A sensor system uses positive closed-loop feedback to provide energy waves into a medium. A sensor comprises a transducer (604), a propagating structure (602), and a reflecting surface (606). A parameter is applied to the propagating structure that affects the medium. The sensor is coupled to a propagation tuned oscillator (416) that forms the positive closed-loop feedback path with the sensor. The propagation tuned oscillator (416) includes an edge-detect receiver (200) that generates a pulse upon sensing a wave front of an energy wave in propagating structure (602). The edge-detect receiver (100) is in the feedback path that continues emitting energy waves into the propagating structure (602). The edge-detect receiver (200) comprises a preamplifier (212), a differentiator (214), a digital pulse circuit (216), and a deblank circuit (218). The transit time, phase, or frequency is measured of the propagating energy waves and correlated to the parameter being measured.