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6. Transmitter System
6-1. Microphone Amplifier
The signal from the noise canceller amplifier (control unit
IC3) passes through the MIC changeover circuit (Q711), the
mute switch (Q713) and the AGC circuit, and goes to the
microphone amplifier IC (IC602).
When an accessory speaker microphone (SP-MIC) is not
installed, the microphone switching terminal (MSW/CTS)
goes high and the microphone changeover switch (Q711)
turns on. When the SP-MIC is installed, the MSW/CTS is
connected to GND in the SP-MIC, Q711 turns off, the inter-
nal microphone is muted and only the external microphone
input is supplied to the microphone amplifier of the TX-RX
unit.
The AGC circuit consists of IC602 (1/2), D600, D601,
Q600 and Q601. The AGC is operated by using the current
obtained by detecting positive or negative polarity of the au-
dio signal amplified by IC602 (1/2) and controlling the posi-
tive (+) and negative (–) level of the amplifier.
The transmit audio signal output from IC602 (2/2) is input
to pin 3 (AINL) of the codec IC (IC608) and converted from
analog to digital. The digitalized transmit audio signal under-
goes AGC processing, pre-emphasizing, filtering, vocoding
(in APCO mode), and returns to the codec IC (IC608). The
signal is converted from digital to analog and an analog sig-
nal (C4FM base band signal in APCO mode) is output from
pin 16 (AOUTR).
The audio signal that is DSP-processed by the codec IC
(IC608) and DSP (IC611) passes through the analog switch
(IC706) and amplifier (IC606, IC604) and goes to the D/A
converter (IC605). The audio signal whose maximum devia-
tion is adjusted by the D/A converter passes through the AF
switch (Q202 is off in TX mode) and goes to VCO modula-
tion input. The audio signal whose modulation balance is
adjusted by the D/A converter passes through a buffer am-
plifier (IC607) and goes to VCXO modulation input.
EXT.
MIC
(IC703)
MUTE
IC3
Q711
MAIN
SW
SW
MIC
N/C
MIC
AMP
Q710
(CONT)
SUB
MSW/CTS
IC706
IC606
IC604
SW
AMP
AMP
Fig. 8 Microphone amplifier
CIRCUIT DESCRIPTION
IC611
D600
D601
DSP
DET
ATT
AMP
CODEC
Q600
IC602
IC608
Q601
IC605
Q202
L702
SW
VCO
D/A
5R
BUFF
VCXO
AMP
IC607
X201
6-2. Noise Cancelling Microphone Circuit
The two signals from INT MIC (Main & Sub) are fed to the
positive (+) input (Sub) and to the negative (–) input (Main) of
the IC3. If the same signals is fed to both Main and Sub, the
Main signal is terminated at the output of IC3 (pin7). In an-
other words, noise from nearby sources not directly con-
nected to the transceiver enters the Main and Sub input at
the same signals and is therefore canceled out.
When a signal is only fed to the Main and there is no
signal at the Sub, IC3 (pin 7) outputs the Main signal as it is.
In other words, only the voice audio of the operator to the
Main MIC is input to the Main so that "N/C" switch is set to
"L", transistor Q14 is turned off and the Sub microphone
also is turned off and the operation is same as above.
6-3. Drive and Final Amplifier
The signal from the T/R switch (D101 is active) is ampli-
fied by the pre-drive (Q101) and drive amplifier (Q103) to
50mW. The drive amplifier output is amplified by the RF
power amplifier (Q1, Q2, Q3) to 3W (1W when the power is
low).
The RF power amplifier has two-stage MOS FET transis-
tor. The output of the RF power amplifier is then passed
through the Transmit-Receive (TX-RX) antenna switching
(D103 is active) and low-pass filter (LPF) and applied to the
antenna terminal.
6-4. APC Circuit
The APC circuit always monitors the current flow through
the RF power amplifier (Q1, Q2 or Q3) and maintains a con-
stant current. The voltage drop at R101, R102 and R103 is
caused by the current flow through the RF power amplifier
and this voltage is applied to the differential amplifier (IC101
1/2).
IC101 (2/2) compares the output voltage of IC101 (1/2)
with the reference voltage from IC605. The output of IC101
(2/2) controls the voltages the VGG of the RF power ampli-
fier to make the both voltages to same voltage.
The power high/low switching is carried out by changing
the reference voltage. The Q102, Q104 and Q105 are
turned on in transmit and the APC circuit is active.
Q101
Q103
From
Pre
DRIVE
T/R SW
DRIVE
AMP
(D101)
AMP
VDD
R101
+B
R102
R103
–
+
Q104
REF
VOL
SW
(IC605)
Fig. 9 Drive and final amplifier and APC circuit
TK-5400
ANT
Q1,2,3
D103
RF
TX-RX
POWER
ANT
LPF
AMP
SW
Q105
VGG
SW
Q102
–
SW
+
IC101
IC101
(1/2)
(2/2)
11
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