| 摘要 |
PURPOSE:To improve the S/N of a received echo signal from long distance by narrowing down the reception band width of a reception system, and lowering a passing center frequency with time, and also reducing, gradually the high frequency component of a signal to be received with time. CONSTITUTION:A variable band-pass filter 26 is inserted into the output side of a sampling circuit 21, and controlled by a controller 11 to narrow down the frequency band from the head of a frequency-lowered receive frame, i.e. output frame of the sampling circuit 21 with time and also lower its center frequency. The loss characteristics in the propagation of a radio wave in the ground is influenced by the frequency and the attenuation amount of, specially, high frequency is large, but the passing band width of the reception system narrows down and also goes to lower in this case as the received high frequency component is reduced, so the noise component in the high frequency range is cut off so as to perform reception with a high S/N. |
