Linear oscillator for proximity sensor

In an oscillator having a parallel resonant circuit wherein the effective impedance and Q, and thus power loss at resonance, is a function of the distance of a part of the resonant circuit from a conductive body, the improvement comprising:

• a source of current and voltage components of electrical power, said power source including
• means for providing a reference current of a selected magnitude and
• a controlled current source having an input and an output for providing a constant current to the resonant circuit through said output proportional to the reference current when applied to said input, said constant current being substantially independent of changes of load impedance Q and the voltage amplitude across the resonant circuit varying with load impedance Q;
• and means for switching said power source to provide input power to the resonant circuit, said switching means including
• a switch connected between said reference current providing means and the input of the controlled current source, said switch having one state in which the reference current is applied to said input and a second state in which the reference current is not applied to said input, and
• control means responsive to the voltage across the resonant circuit for changing the state of said switch in accordance therewith, the voltage across said resonant circuit having an amplitude inversely proportional to the power loss of the resonant circuit and varying linearly with the distance of the resonant circuit part from a conductive body.

In an oscillator having a parallel resonant circuit wherein the effective impedance and Q and thus power loss at resonance is a function of the distance of a part of the resonant circuit from a conductive body, the improvement comprising:

• a source of constant current;
• a voltage controlled differential switch including two switch transistors connected in parallel and connected in series with said constant current source, said switch transistors being alternately conductive;
• means connecting one of said switch transistors to the resonant circuit of said oscillator to provide a constant current input to the resonant circuit;
• a common bias circuit for said two transistor switches;
• and
• a circuit connected between said resonant circuit and said differential switch to cause said one switch transistor to conduct periodically and provide constant current input to said resonant circuit.

In an oscillator having a parallel resonant circuit wherein the effective impedance and Q and thus power loss of resonance is a function of the distance of a part of the resonant circuit from the conductive body, the improvement comprising:

• a source of constant current including a transistor connected in series with a resistor across a DC power source;
• a control circuit for said constant current transistor including a series regulated current source connected with the base of the constant current transistor;
• and
• means for switching said constant current source to the resonant circuit to establish oscillations therein.