Apparatus for measuring the time between received pulses

For use with a system in which successive pulses, each composed of a train of high frequency oscillations, are emitted by a transmitting transducer, propagated through a transmission medium, and received by a receiving transducer having a given transient behavior, a starting transient period followed by a steady-state response, and a given response band width, apparatus for measuring the time interval between reception of successive pulses by the receiving transducer and for preventing erroneous measurements resulting from propagation of the pulses over indirect paths through the transmission medium, said apparatus comprising:

• threshold value detecting means connected to said receiving transducer for receiving the output signal therefrom representative of each pulse received by said receiving transducer, said detecting means having an associated threshold value which is a function of the peak value of the output signal produced by said receiving transducer during steady-state response to a pulse, and said detecting means producing a signal when the output signal from said receiving transducer exceeds said threshold value;
• rise filter means including a comparator connected to receive the output signal from said receiving transducer and to produce a trigger signal when the output signal from said receiving transducer exceeds a comparision value which is substantially less than the peak value of the output signal produced by said receiving transducer during steady-state response to a pulse, said rise filter means further including a timer member connected to receive trigger signals from said comparator for producing, in response to each such trigger signal, a rise pulse having a period at least equal to the reciprocal of the band width of said receiving transducer and not greater than the starting transient period of said receiving transducer;
• signal generator means connected to said threshold value detecting means and to said rise filter means for producing a signal indicating good reception of a pulse each time said detecting means produces a signal in time coincidence with a rise pulse;
• and
• measuring means connected to receive the signals indicating good reception from said signal generator means and responsive thereto for providing an indication of the time interval between reception of successive pulses by said receiving transducer.