| 摘要 |
PURPOSE:To improve the frequency holding range of an AFC circuit by adding a simple circuit to the AFC circuit. CONSTITUTION:Demodulation signals and a DC voltage proportional to detuning are outputted from FM detector 4. These signals are caused to pass through a LPF consisting of resistance 10 and capacities 8 and 9, and only the DC voltage is taken out to feed back negatively the AFC voltage, which is applied to the varactor diode of local oscillator 5, to amplifier 1. Meanwhile, by applying the electronic tuning voltage to the base of transistor 12 from frequency control circuit 6, the collector resistance of transistor 12 is changed to control the feedback quantity of AFC, and the feedback quantity is reduced when the tuning frequency becomes high, thereby making the holding range narrow. When transistor 12 is operated in a sufficient linear region, the holding range becomes fixed. |
