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2.4.4 Transmitter System
2.4.4.1
Audio Band Circuit
The signal from microphone is amplified and converted to digital signal by IC607. IC607 includes AGC function.
Digital signal is transferred to IC404.
2.4.4.2
Baseband Circuit
The audio signal transferred from IC607 is processed at IC404.Voice signals of 300Hz or lower and frequencies of 3 kHz or higher are
cut off and an audio range 300Hz to 3 kHz 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 IC302. The DTMF and MSK
baseband signals are also generated by IC404.
In Digital mode, the audio signal is converted to the 4-Level FSK baseband signal and is transferred to PLL Frequency Synthesizer
block in IC302.
2.4.4.3
Drive and Final amplifier
The RF signal from IC302 is amplified by pre-drive amplifier (IC202). The output of the pre-drive amplifier is amplified by the drive
amplifier (Q201) and RF Power module (IC201) to 50W [NX-3720H], 45W [NX-3820H], 25W [NX-3720, NX-3820] (5W when the power
is low). IC202 is MMIC.Q201 is MOS FET. The output of the final amplifier is then passed through the coupler, antenna switch (D205,
D206 and D207), harmonic filter (LPF) and applied to the antenna terminal.
2.4.4.4
APC circuit
The Automatic transmission power control (APC) circuit 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 (IC205).
The power control IC output is given as a drive source of the RF power module's gate (IC201), and completes APC feedback loop.
When a normal antenna load is connected, forward RF Power is detected by the coupler and direct in power-control IC (IC205).
Forward RF Power is converted into DC voltage in IC205 and compared with the SPC.
The output voltage goes through DC amp (IC203 2/2) to Drive amp (Q201) and RF Power module (IC201).
The voltage controls the gate bias voltage of Q201 and IC201, and keeps transmission output stable.
IC202
IC302
Pre-Drive
2.4.5 PLL Frequency Synthesizer
2.4.5.1
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 (IC302).
The oscillation frequency is adjusted TCXO directly by controlled the D/A converter of FCNT.
IC201
Q201
Drive
Final
Gate Bias 1
Gate Bias 2
Fig.3 Drive and Final amplifier and APC circuit
Coupler
IC205
PA
REV
SPC
CTRL
DET
DC
DC
AMP
AMP
IC203
D205,D206
D207
D208,D209
ANT
ANT
SW
LPF
D204
REV
(No.RA054<Rev.001>)1-17
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