FM tuner with band extraction and optimization of a PLL FM demodulator

摘要

An FM tuner including a PLLFM demodulation circuit is provided, which selectively employs one of two BPFs having different extraction bandwidths as an IFBPF. This FM tuner is configured such that by employing a relationship between a lock range and an input signal level of the PLLFM demodulation circuit, an optimum value of the lock range of the demodulation circuit is set for a bandwidth of an input signal, even if any of the BPFs is selected. When the BPF having a narrower bandwidth is selected as the IFBPF, a signal to be inputted to the PLLFM demodulation circuit is attenuated to decrease the level of the input signal of the demodulation circuit to be lower that the level when the other BPF with a larger bandwidth is selected as the IFBPF. As a result, a lock range width of the PLLFM demodulation circuit is reduced.