RED PT500-2, Руководство по обслуживанию

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PT500-2 Service Manual
20
Squelch Circuit
Part of the AF signal from the IC200 enters the FM IC again,and the noise component is
amplified and rectified by a filter and an amplifier to produce a DC voltage corresponding to
the noise level.
The DC signal from the FM IC goes to the analog port of the microprocessor(IC403).IC403
determines whether to output sounds from the speaker by checking whether the input
voltage is higher or lower than the preset value.
To output sounds from the speaker,IC403 sends a high signal to the MUTE and AFCO lines
and turns IC302 on through Q302 Q304 and Q305.
Audio Power Amplification
The audio power amplifying circuit consists of IC302 and the peripheral components.
The signals are amplified by audio power amplifier to drive the speaker after collecting the
receiving audio signals, voice signals and warning tone signals. The warning tone has no
volume limitation.
When AFCO is high level, Q304 is on, IC302 begins to work and the speaker sounds.
Speaker Impedance: 16ohm
CTCSS Signal filtering
The audio signals after demodulation in IC200 may contain CTCSS (continuous tone control
squelch system) or DCS
（
digital squelch
）
signals. The spectrum component of
CTCSS/DCS is 67 to 250Hz. The filtering circuit composed of IC301 can filter out signals
except CTCSS/DCS spectrum, which makes MCU decode the CTCSS/DCS more
accurately.
5.3 Transmitter (Tx)
Transmitter Power Amplifier