Pll Circuits - Icom IC-91A Service Manual

5-2-4 APC CIRCUIT (RF UNIT)
The APC (Automatic Power Control) circuit stabilizes transmit output
power and controls it High or Low. The APC of this transceiver is a
current monitoring type that detects the transmit signal current of
the transmit amplifiers and controls the transmit output power.
While transmitting, the voltagedrop in R161 is detected by the
current detection circuit (Q150) and applied to the APC controller
(IC150, pin 6). Also, "PSET" signal (power setting reference voltage)
from the D/A converter (IC950, pin 19) is applied to another input
(pin 5).
Then the voltage difference between pins 5 and 6 is output from
pin 7, and the transmit output power is controlled by varying of the
gate voltage of the power amplifier (Q100).
When the transmit output power is set to Low, the driver gate
voltage "DSET" is controlled at the same time as the power setting
reference voltage "PSET" and the dynamic range of the power
control is ensured. Thus, the APC circuit maintains a constant
transmit output power.
• APC CIRCUIT
R161
VCC
Q150
Current
from the
detection
pre-driver
(Q102)
IC150
3
DSET
5
APC
PSET
controller
6
5-2-5 TRANSMIT FILTERS (RF UNIT)
The power amplified transmit signal from the power amplifier is
filtered at the transmit filters. The transmit filters prevent unwanted
RF signals being emitted to the air.
While transmitting in 144 MHz band, the power amplified transmit
signal is passed through the LPF (L102, L103, C106, C107), antenna
switch (D8) and two LPFs (L5–L7, C10–C14; L1–L3, C1–C6) before
being applied to the antenna connector (CHASSIS; J1).
• 144 MHz
from the
power amplifier
D8
(Q100)
ANT
LPF
SW
While transmitting in 430 MHz band, the power amplified transmit
signal is passed through the HPF (L100, L101, C100–C103),
antenna switch (D1, D2), HPF (L4, C7–C9) and LPF (L1–L3, C1–
C6) before being applied to the antenna connector (CHASSIS; J1).
• 430 MHz
from the
power amplifier
D1, D2
(Q100)
ANT
HPF
SW
Q101 Q100
Driver Power
amp. amp.
to the
transmit
filters
1
7
LPF LPF
to the antenna
HPF LPF
to the antenna

5-3 PLL CIRCUITS

5-3-1 VCO CIRCUITS (VCO UNIT AND MAIN UNIT)
This transceiver has 5 VCOs; BC BAND VCO, VHF BAND VCO,
UHF BAND VCO, B_BAND VBVCO and B_BAND UBVCO. The
BC BAND VCO, B_BAND VBVCO and B_BAND UBVCO oscillate
the 1st LO signals. The VHF BAND VCO and UHF BAND VCO
oscillate both transmit output signal and 1st LO signals.
- A_BAND -
• BC BAND VCO
The BC BAND VCO (VCO UNIT; Q1, D1, D3, D5, D6) generates
the 1st LO signals for receiving 0.495–76 MHz signals. The output
signals are amplified at the buffer amplifiers (VCO UNIT; Q200,
Q201), then applied to the 1st mixer (RF UNIT; IC900, pin 3) via
TX/RX switch (RF UNIT; D850).
• VHF BAND VCO
The VHF BAND VCO (VCO UNIT; Q51, D51, D54, D55) generates
both of transmit output signal for 144 MHz band and 1st LO signals
for receiving 76–260 MHz.
While receiving, the VCO oscillates the 1st LO frequency, and the
output signals are amplified at the buffer amplifiers (VCO UNIT;
Q200, Q201). The buffer-amplified signals are applied to the 1st
mixer (RF UNIT; IC900, pin 3) via TX/RX switch (RF UNIT; D850).
While transmitting, the VCO oscillates the transmit frequency,
and the output signal is amplified at the buffer amplifiers (VCO
UNIT; Q200, Q201). The buffer-amplified signals are applied to the
transmit amplifiers via TX/RX switch (RF UNIT; D100).
• UHF BAND VCO
The UHF BAND VCO (Q101, D100, D101, D104) generates both
of the transmit output signal for 430 MHz band and 1st LO signals
for receiving 260–999 MHz.
While receiving, the VCO oscillates the 1st LO frequency, and the
output signals are amplified by the buffer amplifiers (VCO UNIT;
Q200, Q201). If the receiving frequency is 600 MHz and below,
the buffer-amplified signals are applied to the 1st mixer (RF UNIT;
IC900, pin 3) via TX/RX switch (RF UNIT; D850). If the receiving
frequency is 600 MHz and higher, the buffer-amplified signals are
applied to the doubler circuit (RF UNIT; Q850), then the doubled
signals are applied to the 1st mixer (RF UNIT; IC900, pin 3).
While transmitting, the VCO oscillates the transmit frequency, and
the output signal is amplified at the buffer amplifiers (VCO UNIT;
Q200, Q201), then applied to the transmit amplifiers via TX/RX
switch (RF UNIT; D100).
A portion of the VCO output signals generated at BC/VHF/UHF
BAND VCO are applied to the PLL IC (RF UNIT; IC800, pin 4) via
buffer amplifiers (VCO UNIT; Q200, Q202) for comparison signal.
- B_BAND -
• VBVCO
The VBVCO (MAIN UNIT; Q301, D300, D301) generates the 1st
LO signals for receiving 118–174 MHz signals. The output signals
are amplified at the buffer amplifier (MAIN UNIT; Q353), then
applied to the 1st mixer (MAIN UNIT; IC50, pin 3).
• UBVCO
The UBVCO (MAIN UNIT; Q351, D350, D352) generates the 1st
LO signals for receiving 350–470 MHz signals. The output signals
are amplified at the buffer amplifier (MAIN UNIT; Q353), then
applied to the 1st mixer (MAIN UNIT; IC50, pin 3).
A portion of the VCO output signals generated at VBVCO/UBVCO
are applied to the PLL IC (MAIN UNIT; IC400, pin 8) via buffer
amplifier (MAIN UNIT; Q353) for comparison signal.
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