AM radio pre-circuit using varactor diode

Shown is an AM radio pre-circuit using a varactor diode . The function of this circuit is to amplify the AM radio signal and convert the amplified
signal into an intermediate frequency signal for output. The AM radio circuit is composed of high-frequency amplifier, mixing and local oscillator circuits. A varactor is used in the tuning circuit , so electrical tuning can be used.     The AM high-frequency broadcast signal received by the antenna is sent to the gate of the high-frequency amplification transistor VTJ for amplification. There is a frequency selective tuning circuit in the drain circuit of VTI (Ti primary winding and Di resonance). The amplified signal of VT1 is coupled to the base of VT4 through T]. The signal of the local oscillator VT3 is also coupled to the base of VT4 through the 22 pF capacitor. Base, the two signals are mixed in VT4. The output of VT4 is frequency selected and output (465 kHz) through the intermediate frequency transformer Tz. Varactor diodes Di, D2, and D3 are integrated varactor diodes (KV1210).