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 theory behind this occurrence in detail?

thanks in advance

  • 1
    \$\begingroup\$ I think we need more information... A circuit diagram would be helpful. The IC part number and a link to a datasheet. Having said that, what is the oscillating frequency with and without the target? \$\endgroup\$
    – BobT
    May 7, 2020 at 1:09
  • \$\begingroup\$ I use LX3302A IC. Oscillation frequency with metal target 5.53 MHz, without metal target 5.28 MHz. Oscillating signal amplitude with metal target 6.65V without metal target 11.2 V. we supply voltage (5V) from IC to tank circuit and maintain the oscillations from damping due to the parasitic resistance . microsemi.com/product-directory/sensor-interface/… \$\endgroup\$
    – umh
    May 7, 2020 at 1:23

1 Answer 1


The inductance is reduced by proximity to conductive metal. Effectively the metal acts as a shorted turn.

The Q is also reduced because the inductance goes down but the series resistance of the coil remains the same.

For a given voltage the current will go up because of the shorted turn effect. So it depends on the oscillator circuit and whether there is some form of AGC (automatic gain control) and what the limits are on output drive power/current.


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