| 摘要 |
PROBLEM TO BE SOLVED: To provide an automatic frequency control system which is capable of reducing a circuit markedly in scale without deteriorating a frequency resolution. SOLUTION: The automatic frequency control system of this embodiment is equipped with an antenna 10, a tuner circuit 100, an image SAW filter 14, an image intermediate frequency amplifier 20, an image detector 21, an image amplifier 22, an image PLL circuit 102, an AFT control circuit 36, a microcomputer 15, a memory 16, moreover a reference PLL circuit 103 for reducing a variation in the free run frequency of an image voltage-controlled oscillator 28, a comparator 35 that determine which is higher, the received image frequency or a normal image frequency, a mixer 37 which mixes output signals of an image voltage-controlled oscillator 28 with those of a reference voltage-controlled oscillating circuit 30 and extracts sum signals and difference signals of the frequency, and a mixer low-pass filter 38 which feeds only the difference signals of the frequency to a subsequent stage excluding the sum signals of the frequency. COPYRIGHT: (C)2006,JPO&NCIPI
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