C-field again

For now I have driven the C-field solenoid using my Fluke 732A and a Vishay Z201 2.5k resistor.

But is that good enough ?

Maybe not:


In this test I put a very light piece of plastic foam over the physics package which raises the outside temperature roughly 3°C.

That makes the current through the C-field coil drop 72 nA

A C-field shift of 72nA will cause a frequency shift of approx 3.4e-14.

For an active C-field driver to do it better, it must have total noise, all causes, less than 6 PPM, which isn’t trivial.

72nA change on top of 4mA at 10V is a change in total resistance of 0.046 Ohm.

The resistance of the C-field solenoid is approximately 12 Ohms, so that is approx 0.046 / 12 = 3833 PPM

Copper has horrid tempco, around 3900 PPM/K, so it is pretty good guess that the solenoid got approx one degree warmer.