Abstract: A circuit for measuring a parameter to be sensed by means of at least one variable impedance sensor responsive to such parameter. The circuit includes a frequency determining means coupled to the variable sensor for providing an oscillatory frequency signal responsive to the impedance of the sensor, a signal amplifier coupled to the frequency determining means for amplifying the frequency signal to provide an output signal representative of the frequency signal and to sustain circuit oscillation, a signal detector coupled to the output signal to provide a control signal representative of such output signal, and a control coupled to receive the control signal and coupled to the frequency determining means thus allowing the oscillatory frequency signal of the circuit to oscillate at a frequency representative of the measured parameter.

Abstract: The oscillator includes a high gain differential amplifier, the output of which alternately drives two sets of transistors which together provide the oscillator output signal. A first set of transistors operates during one-half cycle of the output while the other set is operating during the other half-cycle. A DC feedback path provides degenerative DC feedback to minimize the DC component of the output signal, while an AC feedback path, which includes a filter network which passes the circuit operating frequency at a 180.degree. phase shift relative to the phase of the output signal, provides regenerative AC feedback to maintain circuit oscillation.

Abstract: A low frequency modulating signal and a high frequency carrier signal circuit employing a twin-T oscillator having an integrated circuit operational amplifier for producing the modulating signal and an L-C resonant feedback oscillator having a transistor amplifier for producing the carrier signal, and a monitoring shunt regulator for supplying a reference potential to the twin-T oscillator and biasing and operating potentials to the L-C resonant feedback oscillator and for deactivating the L-C resonant feedback oscillator when an increase in resistance occurs in the shunt regulator.