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NX-900
19
4. Transmitter System
4-1. Audio Band Circuit
The signal from the microphone goes through the
mute switch (Q706), the Mic-Mute signal (MM) becomes
Low, and the mute switch (Q706) is turned off. The sig-
nal from microphone goes through the Mic-AGC (IC709
(2/2),Q704,Q705,D704,D705), then through the switch IC
(IC708), and is amplifi ed by Mic-Amp IC706 (2/2). LPF IC706
(1/2) works as an anti-aliasing fi lter.
If an optional scrambler board is installed, the switch
(IC708) adjusts the signal path so that the audio signal is in-
put to the scrambler board.
4-2. Baseband Circuit
The audio signal output from the baseband circuit is con-
verted to digital data of a sampling frequency of 48kHz. This
digital data is sent to the DSP (IC717), and 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. In Digi-
tal mode, the audio signal is converted to the 4-Level FSK
baseband signal. The DTMF and MSK baseband signals are
also generated by the DSP. The processed baseband signal
is output from IC717 and IC510.
The signal from IC717 directly modulates the PLL IC (IC3).
The signal from IC510 is filtered with the LPF (IC702)
working as a smoothing fi lter. The DAC (IC719) assigns the
baseband signal to the VCO. At this time, the level output
according to the transmit carrier is fi ne-adjusted.
CIRCUIT DESCRIPTION
SW
Q706
IC708
MM
AMP
IC706
(2/2)
LPF
IC706
(1/2)
LPF
IC702
(1/2)
AMP
IC719
IC709
(1/2)
IC510
IC717
DSP
LPF
IC702
(2/2)
AMP
IC2
(no use)
(no use)
AMP
VCTCXO
X1
VCO
IC5 (2/2)
PLL
IC3
AGC
D704,D705
Q704,Q705
IC709 (2/2)
SW
/SCSW
Optional Scrambler Board
MIC
Fig. 5
4-3. Drive and Final amplifi er
The transmit signal obtained from the TX VCO buffer
amplifi er Q9, is amplifi ed to approxi17dBm by the
driver amplifi ers Q10, Q420, Q150 and Q151.
This amplified signal is passed to the power amplifier
module (power module) IC102, which consists of a MOS-
FET amplifi er, and is capable of transmitting output power.
4-4. APC circuit
The automatic power control (APC) circuit stabilizes the
transmitter output power at a predetermined level, and con-
sists of forward/refl ected power detector circuits.
The forward/refl ected power detector circuits detect for-
ward RF power and refl ected RF power to DC voltage, and
consist of a CM coupling type detection circuit formed by a
CM coupler.
The voltage comparator (IC103) compares the voltage
obtained by the above detected voltage with reference volt-
age set using ASIC (IC510).
An APC voltage proportional to the difference between
the sensed voltage and the reference voltage appears at the
output of IC103. This output voltage controls the gate volt-
age to the power amplifi er module (power module) IC102
(RA45H76871101), the Pre-Drive amplifier Q150 and the
Drive amplifi er Q151.
TX VCO
AMP
Q10
Buff AMP
Q9
AMP
Q420
Pre-Drive
AMP
Q150
Drive
AMP
Q151
Power
Module
IC102
MOD
APC
APC
MP
SW
ANT
LPF
FWD
DET
D103,
D925,D926
D920,D921
ANT
VGG
IC103
CM
COUPLER
REV
DET
D109
D108
Fig. 6
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