Pll Circuits; Apc Circuit (Main Unit); Vco Circuit (Main Unit) - Icom IC-2200H Service Manual

4-2-7 APC CIRCUIT (MAIN UNIT)

The APC circuit protects the power amplifier from a mis-
matched output load and stabilizes the output power.
The APC detector circuit (D14, D17, L39) detects forward
signals and reflection signals at D14 and D17 respectively.
The combined voltage is at minimum level when the anten-
na impedance is matched at 50 Ω and is increased when it
is mismatched.
The detected voltage is applied to the differential amplifier
(IC6, pin 2), and the power setting voltage is applied to the
other input (pin 1) for the reference.
When antenna impedance is mismatched, the detected volt-
age exceeds the power setting voltage. The output voltage
of the differential amplifier (IC6, pin 1) controls the input cur-
rent of the power module (IC10) to reduce the output power.

4-3 PLL CIRCUITS

4-3-1 PLL CIRCUIT
A PLL circuit provides stable oscillation of the transmit fre-
quency and the receive 1st LO frequency. The PLL circuit
compares the phase of the divided VCO frequency to the
reference frequency. The PLL output frequency is controlled
by the divided ratio (N-data) of a programmable divider.
• APC CIRCUIT
HV
T8
RF signal
Q26, Q39, Q40
TXC
T1
IC11
APC control circuit
• PLL CIRCUIT
Loop
filter
21.25 MHz 2nd LO signal
to the FM IF IC (IC4, pin 2)
LPF
Q17
Pre
drive
—
IC6
APC
amp.
+
RX VCO
Q7, D5
TX VCO
Q6, D2, D4
Charge Phase
9
detector
pump
Reference
divider
16
1
An oscillated signal from the VCO passes thorough the LO
and buffer amplifiers (Q9, Q12) is applied to the PLL IC (IC1,
pin 6) and is prescaled in the PLL IC based on the divided
ratio (N-data). The reference signal is generated at the ref-
erence oscillator (X1) and is also applied to the PLL IC. The
PLL IC detects the out-of-step phase using the reference
frequency and outputs it from pin 15. The output signal is
passed through the loop filter (Q2, R6, R11–R15, C11, C12)
and is then applied to the VCO circuit as the lock voltage via
the "LV" line.

4-3-2 VCO CIRCUIT (MAIN unit)

The VCO circuit contains a separate TX-VCO (Q6, D2, D4)
and RX-VCO (Q7, D5, D38). The oscillated signal is ampli-
fied at the LO (Q9) and buffer (Q11) amplifiers, and is then
applied to the T/R switching circuit (D6, D7). Then the Tx
and Rx signals are applied to the pre-driver (Q17) and 1st
mixer (Q19) respectively.
A portion of the signal from Q4 is amplified at the buffer
amplifier (Q6) and is then fed back to the PLL IC (IC1 pin 2)
as the comparison signal.
Q35
Driver
Power
amp. amp.
Buffer
Q9
LO
Buffer
IC1 LV2105V
Programmable
Prescaler
divider
Shift register
X1
21.25 MHz
4 - 4
Downloaded by
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Power detector circuit
IC10
L39
to the antenna
D14 D17
Q11
D7
to 1st mixer circuit
D6
to transmitter circuit
Q12
LPF
6
2
PLLCK
3
PLLDATA
4
PLLSTB