Small FM radio circuit

Shown is a small FM radio circuit. As can be seen from the figure, there are multiple resonant circuits . L connected to the antenna end and vc form
a resonant circuit. It resonates on a certain program carrier frequency of the tuned broadcast and is used to select the FM program of the radio station. The signal received by the antenna is frequency
selected by the resonant circuit and then sent to the base of the mixing transistor . At the same time, the local oscillator signal is also sent to the base of the mixing transistor.
There is also a resonant circuit L2 and vc in the base circuit of the local oscillator circuit transistor . It resonates at the local oscillator frequency, which is 10.7 MHz higher than the carrier frequency of the broadcast program. Mixing transistors
amplify the sum and difference frequencies of two signals. An LC resonant circuit (L3) is interleaved in the collector load circuit of the mixer amplifier transistor
. The resonant frequency of L3 is 10.7 MHz. In this way , the difference frequency (10.7 MHz) of the first two mixing signals is selectively amplified through the La resonant circuit
, and the RF signal is turned into an intermediate frequency signal and sent to the next stage intermediate frequency amplifier. After the intermediate frequency is amplified, the frequency discrimination integrated circuit extracts the audio signal from the intermediate frequency carrier
, and finally drives the speaker through the audio amplifier to restore the sound signal.