Kenwood TH-D72A Service Manual page 7

4. Transmitter System
4-1. Transmitter circuit
The VHF TX signal of the VCO B output through the RF
amplifi er (Q201) passes through the band switches (D202,
D205), is amplifi ed with the Pre-drive amplifi er (IC202) and
Drive amplifier (Q214), passes through the band switch
(D211) and is amplified to the final output level with the
Final amplifi er (Q215). The signal then passes through the
antenna switch (D215, D216), Duplexer and LPF, and is fed
to the antenna.
The UHF TX signal of the VCO B output through the RF
amplifi er (Q201) passes through the band switches (D201,
D204), is amplifi ed with the Pre-drive amplifi er (IC202) and
Drive amplifier (Q214), passes through the band switch
(D212) and is amplified to the final output level with the
Final amplifi er (Q215). The signal then passes through the
antenna switch (D217), Duplexer and LPF, and is fed to the
antenna.
D202
RF
SW
Q201
430T
From
VCO B
144T
RF
SW
D201
VHF
LPF
To
Q214
IC201
Q215
R216 –
R217 +
IC737
+B
Main
MCU
5. VCO-PLL Circuit
5-1. Oscillator circuit
The reference frequency (16.800MHz) generated with
the TCXO (X431) of the Band A oscillation circuit is divided
frequency in the PLL IC (IC431) and obtains the comparison
frequencies of 5kHz or 6.25kHz. Similarly, the reference
frequency (16.800MHz) generated with the TCXO (X511) of
the Band B oscillation circuit is divided frequency in the PLL
IC (IC513) and obtains the comparison frequencies of 5kHz
or 6.25kHz. The reference oscillator of Band A generated to
three times with the RF amplifi er (Q357) is used also for the
second-local oscillator for the reception of Band A.
The Band A VCO oscillates and is amplifi ed with oscilla-
tion amplifi er (Q435), and output to the F-in amplifi er (Q434)
CIRCUIT DESCRIPTION
D205 D211
RF
SW
IC202 Q214
RF
SW
D204 D212
Q207
IC203
– RF
+ SW
IC721
Q212
E-
Volume
Figure 3 Transmitter circuit
TH-D72A/D72E
4-2. APC circuit
The Automatic transmission Power Control (APC) is a cir-
cuit to obtain a steady TX Power, and controls the TX Power
detecting the drain current of the Drive amplifi er (Q214) and
Final amplifi er (Q215). The voltage difference generated by
the drain current in the resisters (R216, R217) is amplifi ed
with the OP-Amplifi er (IC201) and DC Amplifi er (Q207), and
the detecting voltage is obtained. The control voltage output
from E-volume (IC721) controlled with the Main MCU (IC737)
according to each band and TX Power is prepared. These
two obtained voltages are compared with the OP-Amplifi er
(IC203) and obtains the APC voltage. The Gate voltage
of the Drive amplifi er (Q214) and Final amplifi er (Q215) is
changed depending on this APC voltage and the TX Power
is controlled.
4-3. Thermal protection circuit
The Sub MCU (IC611) observes the detecting voltage
by the Thermistor (TH201) arranged to prevent the thermal
fracture of the Final amplifier (Q215). The APC voltage is
changed when the preset temperature is exceeded, and
excessive generation of heat is prevented by controlling the
TX Power.
VHF VHF
LPF LPF
RF
SW
Q215
430T
144T
RF
SW
UHF UHF
HPF HPF
IC611 TH201
D227
Sub D228
TH
MCU
RF
SW
D225,D226
Voltage
shift
and F-out amplifier (Q437) through the Buffer amplifier
(Q436). The Band B VCO oscillates and is amplified with
two oscillation amplifi ers (Q522, Q523), and output to the
F-in amplifi er (Q521) and F-out amplifi er (Q201) through the
Buffer amplifi er (Q524). To improve the modulation charac-
teristics of the DCS and 9600bps packet signal, the modula-
tion is put on the TCXO (X431) of Band B.
5-2. Phase comparator
The PLL ICs (IC431, IC513) of the pulse-swallow method
divides the input oscillator frequency and reference fre-
quency according to the PLL Data from the Sub MCU (IC611),
and achieves the PLL synthesizer corresponding to the step
frequency by comparing phases.
D215,D216 VHF
D219,D221
LPF
RF
SW
430T
144T
RF
SW
D217
UHF UHF
LPF D220,D222 HPF
D214
D223
RF
SW
144T
UHF
LPF
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