Parallel T impedance measurement circuit for use with variable impedance sensor

A circuit for measuring the capacitance of a variable capacitor which is responsive to an external stimulus, the circuit comprising:

• a parallel T network including the variable capacitor, the network having an input, an output and a characteristic impedance, for providing a frequency signal at the output which has a frequency and amplitude representative of the external stimulus;
• signal amplification means coupled to output of the parallel T network for providing to the input of the parallel T network an alternating current output signal representative of the frequency signal;
• detection means coupled to the amplification means for rectifying the alternating current output signal to produce a direct current control signal which is a function of the amplitude of the output signal;
• and
• control means coupled to the parallel T network for changing the characteristic impedance of the parallel T network responsive to the control signal to enable the frequency signal to oscillate at a frequency representative of the external stimulus.

A circuit for providing an output which is a function of first and second parameters, the circuit comprising

• a first circuit for producing a first parameter output signal having a frequency representative of the first parameter, the first circuit comprising:
• first variable impedance sensor means for sensing the first parameter, the first variable impedance sensor means having a first variable impedance which is a function of the first parameter;
• first frequency determining means, coupled to the first variable impedance sensor means to form a first parallel T circuit having an input and output, for providing at the output a first oscillatory frequency signal having a frequency which is a function of the first variable impedance;
• first signal amplification means for amplifying the first oscillatory frequency signal to produce the first parameter output signal, the first signal amplification means having an input coupled to the output of the first parallel T circuit for receiving the first oscillatory frequency signal and having an output for providing the first parameter output signal, the output of the first signal amplification means being coupled to the input of the first parallel T circuit to sustain oscillation of the first parallel T circuit;
• first detection means coupled to the output of the first signal amplification means for providing a direct current first control signal which is a function of amplitude of the first parameter output signal;
• and
• first control means responsive to the first control signal and coupled to the first frequency determining means for enabling the first oscillatory frequency signal to oscillate at a frequency representative of the first variable impedance;
• a second circuit for producing a second parameter output signal having a frequency representative of the second parameter, the second circuit comprising:
• second variable impedance sensor means for sensing the second parameter, the second variable impedance sensor means having a second variable impedance which is a function of the second parameter;
• second frequency determining means, coupled to the second variable impedance sensor means to form a second parallel T circuit having an input and output, for providing at the output a second oscillatory frequency signal having a frequency which is a function of the second variable impedance;
• second signal amplification means for amplifying the second oscillatory frequency signal to produce the second parameter output signal, the second signal amplification means having an input coupled to the output of the second parallel T circuit for receiving the oscillatory frequency signal and having an output for providing the second parameter output signal, the ouput of the second signal amplification means being coupled to the input of the second parallel T circuit to sustain oscillation of the second parallel T circuit;
• second detection means coupled to the output of the second signal amplification means for providing a direct current second control signal which is a function of amplitude of the second parameter output signal;
• and
• second control means responsive to the second control signal and coupled to the second frequency determining means for enabling the second oscillatory frequency signal to oscillate at a frequency representative of the second variable impedance;
• and
• processor means responsive to the first and second parameter output signals for providing the output which is a function of the first and second parameters.

A circuit for providing an output signal having a frequency which is representative of a sensed parameter, the circuit comprising:

• sensing means for sensing the parameter, the sensing means exhibiting a variable impedance which is a function of the parameter;
• controllable impedance means for providing a controllable impedance which is a function of a direct current control signal;
• frequency determining means, connected to the sensing means and the controllable impedance means to form a parallel T circuit, for receiving the output signal and providing an oscillatory frequency signal which has a frequency which is a function of the variable impedance and the controllable impedance;
• signal amplification means for amplifying the oscillatory frequency signal to produce the output signal;
• and
• detection means for providing the direct current control signal to the controllable impedance means as a function of amplitude of the output signal.