FM modulator strong-weak signal switching circuit a

Shown is a strong and weak signal switching circuit for an FM modulator . Circle 3-14 (a) in this circuit shows a mechanical switch and a manual selection switch, which is selected according to the signal strength. Figure 3-14 (b) shows the use of diodes for switching control. Figure 3-14 (c) shows the use of the signal level received by the intermediate frequency amplifier circuit to control the diode. VT3 controls the bias voltage of the diode . When forward biased, the antenna signal is directly sent to the resonant circuit. When reverse biased, it is induced into the resonant circuit through the inductor. There is an attenuation difference of about 12 to 15 dB between the two. This circuit can generally be used as an attenuation switch for antenna inputs that receive strong signals (local broadcasts) and weak signals (foreign signals).