Icom IC-208H Service Manual page 11

• IN CASE OF FM SIGNAL RECEIVING
The signal is then amplified at the limiter amplifier section
(IC5, pin 5) and applied to the FM detector section (X2, IC5,
pins 10, 11) for demodulation the 2nd IF signal into AF sig-
nals.
The FM detector circuit employs a quadrature detection
method (liner phase detection), which uses a ceramic dis-
criminator (X2) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC5 (pin 9) passes through the
AM/FM selector circuit (IC5, pins 7, 1), and is then applied
to the AF circuit.
• IN CASE OF AM SIGNAL RECEIVING
The signal is amplified at the amplifier (Q16), and is then
applied to the AM detector circuit (Q13) for demodulation the
2nd IF signal into AF signals.
The detected signal from Q13 passes through the AM/FM
selector circuit (IC5, pins 6, 1), and is then applied to the AF
circuit.
4-1-8 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the detected signals to
drive a speaker. The AF circuit includes an AF mute circuit
for the squelch.
The detected AM or FM AF signals are applied to AF mute
circuit (Q14). The output signals pass through the low-pass
filter (Q5), and are then applied to the electric volume circuit
(IC508, pin 1). The signals are applied to the other AF mute
circuit (Q512). The signals are level adjusted at the volume
control IC (IC508), and are amplified at the AF power ampli-
fier (IC510, pin 1) after being passed through the other AF
mute circuit (Q512).
The output signal from IC510 (pin 4) drives the external or
internal speaker.
A part of signals from the AM/FM selector circuit (IC5, pin 1)
pass through low-pass filters.
When the signal passes through the low-pass filter (Q3), the
filtered signal is applied to the MAIN CPU (IC505, pin 39) via
the "DTCS_IN" signal to analyze the DTCS signal.
• AGC CIRCUIT
D19—D21,
D525
AGC
AM/FM
AF signal
selector
2nd IF filter
450 kHz
FI1 FI2
Q13,
Q16
W/N selector
AM
DET
FM IF IC
IC8
When the signal passes through the other low-pass filter
(C52, C53, R46, R52), the filtered signal is applied to the
MAIN CPU (IC505, pin 42) via the "WXALT" signal to detect
WX alert signal.
A part of signals from the AF mute circuit (Q14) is applied to
the 6-pins data jack as data signal via the "DATAOUT" sig-
nal.
4-1-9 SQUELCH CIRCUIT (MAIN UNIT)
• NOISE SQUELCH
A noise squelch circuit cuts out AF signals when no RF sig-
nals are received. By detecting noise components in the AF
signals, the squelch circuit switches the AF mute switch.
Some of the noise components in the AF signals from the
FM IF IC (IC8, pin 9) are passed through the active filter sec-
tion (IC8, pins 8, 7), and then applied to the noise detector
section. The detected noise signals are applied to the MAIN
CPU (IC505, pin 38) via the "SQL" line.
Even when the squelch is closed, the AF mute switch
(Q512) opens at the moment of emitting beep tone.
• TONE SQUELCH
The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS). When tone squelch is in use, and
a signal with a mismatched or no subaudible tone is
received, the tone squelch circuit mutes the AF signals even
when noise squelch is open.
A portion of the AF signals from the FM IF IC (IC8, pin 9)
passes through the low-pass filter (Q3) to remove AF (voice)
signals via the AM/FM AF selector (IC5, pins 7, 1), and is
then applied to the CTCSS decoder inside the CPU (IC505,
pin 39) via the "DTCS_IN" line to control the AF mute switch.
4-1-10 AGC CIRCUIT (MAIN UNIT)
The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain to keep the audio output at a constant level. The
receiver gain is determined by the voltage on the AGC line
(Q13 collector).
RX
SW
IC9,
IC10,
Q21
RX
SW
Q18
RX
IF
SW
4 - 3
Antenna
1st
mixer
RF
RF
RF RF
RF
RF