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TK-5400

CIRCUIT DESCRIPTION

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.

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.

Fig. 8

Microphone amplifier

MSW/CTS

CODEC

DSP

MIC

MAIN

SUB

Q711

IC3