Icom IC-PCR1500 Service Manual page 14

<3rd IF AND DEMODULATOR CIRCUITS (FM)>
3rd IF signal to the AM demodulator circuits
2
IC8
D/A converter
1
FM-demodulated signals
to the AF circuits
<2nd IF AND DEMODULATOR CIRCUITS (WFM)>
FM-demodulated signals
to the AF circuits
4-1-6 AF CIRCUITS (LOGIC UNIT)
The demodulated AF signals from the demodulator circuits are
amplifi ed and fi ltered in the AF circuits.
In FM mode, the demodulated AF signals from the
demodulator circuits are passed through the AF select
switch (IC4, pins 1, 7) and BPF (IC27, pins 1, 3 and 5, 7),
then applied to the AF amplifi er (Q18).
In other than FM mode, the demodulated AF signals from
the demodulator circuits are passed through the AF select
switch (MAIN-A/B; UNIT ;IC27, pins 8, 9; WFM, pins 3, 4;
AM, pins 1, 2; SSB), then applied to the AF amplifi er (Q18).
The amplified AF signal are applied to the electric volume
(IC17, pins 6, 7) for level adjustment. The level adjusted
AF signals are passed through the speaker mute switch
(Q12) and applied to the AF power amplifier (IC12, pin 1).
The power-amplified AF signals are applied to the internal
speaker via J1 (J6) or an external speaker/ear phone via the
phone switch (S1), attenuator (R19, R20) and J1 (J6).
• AF CIRCUITS
IC4
FM
1
IF IC
9
(IC24)
7
AF
selector
WFM
IF IC
8
(IC16)
8
AM
3
AM demodulator
(D75)
SSB 1
From the BFO
AF mixer (IC23)*
8
Noise
Amp.
Limit'r
Quodrature
Amp.
Detector
9
10 11
+5V
X2
Q73
Limit'r
Amp.
Quodrature
Detector
9
12 11
X3
IC27 IC17
Q18
5 1
AF
BPF
Amp.
6
Ele. VR
7 3
IC27
9
Q15
4
AF
Amp.
2
AF
selector
MAIN-A/B UNIT LOGIC UNIT
D96 D94
BPF
D73 FI9
D69
BPF
Q62
D95 FI8
D93
BPF
3rd IF amp.
D72 FI7
D68
BPF
7 5
Noise
Detector
RSSI
12 13
"NOISE" signal to the CPU (IC18, pin 16)
"RSSI" signal to theAGC amplifier (Q67)
Limit'r
Limit'r
Amp.
Amp.
RSSI
17 2
2nd IF signal from the 2nd IF circuits
"RSSI" signal to the NB circuits
In the USB (Universal Serial Bus) audio output mode, the AF
signals from the AF amplifi er (Q18) are applied to another AF
amplifi er (Q15). The amplifi ed AF signals are passed through
the USB audio selector (IC2038, pins 1, 3), then applied
to the USB audio IC (IC16, pin 12) then converted into the
USB audio data. The converted audio data is applied to the
connected PC via the USB HUB (IC32, pins 5, 6 and 2, 3).
4-1-7 SQUELCH CIRCUIT
• NOISE SQUELCH (MAIN-A/B UNITS)
The noise squelch mutes the AF output signals when no RF
signals are received. By detecting noise components in the
demodulated AF signals, the squelch circuit toggles the AF
power amplifi er ON and OFF.
A portion of the demodulated signals from the IF IC (IC24,
pin 9) are applied to the D/A converter (IC8, pin 1) for level
adjustment. The level-adjusted AF signals are output from
pin 2, and passed through the noise filter (R421, R417,
C520, C523). The fi ltered noise signals are then applied to
the noise amplifier (IC24, pins 7, 8) to be amplified noise
components only.
The amplified noise components are converted into the
IC12
Q12, D8
Power
Amp.
7 1 4
SP
mute sw.
IC16 IC32
1 6
D+ 2 D+
USB
USB
12
2 D– 5 3 D–
audio HUB
4 - 3
10.25 MHz
3rd LO signal
REF
amp.
Q71 Q80
3 2
IF IC (IC24)
16
2nd IF signal from the 2nd IF circuits
IF IC (IC16)
Phone switch (S1)
(CHASSIS; J6)
ATT
Speaker (CHASSIS; SP1)
To the PC
USB connector
(J2)
*MAIN-A UNIT only
TCXO
REF
amp.
X2