Kenwood NX-900 Service Manual page 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.
MIC
LPF
IC702
(2/2)
4-3. Drive and Final amplifi er
The transmit signal obtained from the TX VCO buffer
amplifi er Q9, is amplifi ed to approximately +17dBm 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.
Buff AMP
Q9
MOD TX VCO
CIRCUIT DESCRIPTION
Optional Scrambler Board
D704,D705
Q704,Q705
Q706 IC709 (2/2) IC708
SW AGC
MM /SCSW
LPF AMP IC719
IC702 IC709
(1/2) (1/2)
Pre-Drive
AMP AMP AMP
Q10 Q420 Q150
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.
IC706 IC706
(2/2) (1/2)
SW AMP LPF
IC2
(no use) (no use)
AMP
AMP
IC5 (2/2)
Fig. 5
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.
Drive Power
AMP Module
Q151 IC102
CM
COUPLER
VGG D109
FWD
DET
IC103
APC
APC
MP
Fig. 6
NX-900
IC717
DSP
IC510
X1
VCTCXO PLL
IC3
VCO
D103,
ANT
D925,D926
D920,D921
ANT
SW
LPF
D108
REV
DET
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