Pll Circuits - Icom IC-F4021T Service Manual

5-3 PLL CIRCUITS

5-3-1 VOLTAGE CONTROLLED OSCILLATORS
(VCOs; MAIN UNIT)
VCO is an oscillator whose oscillating frequency is controlled
by adding voltage (lock voltage).
This transceiver has 2 VCOs RX VCO (Q17, D9, D11) and
TX VCO (Q16, D10, D13). The RX VCO oscillates the 1st LO
signals, and the TX VCO oscillates the transmit signal.
• RX VCO
The output signals are amplifi ed by the buffer amplifi ers (Q15,
Q29), and applied to the 1st mixer (Q6) via TX/RX switches
(D16 is OFF, D17 is ON) and LPF (L46, C396, C397), to be
mixed with the received signals to produce the 46.35 MHz 1st
IF signal.
• TX VCO
The output signal is applied to the transmit amplifi ers via the
buffer amplifi ers (Q15, Q29) and TX/RX switches (D16 is ON,
D17 is OFF).
A portion of each VCO output is applied to the PLL IC (IC2, pin
8) via the buffer amplifi er (Q15), doubler (Q14) and BPF (L32,
L34, C196, C197, C205).
• PLL CIRCUITS
D12
Loop
filter
Modulation signals
from the D/A converter
(IC12, pin10)
PLL unlock signal
to the CPU (IC22, pin 34)
TX VCO
Q16, D10, D13
RX VCO
Q17, D9, D11
5
Charge Programmable
pump divider
Phase
detector
7
Reference
counter
5-3-2 PLL IC (MAIN UNIT)
The PLL circuit provides stable oscillation of the transmit
frequency and receive 1st LO frequency. The PLL output
frequency is controlled by the divided ratio (N-data) from the
CPU.
The VCO output signal from the BPF (L32, L34, C196, C197,
C205) is applied to the PLL IC (IC2, pin 8). The applied
signal is divided at the prescaler and programmable counter
according to the "SSO" signal from the CPU (IC22, pin 99).
The divided signal is phase-compared with the reference
frequency signal from the reference frequency oscillator (X2),
at the phase detector.
The phase difference is output from pin 5 as a pulse type
signal after being passed through the internal charge pump.
The output signal is converted into the DC voltage (lock
voltage) by passing through the loop filter (R94–R96, C16,
C17, C146). The lock voltage is applied to the varactors (D9
and D11 of RX VCO, D10 and D13 of TX VCO) and locked to
keep the VCO frequency constant.
If the oscillated signal drifts, its phase changes from that of
the reference frequency, causing a lock voltage change to
compensate for the drift in the VCO oscillating frequency.
Buffer
Buffer
Q15
PLL IC (IC2)
8
Prescaler
9
SCK
10
Shift register
SSO
11
PLST
15.3 MHz
1
reference frequency signal
5 - 4
D16
to the transmit amplifiers
Q29
D17
to the1st IF circuits
×2
Q14
BPF
PLL control signals from the CPU (IC22)
TCXO
X2