1-20 (No.RA075<Rev.001>)
2.4.4 Transmitter System
2.4.4.1
Audio Band Circuit
Refer to the “LEVEL DIAGRAM” in this manual. The signal from microphone is amplified and converted to digital signal by CODEC
(IC902). IC902 includes AGC function.
Digital signal is transferred to MPU/DSP (IC706) through SSI.
2.4.4.2
Baseband Circuit
Refer to the "LEVEL DIAGRAM" in this manual. The audio signal transferred from CODEC (IC902) is processed at MPU/DSP (IC706).
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 CTCSS and DCS, as required, and is
transferred to PLL Frequency Synthesizer block. The DTMF and MDC baseband signals are also generated by MPU/DSP (IC706).
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 1 (IC301) and pre-drive amplifier 2 (Q301). The output of the pre-drive
amplifier is amplified by the drive amplifier (Q308) and RF Power amplifier (Q306 and Q307) to 110W (70W when the power is middle
and 25W when the power is low). IC301 is MMIC. Q301, Q306, Q307 and Q308 is MOS FET. The output of the final amplifier is then
passed through the coupler, antenna switch (D306, D307, D308, D309 and D310), harmonic filter (LPF) and applied to the antenna
terminal.
2.4.4.4
APC Circuit
The Automatic transmission power control circuit (APC) controls the transmitter output in adjusted value by monitoring it with a coupler.
RF detection and comparison functions are included in the power-control IC (IC306).
The power control IC output is given as a drive source of the RF power amplifier’s gate (Q306 and Q307), and completes APC
feedback loop.
When a normal antenna load is connected, the Forward RF Power is referenced by the coupler and sent to power-control IC (IC306).
Forward RF Power is converted into DC voltage at IC306 and is compared with the SPC.
The output voltage goes through DC amplifier (IC305 B/2) to pre-drive amplifier 2 (Q301), Drive amplifier (Q308) and RF Power
amplifier (Q306 and Q307).
The voltage controls the gate bias voltage of Q301, Q306, Q307 and Q308, and keeps transmission output stable.
Fig.3 Drive and Final amplifier and APC circuit
D306
D307,308
D309,310
Pre-
Drive1
Drive Amp
RF Power Amp
ANT
SW
LPF
Coupler
PA
CTRL
REV
DET
DC
AMP
DC
AMP
DC
AMP
IC306
SPC
REV
Gate Bias
Gate Bias
Q307
RF Power Amp
Q306
Q308
IC301
Pre-
Drive2
Q301
IC305
D304
TX VCO
ANT
IC308
IC305
