Method and apparatus for eliminating substantially false echoes produced by electrical noise induced into high gain receiver circuits

An ultrasonic ranging system comprising:

• an ultrasonic transducer;
• transmitter circuitry coupled to said transducer for causing the transducer to generate tone pulses of frequencies within a predetermined bandwidth and at a predetermined pulse repetition rate to a target area, said transducer for receiving echo tone pulses from said target area at times within interpulse periods of said predetermined pulse repetition rate and converting said received echo tone pulses into representative electrical echo pulses;
• receiver circuitry coupled to said transducer for receiving said electrical echo pulses from the transducer, said receiver circuitry susceptible to electrical noise within the frequency bandwidth of the electrical echo pulses to create false echo pulses;
• and
• a filter for eliminating substantially said false echo pulses by accepting only electrical echo pulses which occur synchronously among interpulse periods, wherein the accepted echo pulses are used for range measurements of said target area.