1-8 (No.RA035<Rev.001>)
2.3.2.4
Audio Amplifier Circuit
The demodulated signal from IC400 is sent to an AF amplifier through IC705, and is routed to an audio power amplifier (INT:IC709,
EXT:IC707) where it is amplified and output to the speaker.
Fig.3 Audio amplifier circuit
2.3.2.5
Squelch Circuit
Part of the AF signal from the IC400 enters the FM IC (IC400) 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.
There are 2 noise filters. Analog Narrow band and DMR band will share the same noise filter (W_N control is Low) whereas analog
wide 5k band and analog wide 4k band will use another noise filter (W_N control is HIGH).
The selection of different noise filter is by 2 multiplexer (IC719 and IC720).
The DC signal from the FM IC goes to the analog port of the MCU (IC710). IC710 determines whether or not to output sounds from
the speaker by checking if the input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC710 sends a high signal to the EXTSPSW line and turns IC707 on through Q709, Q710, Q713
and Q716. This explanation is for the external speaker amplifier. For the internal loudspeaker amplifier, IC710 sends a high signal to
the INTSPSW line and turns IC709 on through Q711 and Q714.
2.3.3 Transmitter System
2.3.3.1
Microphone Amplifier Circuit
The signal from the microphone is limited by the AGC circuit, which is composed of D700, D701, Q705, Q706 and the mute switch
(Q704). IC705 is composed of a high-pass filter, low-pass filter and pre-emphasis/IDC circuit.
The signal from the microphone and the low speed data from the MCU (IC710) enter the baseband IC (IC705) and pass through each
path and are mixed inside the IC.
The output signal from the audio processor MOD2 goes to the VCO modulation input. The other output signal from MOD1 goes to the
TCXO modulation input.
Fig.4 Microphone amplifier circuit
2.3.3.2
Drive and Final Amplifier Circuit
The signal from the T/R switch (D15 is on) is amplified by the pre-drive amplifier (Q201).
The output of the pre-drive amplifier is amplified by the drive amplifier (Q206) and the RF final amplifier (Q208) to 5.0W (1W when the
power is low).
The drive amplifier and the RF final amplifier consist of two MOS FET stages.
The output of the RF final amplifier is then passed through the antenna switch (D203 and D204) and harmonic filter (LPF) and is ap-
plied to the antenna terminal.
Baseband
IC
IC705
AF PA
(INT)
IC709
INT
SP
FM IC
IC400
AF PA
(EXT)
IC707
EXT
SP
SPO
MIC
AGC
SW
D700,701
Q705,706
MICM
X1
TCXO
VCO
LSD
OUT
IC705
Q704
MIC
Baseband
MOD1
MOD2
Summary of Contents for TK-D240
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