| 摘要 |
PURPOSE:To attain phase locked loop at the sum and difference frequency of the 1st and 2nd reference signals by interrupting an output signal after frequency conversion for a prescribed time only at the locking of phase locked loop so as to switch the polarity. CONSTITUTION:When it is possible that the frequency of a voltage controlled oscillator is at the side of image frequency fc at power application and fre quency change, a control circuit 8 detects the unlocking of phase locking so as to interrupt the 1st switch 7 over a prescribed time. In restoring the 1st switch 7, the phase locked loop starts locking to lock the frequency to an actual frequency fa. In phase locking to the image frequency fc, after the control circuit 8 throws the 2nd switch 9 to the position of a non-inverting input of an amplifier 6, the phase locking is attained similarly by operating the 1st switch 7.
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