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5.
Audio amplifier circuit
The demodulated signal from U501 is sent to the audio amplifier via U102.The signal is output by AF
volume control and amplified by audio power amplifier (U204) to loudspeaker
6.
Squelch
A part of the AF signal output by the U601 enters the IC, and the noise component is amplified and
modified by the filter and amplifier to generate the DC voltage corresponding to the noise level.The DC
signal is sent to the analog port (U301) of the microprocessor via the U261. U301 determines whether to
output sound from the speaker by detecting whether the input voltage is higher or lower than the preset
value.When the sound is output from the speaker, U301 sends a high level signal to the SP MUTE line and
turns on the U561.（as picture 4）
as picture 3 Wide / narrow switching circuit
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