Icom IC-91A Service Manual page 14

5-3-2 PLL CIRCUITS (RF AND MAIN UNITS)
The PLL circuit provides stable oscillation of the transmit frequency
and receive 1st LO frequency. The PLL circuit compares the phase
of the divided VCO frequency with the reference frequency. The
PLL output frequency is controlled by the divided ratio (N-data)
from the CPU.
• A_BAND PLL CIRCUIT
The buffer amplified signals from the buffer amplifier (VCO UNIT;
Q202) are applied to the PLL IC (RF UNIT; IC800, pin 4). The applied
signals are divided at the prescaler and OSC counter according to
the "PLLDATA" signal from the CPU (LOGIC UNIT; IC1, pin 94).
The divided signal is phasecompared with the reference frequency
at the phase detector.
• A_BAND PLL AND VCO CIRCUITS
VCO BOARD
BC BAND VCO
Q1,D1,D3,D5,D6
ULV BCLV VLV
R815 R811
Loop
filter
"UNLK" signal to the CPU
(LOGIC UNIT; IC1, pin 92)
UHF BAND VCO
Buffer
amp.
Q101,D100,D101,D104
Q200
VHF BAND VCO
Q51,D51,D54,D55
PLL IC (IC800)
Phase
2 Charge
Pump Detector
9 Reference
Counter
6
The phase difference is output from pin 2 as a pulse type signal
after being passed through the internal charge pump. The output
signal is applied to the BC/VHF/UHF BAND VCOs (VCO UNIT)
after being converted into the DC voltage (lock voltage) at the loop
filter (RF UNIT; C820–C823, R806–R808).
The lock voltage for BC BAND VCO ("BCLV") from the loop filter
is applied to the VCO (VCO UNIT) via R815 and pin 1 of J800
(RF UNIT). The lock voltage for VHF BAND VCO ("VLV") from the
loop filter is applied to the VCO (VCO UNIT) via R811 and pin 6
of J800 (RF UNIT). The lock voltage for UHF BAND VCO ("ULV")
from the loop filter is applied to the VCO (VCO UNIT) via pin 16 of
J800 (RF UNIT).
LO
amp.
LOOUT
Q201
Buffer
amp.
PLLOUT
Q202
OSC 4
Pre-
Counter
scaler
10
12
Input Register
13
Reference frequency
signal 15.3 MHz
from the TCXO
(MAIN UNIT; X450)
5 - 7
D100
1
IC100
transmit signal
Doubler
×2
Q850
1st LO signals to the
D850
1st mixer (IC900, pin 3)
"CK"
PLL IC control signals
"PLLDATA"
from the CPU (LOGIC UNIT; IC1)
"APLLSTB"
4
to the transmit
amplifiers
RF UNIT