| 摘要 |
PROBLEM TO BE SOLVED: To perform elimination other than the carrier waves of PPM modulation signals by providing the second detection circuit of a prescribed circuit constitution between the first detection circuit of BPF output signals and a shaping circuit and using the output of the same current mirror circuit for the control signals of a BPF and the second detection circuit. SOLUTION: A second detection circuit 22 is constituted of a frequency selection circuit 20 and a pulse detection circuit 21 and the output terminal 19.4 of the current mirror circuit 19 is turned into a constant power supply of the frequency selection circuit 20, and a current value is linked with a power supply I2 for stipulating the tuning frequency fo of the BPF 3. When signals are inputted from a first detection circuit 24, the frequency selection circuit 20 divides them into low frequencies and a high frequencies and detects only the carrier wave of the PPM modulation signals. At that time, a constant-current source is linked with the power supply I2 and the carrier wave is detected accurately. The pulse detection circuit 21 detects that the signals have been inputted as a stipulated pulse number, delays only stipulated pulses, and then outputs them. Thus, the optical noise of an inverter fluorescent lamp or the like and the noise of short pulses near the horizontal synchronization signals of a TV are eliminated as well. |
