| 摘要 |
PURPOSE:To remove noise except a desirable tone signal by equipping a radio tone signal detection circuit with a low-pass filter whose passing band can be varied. CONSTITUTION:A carrier modulated by a desirable tone signal is received by antenna 10 and then demodulated by receiver 11. Its output is supplied via limiter 12 to N-path filter 13, thereby allowing a frequency 1/N as high as clock freqnency 26 and frequencies obtained by multiplying it by an integer to pass through. Low- pass filter 14 converts a staircase wave into a wave close to a sine wave, and low- pass filter 15 varies in its passing band by control signal 27 from tone assignment circuit 20. The output of filter 15 is supplied via wavefrom-shaping circuit 16 to detector 17, where when an input tone signal is identified as desirable one, the signal is supplied to both control circuit 18 and N-path filter 13 to change its band width, thereby quickening its fall time. |
