Icom IC-A20MK II Service Manual page 10

3-3 PLL CIRCUITS
3-3-1 GENERAL (RF UNIT)
The PLL circuit is designed in a way that allows the desired
frequency to be changed directly by the VCO without a
prescaler by using a high-speed PLL IC (IC2001).
3-3-2 REFERENCE OSCILLATOR (RF UNIT)
To obtain a 25 kHz reference frequency, a 12.8 MHz signal
generated by the reference oscillator section of ІС2001 and
X2001 is divided by 512 at the divider section of IC2001.
3-3-3 VCO CIRCUIT (VCO UNIT)
The VCO (Q2201, Q2202) employs a Colpitts oscillator
circuit. Frequency shifting for transmitting and receiving is
performed by changing the bias voltage of D2201.
Signals generated at the VCO are output as OUT1 and
OUT2. The OUT2 signal is applied to the PLL IC (IC2001,
pin 8), and the OUT1 signal is applied to the AM modulator
(Q2022) or 1st mixer (Q2003).
3-3-4 PROGRAMMABLE DIVIDER (RF UNIT)
The output signal from OUT2 of the VCO UNIT is applied to
the divider (IC2001, pin 8) and divided N times at the
programmable counter section of IC2001. The output signal
from the programmable counter is applied to the phase
detector section of IC2001 and is phase compared.
PLL CIRCUIT BLOCK DIAGRAM
UNLOCK
DATA from
LOGIC UNIT
X2001 2
12.8 MHz
1c2001
ho
The output signal from the phase detector is output from pin
N data is the number of times the desired frequency is
divided by the 25 kHz reference frequency. The desired
frequency is the transmit frequency while transmitting and
the 1st LO frequency while receiving.
Desired frequency
Reference frequency
N data -
The signal passes through a charge pump (Q2012, Q2013,
Q2014) and a lag lead-type loop filter (R2032, R2033,
C2056). It is then applied to the VCO UNIT as lock voltage
(LV).
The output voltage from the charge pump is also applied to
the BPF control circuit (D2008, D2019, Q2023) to change
the center frequency of the receiver bandpass filters.
3-3-5 UNLOCK CIRCUIT (RF UNIT)
When the PLL circuit is unlocked, the phase detector
(ІС2001, pin 7) becomes "LOW," turning the unlock sensor
(Q2010, Q2011) ON via the time constant circuit (R2030,
C2050). Q2011 sends an unlock signal to the CPU
(IC3001, pin 35).
to LOGIC UNIT pin 35
of IC3001
to RX ВРЕ 02003-02006
RF UNIT
VCO UNIT
SHIFT
to 1st
mixer
Q2003
BUFFER
AMP
Q2204
Q2205
to AM mod.
Q2022,
Fig. 3-6