# Air gap variation between metal target and planar Inductor coil changes the coil inductance and oscillating signal's frequency & amplitude?

I am working on planar coil inductive sensor.

I noticed if i change the air gap distance between the metal target and sensor coil. the Inductance of the coil changes, frequency and voltage amplitude change in the oscillating signal(LC Tank Circuit) changes too. Some time it exceeds the working range of IC.

When the target is near to sensing coil ( Less air gap) the frequency of oscillating signal is high and voltage amplitude is low.

When the target is moved away ( More air gap) the oscillating signal's frequency is reduced and voltage amplitude is increased.

Could anyone please explain the theroy behind this occurrence in detail ?
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