The receiving section employs a dual-conversion superheterodyne architecture. Signals input from the antenna pass through a transmit-receive switching circuit and a band-pass filter before undergoing RF amplification. After passing through a second band-pass filter, the amplified RF signal enters the first mixer, where it is mixed with the first local oscillator signal-generated by a Phase-Locked Loop (PLL) frequency synthesizer circuit-to produce a first Intermediate Frequency (IF) signal. This first IF signal then passes through a crystal filter to further suppress adjacent-channel interference and spurious signals. The filtered first IF signal enters an IF processing chip, where it is mixed again with a second local oscillator signal to generate a second IF signal. After passing through a ceramic filter to remove unwanted spurious components, this second IF signal is amplified and demodulated to produce an audio signal. The audio signal then proceeds through circuits for amplification, band-pass filtering, and de-emphasis before entering the volume control circuit and power amplifier; the amplified signal subsequently drives the loudspeaker, enabling the user to receive the desired information.
Regarding signal processing: the user's voice is converted into an electrical audio signal by a microphone. This audio signal passes through an amplifier, a pre-emphasis circuit, and a band-pass filter before directly modulating a Voltage-Controlled Oscillator (VCO).
The CPU generates CTCSS/DTCSS signals which, after amplification and level adjustment, enter the VCO to modulate the carrier wave. A portion of the low-frequency signal obtained after demodulation in the receiver is amplified, filtered, and shaped by a sub-audio band-pass filter before being fed into the CPU. The CPU compares this signal against a preset value to control the output of the audio power amplifier and loudspeaker; specifically, if the received signal matches the preset value, the loudspeaker is enabled; otherwise, the loudspeaker remains muted.
