| 摘要 |
PURPOSE:To prevent generation of a cross spurious beat by shifting an oscillating frequency of a common oscillator within a margin frequency of an intermediate band pass filter of a pre-stage mixer and a post-stage mixer 4. CONSTITUTION:In shifting a frequency of a local oscillator 7 of a mixer converting a frequency fL1 of a VCO61 in a PLL circuit 61 into a frequency fM, the frequency is changed by the same amount of the sifted frequency and the VCO is locked. Thus, when the oscillating frequency of the common oscillator 7 is shifted so as to skip the spurious generating frequency by changing a control voltage of a varactor diode 72 a little before the frequency fL1 causing the cross spurious frequency at the inside of the PLL circuit 6, no spurious frequency is included in the frequency fL1. Further, when the frequency fL1 is changed, an output frequency of a mixer 2 is changed naturally, the component of change is cancelled because a local frequency fL2 of the post-stage mixer 4 is changed in the same direction, no effect is given to the output frequency of the mixer 4. |
