Af Circuits (Logic Unit); Frequency Synthesizer Circuits (Rf Unit) - Icom IC-R6 Service Manual

4-2 AF CIRCUITS (LOGIC UNIT)

FM mode
The demodulated AF signal is passed through the
de-emphasis circuit (R323 and C316) and AF filter (Q301),
and then applied to the AF AMP (Q300) through the AF line
SW (D300).
Wide FM/AM mode
The demodulated AF signal is passed through the AF filter
(R316, R320 and C314) and the AF line SW (Q302), and
then applied to the AF AMP (Q300).
The amplified AF signal is passed through the LPF (R350
and C404), whose frequency response is changed by Q508
according to the user setting (The "AF FILTER"). The filtered
AF signal is passed through the AF mute SW (Q350), and
then adjusted in level by the electric VR (IC451).
The level-adjusted AF signal is applied to the AF power
AMP (IC450).
AUDIO POWER AMPLIFIER
IC450 is a BTL (Bridge-Tied Load) audio power amplifier
which provides high AF output power.
The amplified AF signal is passed through the speaker mute
SW (Q507), and then applied to the internal or external
speaker.
• AF CIRCUITS
LOGIC UNIT
From the demodulator circuits
Q301 D300
FM AF
MODE
FILTER Q300
SW
AF
AMP
Q302
AF
MODE
FILTER
SW
WFM/AM
IC451 Q507,D505
IC450
AF
SPEAKER
Electric
AMP MUTE
VR
Q350
J2
EXT SP
AF
MUTE
SP1(CHASSIS)
LPF
SP1
Q508
AF
FILTER
CTRL
4-3 FREQUENCY SYNTHESIZER CIRCUITS
(RF UNIT)
1st VCO
The 1st VCO is composed of Q528, Q529 and D105,
and generates the 1st LO signal, ranging from 266.8 to
550 MHz. This VCO has a shift switch which changes the
oscillating frequency range. The output signal is amplified
by Q525, and then applied to both the buffer (Q43) and the
doubler circuit.
DOUBLER CIRCUITS FOR THE 1ST LO SIGNAL
IC-R6 has two doubler circuits, "DBL1" and "DBL2". They
double the frequency of the VCO output signal to obtain the
1st LO signal with a wide frequency range.
The "DBL1 (Q31, D43 and D50)" doubles the VCO output
signal ranging from 274.85 to 400 MHz, and the "DBL2 (Q31,
D44 and D47)" doubles the signal ranging from 380.45 to
549.85 MHz. When the fundamental frequency signal is
directly used as the 1st LO signal, the VCO output signal
by-passes both doubler circuits through D42 and D52.
2nd VCO
The 2nd VCO is composed of Q6, D17, L45, C80, C207
and C208, and generates 247.05/286.35 MHz (for FM and
AM modes) and 246.750/286.650 MHz (for WFM mode) 2nd
LO signals.
PLL (Phase Locked Loop) CIRCUIT
A portion of signal from each VCO is fed back to the PLL IC
(IC3). The output signal of the 1st VCO is applied to pin 11
of the IC, through the buffer (Q43), and that of the 2nd VCO
is applied to pin 19.
The applied VCO output signal is divided and phase-
compared with a 19.2 MHz reference frequency signal from
the TCXO (X1), which is also divided. The resulting signal of
the phase comparison is output from pin 5 (to the 1st VCO)
or pin 2 (to the 2nd VCO) through the internal charge pump,
and DC-converted by the loop fi lter, and then applied to the
VCO as the lock voltage.
When the oscillation frequency 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.
• FREQUENCY SYNTHESIZER CIRCUITS
D42-D44
LO
SW
Q31
Q31
X2 X2
D47,D50,D52
LO
SW
1st Lo
266.8-1099.695 MHz
IC1
1st IF MIXER
4 - 3
Q2,Q45
1ST LO VCO
Q525
LOOP
BUFF
FIL
Q528,Q529,D105
IC3 PLL IC
Q43
Charge
BUFF Prescaler
pump
Phase
Divider
comparator
Phase
Divider
comparator
19.200 MHz
TCXO
LOOP
Charge
Prescaler
FIL pump
2ND LO VCO
Q6,D17
LPF
2nd Lo
FM,AM: 247.050/286.350 MHz
WFM : 246.750/286.650 MHz
Q523
2nd IF MIXER
(3rd IF MIXER)
X1
IF IC
IC12