| 摘要 |
PURPOSE:To make unnecessary a virtual drawing prevention circuit, sweep circuit and temperature compensating circuit, by providing a frequency discriminator in which the carrier frequency is set in advance. CONSTITUTION:A frequency discriminating circuit 21 discriminates the frequency of carrier of an input signal S, that is, signals of frequencies other than this frequency only are passed through. Thus, when a phase locked loop circuit (PLL) is drawn in an artificial drawing frequency, the output signal of a voltage controlled oscillator (VCO)5 passes through the frequency discriminating circuit 21, resulting that the output voltage of a low pass filter 23 is increased. AFC is given to the VCO5 with this output voltage and the PLL circuit is outside the artificial drawing frequency. When the PLL circuit is drawn in the carrier drawing frequency, the potential of the output signal of the frequency discriminating circuit 21 is zero, thus the potential of the low pass filter 23 is also zero and no AFC is given to the VCO5. Thus, when drawing is made once to the carrier drawing frequency, the frequency is kept to this drawing frequency. |
