Transmitter Circuits - Icom IC-2GA Service Manual

4-2 TRANSMITTER CIRCUITS
(1) MICROPHONE AMPLIFIER (MAIN UNIT)
AF signals from the built-in condenser microphone or from
the external microphone jack are amplified at a limiter
amplifier (IC205), and are pre-emphasized to 6 dB/octave
(300 Hz—3 kHz). Pin 1 outputs a square wave which con-
tains many high harmonic components. The signals pass
through the splatter filter in IC205 where signals of 3 kHz
and above are attenuated. The signals from pin 7 are
applied to the VCO UNIT for frequency modulation (FM).
(2) DRIVE AMPLIFIER (RF UNIT)
The VCO output is buffer amplified at 012. Passing
through D16, the signals are then amplified at the drive
amplifier (Q10).
(3) RF POWER AMPLIFIER (RF UNIT)
010 output is amplified at IC3 to 7 W (HIGH) or 1 W (LOW).
When transmitting, the RF signals amplified at IC3 pass
through the antenna switching diode (D15) and the low
pass filter where harmonic signals are rejected. The signal
is then applied to the antenna connector.
RX 1st mixer
APC CIRCUIT
RF signal
from Q12
Power mod
Driver amp
Q15
Detected voltage
Reference voltage
4-3 PLL CIRCUITS
(1) GENERAL
The PLL circuit, using a one-chip PLL IC (IC2), directly
generates the desired frequency. ІС2 sets the dividing ratio
based on serial data from the CPU (IC501), and compares
the phases of the VCO signal and the reference oscillator
frequency. it detects the out-of-step phase and outputs it.
(2) REFERENCE OSCILLATOR CIRCUIT
(RF UNIT)
ХЗ oscillates а 10 MHz signal. ІС2 divides this signal with
N-data from the CPU. In this way, one of six tuning steps
(b, 10, 12.5, 15, 20, 25 kHz) can be selected.
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(4) APC CIRCUIT (Automatic Power Control)
(RF UNIT)
IC3 output passes through the APC detector circuit (D13,
D14, L15, etc.) where its RF output level is detected.
The detected output, compared at the differential amplifier
(Q13) to the reference voltage output of the power set
circuit, is applied to the base of Q15.
When the antenna is matched at 50 Q, the detected output
is at a minimum. When mismatched, the detected output
increases. The current at the collectors of Q11 and Q15, as
well as the gain of the driver (Q10), decreases until the
detected output and the reference voltage become equal.
In this way, the power module (IC3) is protected from
damage.
Q17 acts as a switch for the reference voltage used in the
APC circuit. In this way, HIGH/LOW transmit output power
switching is accomplished.
This current is controlled
<+— by the APC circuit.
7W output power
to low-pass filter
ber
Ў АРС DETECTOR CIRCUIT
RF indicator signal
to the LOGIC UNIT
POWER SET CIRCUIT
"С
T+5
LOW
("HIGH" when
low power is selected)
(3) CHARGE PUMP AND LOOP FILTER CIRCUITS
(RF UNIT)
Phase-detected signals from pins 5 and 12 are converted to
DC voltage by the charge pump (Q6~Q8) and the lag-lead
loop filter (В48--В50, C81, C112 and C113).
The frequency at which the VCO oscillates is controlled by
varactor diodes (D703, D704). DC voltage (PLL lock voltage)
is provided through the loop filter. The output of the loop
filter passes through Q9, and is used to control the
bandpass filter (D3, D5, D6) in the receiver RF circuit.
When the power save function is on, the charge pump
control circuit (020—022, 018) cuts off the charge pump
and the VCO.