FM stereo tuning circuit

The tuning circuit is used to tune and select the radio signal to be received from the signal sent by the antenna. The signal input by the antenna is first amplified in the tuner at
high frequency, and then mixed with the local oscillation signal to output an intermediate frequency signal .
    The figure shows a typical FM stereo tuning circuit.
    The signal received by the antenna in the circuit is input from pin ①, and is added to the signal grid of the high-frequency amplified double-gate field effect transistor through the coil and coupling capacitor.
The tuning control voltage is input from pin ⑤. The tuning voltage controls the reverse bias of the varactor diode VDI. By changing the capacitance,
the frequency of the high-frequency amplification stage can be tuned, and the signal of the radio station to be received is selected and sent to the mixing circuit. The mixing circuit also receives the local oscillation constant amplitude wave sent by the local oscillation circuit which is 10.7 MHz
higher Its oscillation frequency is controlled by the reverse bias of the tuning voltage control varactor diode VD2 of. The mixing
circuit outputs the difference frequency between the local oscillation signal and the high-frequency signal, that is, the FM intermediate frequency signal. Since the tuning circuit is at the forefront, in order to ensure reception quality, the
output intermediate frequency signal is required to be stable. Measures are taken in the circuit to make the high-frequency amplifier have automatic gain control AGC function. The high-frequency amplified AOC
controls the voltage and sends it to the control electrode of the double-gate field effect transistor to control its voltage gain. When the received signal is weak, the AGC voltage increases the gain, and vice versa
.