Hi all,

New here.

This one seemed obvious at first, but now I am not so sure.

I have an air-core coil, inductance L, resistance R. The resistance exhibits a small amount of Johnson noise.

If the coil remains open-circuited, will there still be an, admitedly very small, amount of magnetic noise in the coil?

 

An air core inductor in the open would act like an antenna, gathering a much higher signal from radio station emissions than from thermal noise.

What are the values of R and L, and are you trying to use the noise in an RNG, or eliminate it?

 

It is part of a force transducer used within a magnetic structure to give about 15 N/A at frequencies below 30Hz. Used in an instrument similar to http://bnordgren.org/seismo/gif_images.htm

At 1/50Hz we are measuring a force-noise density of somewhere around 160 nanograms/sqrt-Hz and are trying to discover what determines how low we can go. Spice modeling was suggesting that the coil might be able generate a force noise even when not connected, which didn't really seem right.

We're talking about 50 Ohms and a few Hy.

 

So from the pictures, I'm assuming the circuit is inside a conductive container, which is connected to earth ground?

If so, and you are still seeing the noise, I'd look to other parts of the unit first, semiconductors and resistors are much more likely to be noisy compared to a large inductor, while at the same time, the large inductor would pick up EM radiation much more easily if not shielded.

 

We are not actually troubleshooting, but trying to better understand all the potential noise sources in the instrument. Made a Spice model of the electronics and then started removing sections until all that was left was the coil by itself which still showed a small but significant amount of noise in the model, which was what prompted my original question.

One needs to think in terms of frequencies in the 1 Hz to 1 milliHz range, like minutes per cycle.

Electrostatic shielded, magnetic shielded, thermally insulated and enclosed in a pressure-tight case to resist air buoyancy variations from atmospheric pressure noise. Today we saw a small 1/60 Hz peak in the spectrum that went away when a nearby laptop was turned off. It apparently was doing something regularly, once per minute. Probably a magnetic effect.