Transmitter System; Audio Band Circuit; Base Band Circuit; Drive And Final Amplifier - Kenwood NX-800 Service Manual

NX-800

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,
then mute switch (Q706) is turned off. The signal from mi-
crophone goes through Mic-AGC (Q704, Q705, D704, D705),
and goes through switch IC (IC708), and amplified by Mic-
Amp IC706 (1/2), LPF IC706 (2/2) works as anti-aliasing filter.
If an optional scrambler board is installed, the switch
(IC708) adjusts the signal path so that the audio signal is input
to the scrambler board.

4-3. Drive and Final amplifier

The transmit signal obtained from the TX VCO buffer am-
plifier Q9, is amplified to approximately +17dBm by the driver
amplifiers Q10, Q101 and Q102.
This amplified signal is passed to the power amplifier
module(power module)IC102, which consists of a MOS-FET
amplifier and capable of transmission output power.
4-4. APC circuit
The Automatic transmission power control (APC) circuit
stabilizers the transmitter output power at a predetermined
level by detecting the power module output with a diodes
TX VCO
MOD
VCTCXO
MB
19.2MHz
20
CIRCUIT DESCRIPTION
Optional Scrambler Board
D703,D704
Q705,Q706
INT. MIC Q707
SW AGC
MM
IC702(2/2) IC702(1/2) IC709(1/2)
LPF LPF AMP
0dBm
Buff AMP Drive AMP Drive AMP
Q9 Q10 Q101

4-2. Base Band Circuit

The audio signal output from the base band circuit is con-
verted to digital data of a sampling frequency of 48 kHz. This
digital data is sent to the DSP (IC502), 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, and is
then output from the IC510. In Digital mode, the audio signal
is converted to the 4-Level FSK base band signal and output
from the IC510. The DTMF and MSK base band signals are
also generated by the DSP and output by the IC510.
LPF (IC702) works as smoothing filter. The DAC (IC712)
assigns the base band signal to the VCO and VCTCXO (X1).
At this time, the level output according to the transmit carrier
is fine-adjusted according to each modulation method.
IC708 IC706(1/2) IC706(1/2)
SW AMP LPF
SCSW
IC1
IC712 AMP
AMP
IC5(2/2)
Fig. 5
D108, D109 and D110. Diodes D108, D109 and D110 apply a
voltage to DC amplifier IC103 (1/2).
IC103 (2/2) compares the APC control voltage (PC) gener-
ated by microprocessor IC510 and DC amplifier IC101 (1/2,
2/2) with the detection output voltage from IC103 (1/2) to
control the Vgg Pin of IC102, and stabilizes transmission output.
The APC circuit is configured to protect over-current of the
power module due to fluctuations of the load at the antenna
end and to stabilize transmission output at voltage and
temperature variations.
+17dBm
Drive AMP Final AMP
Q102 IC102
Gate
bias
PC
DC DC
AMP AMP
MP
IC101 IC103
Fig. 6
IC502 DSP
IC510
X1
VCTCXO
VCO
D103,D106,
D216
SW ANT Filter
D108 D109 D110
DET DET DET
ANT