| 摘要 |
PURPOSE:To permit an optional band signal of the sharp frequency characteristics to pass through a filter, by permitting a received signal to pass through the filter only when one wavelength of this received signal is between upper limit and lower limit frequencies. CONSTITUTION:A received ultrasonic signal is converted to an electric signal by a wave receiver 1 and is amplified by a receiver 2 and is applied to an A/D converter 3 and a switch 6. Next, set values for upper limit and lower limit pulse widths from a pulse width setting device 5 are compared with the signal from the A/D converter 3 by a digital pulse width comparator 4. When upper limit and lower limit frequencies of a required band of the receiver are denoted as fH and fL respectively, their pulse widths tH and tL become 1/fH and 1/fL respectively; and when the frequency of the received signal is denoted as (f), its pulse width (t) becomes 1/f. If a signal is sent from the comparator 4 for tH<= t<=tL on a basis of these relations to turn on the switch 6, the output signal of the receiver 2 is carried to the next stage only when fL<=f<=fH is satisfied. |
