A sensor system uses positive closed-loop feedback to provide energy waves into a medium. It comprises a transducer (604), a propagating structure (602), and a transducer (606). A parameter is applied to the propagating structure that affects the medium. A sensor is coupled to a propagation tuned oscillator (416) that forms a positive closed-loop feedback path. The propagation tuned oscillator (416) includes a zero-crossing receiver (200) that generates a pulse upon sensing a transition of an energy wave from the propagating structure (602). The zero-crossing receiver (200) is in the feedback path that maintains the emission of energy waves into the propagating structure (602). The zero-crossing receiver (200) comprises a preamplifier (206), a filter (208), an offset adjustment circuit (210), a comparator (212) and a pulse circuit (218). The transit time, phase, or frequency is measured of the propagating energy waves and correlated to the parameter being measured.
