Kenwood NX-3400 Service Manual page 13

2.3.4 Transmitter System
2.3.4.1 Audio Band Circuit
The signal from microphone is amplified and converted to digital signal by IC601, IC601 includes AGC function. Digital signal is
transferred to MPU/DSP (IC401).
2.3.4.2 Baseband Circuit
The audio signal transferred from CODEC (IC601) is processed at MPU/DSP (IC401). Voice signals of 300Hz or lower and frequencies
of 3kHz or higher are cut off and an audio range 300Hz to 3kHz is extracted. The audio signal is then pre-emphasized in FM mode
and synthesized with the signals, such as QT and DQT, as required, and is transferred to PLL Frequency Synthesizer block in
Transceiver IC (IC303). The DTMF and MSK baseband signals are also generated by MPU/DSP (IC401).
In Digital mode, the audio signal is converted to the 4-Level FSK baseband signal and is transferred to PLL Frequency Synthesizer
block in Transceiver IC (IC303).
Transceiver IC
IC303
2.3.4.3 Drive and Final Amplifier
The signal from the Transceiver IC (IC303) is amplified by pre-drive amplifier (IC202). The output of the pre-drive amplifier is amplified
by the drive amplifier (Q201) and final amplifier (Q202) to 3W (1W when the power is low). IC202 is MMIC. Q201 and Q202 are MOS
FET. The output of the final amplifier is then passed through the harmonic filter (LPF) and antenna switch (D201 and D207, D202 and
D203) and applied to the antenna terminal.
2.3.4.4 APC Circuit
The APC circuit always monitors the current flowing through the drive amplifier (Q201) and final amplifier (Q202). And APC keeps
current constant.
The voltage drop at R248 and R250 is caused by the current flowing through the RF power amplifier and this voltage is applied to APC
circuit. Output voltage from APC controls the VGG of Q201 and Q202 to keep the current constant. The change of power high/low is
carried out by the change of the reference voltage.
IC202
TX
Pre-Drive
2.3.5 PLL Frequency Synthesizer
2.3.5.1 TCXO (X401)
TCXO (X401) generates a reference frequency of 19.2MHz for the PLL frequency synthesizer. This reference signal is buffered by
Q401. And it is distributed to PLL (incorporated in Transceiver IC (IC303)), MPU/DSP (IC401) and IC351.
The oscillation frequency is adjusted TCXO directly by controlled the D/A converter of FCNT.
Serial
I/F
DSP
MPU/DSP
IC401
Fig.4 Audio and Baseband circuit
Q201
Drive
Fig.5 Drive and Final amplifier and APC circuit
I2S
AD
I2C
CODEC
IC601
Q202
Final
+B
Internal MIC
External MIC
D201, D202,
D203, D207
ANT SW
APC
IC203
APC reference voltage
(No.RA070<Rev.001>)1-13