a flowmeter system having circuitry defining a path for confining the flow of a fluid medium therethrough , first and second transducers disposed along said flow path for generating and receiving acoustic compression waves in the fluid medium between the transducers , a phase lock loop receiver / transmitter system including a voltage controlled oscillator for adjusting the frequency of the acoustic compression waves to maintain the compression wave length constant , a phase detector for measuring the phase difference of the received acoustic compression waves relative to that transmitted and for producing a sum signal proportional to the sum of the measured phase differences to vary the output of said voltage controlled oscillator , circuitry for producing a difference signal proportional to the difference of the measured phase differences representing the direction and magnitude of the flow of the fluid medium as well as changes in its composition and an active filter in the loop for increasing the loop gain of thesystem .