The CTCSS/CDCSS signal generated by CPU is amplified and adjusted, and then enters the voltage controlled oscillator for modulation. A part of low-frequency signals obtained after receiving frequency discrimination are amplified, filtered and shaped by a sub-audio bandpass filter.

Enter the CPU, compare with the preset value, and control the output of audio power amplifier and speaker with the result. That is, if it is the same as the preset value, turn on the speaker; If not, please turn off the speakers. ?

Receiving part:

The receiving part mixes an amplified signal from a radio frequency with a first local oscillator signal from a PLL frequency synthesizer circuit at a first mixer, and generates a first intermediate frequency signal.

The first intermediate frequency signal further eliminates the clutter signals of adjacent channels through the crystal filter. The filtered first intermediate frequency signal enters the intermediate frequency processing chip and is mixed with the second local oscillator signal again to generate the second intermediate frequency signal.

The second IF signal is filtered by a ceramic filter and then amplified and frequency-discriminated to generate an audio signal. After amplification, band-pass filtering, de-emphasis and other circuits, the audio signal enters the volume control circuit and power amplifier for amplification, driving the speaker and getting the information people need.

Refer to the above? Baidu encyclopedia-interphone