| 摘要 |
To determine the exact instant (Ts) between the instant of transmission (T0) of the leading-edge of a square-wave pulse (S1) transmitted by a transmitter (4) and the instant (T1) of the first appearance of the received signal (S2), the latter is scanned with a threshold voltage (V) specifiable in steps, so that with n-time pulse transmission for repetition of the transmitting and receiving operation the threshold voltage is decreased by a value of DELTA V in each transmitting and receiving operation. The instantaneous value of the threshold voltage (V) is compared with the peak values (VA, VB, VC, VD) of the amplitudes of the positive half-waves of the received signal (S2). The instant at which the value of the threshold voltage (V) falls below the peak value (VA, VB, VC, VD) of the positive half-wave of the received signal (S2) which follows each scanned half-wave is determined and the measurement of the propagation time delay for the positive half-wave is stopped at the following zero-crossover point (T A to TE) of the received signal so that a group of threshold-voltage values (A, B, C, D) corresponds to each half wave of the received signal. From the maximum values of each group, the envelope (7) of the group of waves increasing in amplitude of the received signal (S2) is formed and the point of intersection of the envelope with the baseline 0 of the group of waves of the received signal (S2) is defined as the instant (T1) of the first appearance of the received signal (S2) in the receiver (5).
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