I am building an instrument intended to pick up weak (~100 picotesla) 25 kHz oscillating magnetic fields with Faraday induction.
As shown in the photo, I have a 4-layer solenoid coil for pickup followed by a two-stage preamplifier that amplifies ~1000x for frequencies above 10 kHz. The pickup coil is not tuned and the circuit looks like below:
Signal/noise after Fourier transform of the time-domain signal, 1500 nT field at 26 kHz. 500 ms acquisition time.
Presently the setup can detect oscillating fields down to 2 nanotesla. There is a significant change in noise floor when I connect/disconnect the pickup solenoid coil, which suggests sensitivity can be improved. A possible solution is tune the inductor to the frequency of interest. My question is what is the best way to approach this, and general comments about how to improve the sensitivity are also welcome.
EDIT Suggestion by Henry Crun: tune an LC circuit to 25 kHz and use as the pickup coil. I'm limited to an air-core inductor for pickup. So I attempted this with the following circuit
And this was the result: a pickup coil tuned at 24 kHz. So, the question now is -- what is the best way to increase the Q factor of the coil, which would increase the sensitivity. I have a detection limit of ~100 pT/Hz^1/2 and want to improve on that by another factor of 10 to 100.