Kenwood NX-840, Service Manual

Page: 12 / 54

1-12 (No.RA011<Rev.001>)
2.4.3.3 APC Circuit
The automatic transmission power control (APC) circuit detects part of a final amplifier output with a coupler circuit and applies a volt-
age to IC301.
IC301 compares the APC control voltage (APCC) generated by the baseband IC (IC709) and DC amplifier (IC503) with the detection
output voltage.
IC301 generates the voltage to control IC300 and stabilizes transmission output.
The APC circuit is configured to protect over current of Q300 and IC300 due to fluctuations of the load at the antenna end and to
stabilize transmission output at voltage and temperature variations.
Fig.6 APC circuit and Power amplifier circuit
2.4.4 PLL Frequency Synthesizer
The PLL circuit generates the first local oscillator signal for reception and the RF signal for transmission.
2.4.4.1 PLL Circuit
The frequency step of the PLL is 2.5, 5.0, 6.25 or 7.5 kHz. A 16.8MHz reference signal is divided at IC2 by a fixed counter to produce
the reference frequency.
The voltage controlled oscillator (VCO) feedback output is divided by a programmable counter in IC2.
The 2 signals are phase compared, filtered through a low pass filter and passed to VCO to control the oscillator frequency.
2.4.4.2 VCO Circuit
The operating frequency is generated by Q7 in transmit mode and Q5 in receive mode.
The oscillator frequency is controlled by applying the control voltage, which obtained from the phase comparator, to varactor diodes
(D5, D6, D9, D10, D11 in transmit mode and D7, D8, D12, D13, D14 in receive mode).
The TX/RX pin is set “High” in receive mode causing turn on Q9. And T/R pin is set “Low” in transmit mode causing turn on Q6.
Fig.7 PLL and VCO circuit
2.4.4.3 Unlock Circuit
If low level appears at the “LD” pin of IC2, then PLL an unlock condition occurs.
It causes the voltage applied to the “PLL_UL” pin of the microprocessor to go low.
When the microprocessor detects this condition, the transmitter is disabled by ignoring the push-to-talk switch input signal.
IC300 Q300
DRIVE AMP
RF POWER
MODULE
DC
AMP
IC503
ANT
SW
D305,D306
D309,D310
LPF
ANT
IC301
APC
control
Coupler
circuit
D16
APCC
IC709
pin 32
D5,D6
D9,D10,D11
D12,D13,D14
ASTC
IC2
Assist
voltage
Q7
TX VCO
Q11
BUFF
AMP
D7,D8
Q5
RX VCO
Q6,Q9
T/R SW
Charge
pump
OP
AMP
LPF
Phase
comparator
1/M
1/N
REF
OSC
16.8MHz
PLL_DATA
PLL_UL
MCU
IC2: PLL IC
Q2
AMP