| 摘要 |
PROBLEM TO BE SOLVED: To enhance the accuracy of frequency without deteriorating the accuracy of distance by enlarging the analytic frame as the unit time for indexing the reflection echo in fast Fourier transform(FFT). SOLUTION: A transmission signal from a transmitting section 2 is converted through a transmitter-receiver 1 and radiated, as a transmission sound wave 10, into a medium. A reflection echo 102 from the medium is converted through the transmitter-receiver 1 and delivered, as a receiving signal, to a receiving- amplifying section 4. The amplifier 4 amplifies the receiving signal and delivers the amplified signal to an analog processing section 5. A digital signal converted through an A/D converter 6 is fed to an FFT processing unit 7 for the purpose of FFT analysis. The unit time for indexing the reflection echo in FFT, i.e., the analytic frame T0 , is enlarged to T0×2<m> (m is a positive integer). In the enlarged time region T1 in the analytic frame, FFT is performed at zero or negligible signal level. Since the basic frequency is decreased by a factor of 2m, the accuracy of frequency is enhanced by a factor of 2<m> . |
