Transmitter System; Audio Amplifier Circuit; Baseband Circuit; Apc Circuit - Kenwood NX-5400 Service Manual

2.4.3.3

Audio Amplifier Circuit

Audio processing (high-pass filter, low-pass filter, de-emphasized and so on) at FM mode and decoding at P25 mode are processed
by DSP. SSI signal from DSP is converted to audio signal at IC902. The signal goes to amplifier (IC904 or IC905).
While INTAMT is High, IC904 is activated and audio is heard from internal speaker.
While EXTAMT is High, IC905 is activated and audio is heard from external speaker.
ANT D350
D370
D371
ANT
LPF
SW
DSP
IC702

2.4.4 Transmitter System

2.4.4.1 Audio Band Circuit
The signal from microphone is amplified and converted to digital signal by IC902. IC902 includes AGC function.
Digital signal is transferred to IC702 through SSI.
2.4.4.2

Baseband Circuit

The audio signal transferred from IC902 is processed at IC702. 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. The DTMF and MSK baseband
signals are also generated by IC702.
In Digital mode, the audio signal is converted to the 4-Level FSK baseband signal and is transferred to PLL Frequency Synthesizer
block.
The output level according to the transmit carrier is fine-adjusted according to each modulation method.
2.4.4.3 Drive and Final Amplifier
The signal from the TX PLL is amplified by pre-drive amplifier (IC300 and Q310). The output of the pre-drive amplifier is amplified by
the drive amplifier (Q320) and final amplifier (Q330) to 2.9W at 700MHz band or 3.0W at 800MHz band (0.95W when the power is
low). IC300 is MMIC. Q310, Q320 and Q330 are MOS FET. The output of the final amplifier is then passed through the harmonic filter
(LPF) and antenna switch (D350, D370 and D371) and applied to the antenna terminal.
2.4.4.4

APC Circuit

The APC circuit always monitors the current flowing through the drive amplifier (Q320) and final amplifier (Q330). The APC keeps a
current constant.
The voltage drop at R400 and R402 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 Q310, Q320 and Q330 to keep the current constant. The change of power high/
low is carried out by the change of the reference voltage.
IC300
TX PLL
Pre-
Drive 1
1-12 (No.RA016<Rev.001>)
L536 Q530
IC570
BPF
RF AMP
SW
L556 Q550
BPF
RF AMP
CODEC
IC902
IC905
INTAMT
EXTAMT
Fig.2 RF and IF circuit
Q310
Pre-
Drive 2
Fig.3 Drive and Final amplifier and APC circuit
L530
IC520
BPF
IC500
SW
L550
BPF
RX PLL
Internal Speaker
IC904
External Speaker
Q330
Q320
Drive Final
XF670 Q670 Q660
BPF
MCF IF AMP IF AMP
ANT
SW
APC
IC400
+B APC reference voltage
2nd
Q600
VCO
SSI
IF IC
IC600