between 2.5 and 6 mA.
In addition, the relationship between current and frequency variation of the HP5065A is not linear.
If the pot is short-circuit, the current is 6 mA and the overall TC is that of the imaginary generator ie zero, in our real case 1ppm / C (source+fixed resistors).
So the pot TC is completely irrelevant.
When the pot is fully inserted the current will be 2.5mA, so the total current variation is 3.5mA.
The whole thefull range of pot resistance generates a 3.5mA current variation. This variation generates a frequency fine adjustment of 2E-9.
As a result, the influence of the pot TC is directly correlated to the current stability with a nonlinear frequency variation between zero and X  in the range of 6- 2.5mA.
Or not?

best regards,
Luciano
timeok

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A "Discussion of precise time and frequency measurement" time-nuts at febo.com
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Data Tue, 21 Nov 2017 15:16:36 -0500
Oggetto Re: [time-nuts] HP5065A C-field mods and optical unit mods
Luciano wrote:

> what you refer is the resistance relation referred to the LM299 source around 14VDc but you have to consider
> the tuning range of the pot, in the original HP5065A is 2E10-9.
> Of course, it depends on the setting value of the variable resistance, but you have to consider a possible variation of 20PPM/c on 2E10-9.

No, you don't. The tempco that matters is the aggregate tempco of the
series string. Only a very small portion of the string voltage appears
across the pot, so a 20ppm tempco of that small part will cause a much
smaller change in overall tempco of the series string.

Perhaps Corby will tell us what value the pot is, and then you can
calculate the aggregate tempco of the string at various pot settings and
see for yourself.

Charles

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