Paper thickness detecting apparatus having a resonator with a resonance point set by a capacitance detecting unit

A paper thickness detecting apparatus comprising:

• a detecting unit including first and second detecting electrodes arranged along a running path of paper each opposing a ground electrode;
• a signal oscillating circuit for generating an oscillation frequency signal said oscillating circuit including an oscillating circuit unit;
• a first sensor circuit including a first resonator with a first impedance for shifting a resonance point in response to a change in electrostatic capacitance detected by said first detecting electrode said first sensor circuit also having a first wave detecting circuit for envelope-detecting an output of said first resonator;
• a second sensor circuit including a second resonator with a second impedance for shifting a resonance point in response to a change in electrostatic capacitance detected by said second detecting electrode said second sensor circuit having a second wave detecting circuit for envelope-detecting an output of said second resonator;
• a high-impedance converting circuit connected between said oscillating circuit unit and said first and second sensor circuits, the impedance of said high-impedance converting circuit being greater than said impedance of said first and second resonators;
• connecting means for connecting said high-impedance converting circuit and said first and second sensor circuits;
• and
• an output circuit for outputting a signal indicating a difference between the detection signals from said first and second sensor circuits.