| 摘要 |
PURPOSE:To eliminate the possibility of false announcing occurring due to such as higher harmonic noises generated synchronously with an electric power source by holding an electric power source frequency at non-integer times with a frequency conversion circuit thereby obtaining a detection signal. CONSTITUTION:A frequency conversion circuit 1a has a multiplying circuit, generates the signal of the frequency non-integer times the electric power source frequency and supplies the same to an oscillator 1b. The oscillator 1b oscillates at the output frequency of the circuit 1a and applies the output thereof as a detection signal to an exciting line 2. In this way, the frequency of the detection signal is fixed at non-integer times of the power source frequency, whereby the detection signal is positioned approximately at the center of the frequency of the higher harmonic noise of the power source, and a beat is set so relatively high frequency as to be blocked with a BPF 6; therefore, the possibility of causing malfunction of an announcing circuit by the beat in accordance with the higher harmonic noises is eliminated. |
