Icom IC-F33GT Service Manual page 10

The 2nd IF signal from the 2nd mixer (IC9, pin 3) passes
through the ceramic filter (FI2) to remove unwanted hetero-
dyned frequencies. It is then amplified at the limiter amplifier
section (IC9, pin 5) and applied to the quadrature detector
section (IC9, pins 10, 11) to demodulate the 2nd IF signal
into AF signals.
The demodulated AF signals are output from pin 9 (IC9) and
applied to the base band IC (IC14).
4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker. This transceiver employs the base
band IC which is composed of pre-amplifier, expander,
scrambler, MSK de-modulator, etc. at the AF amplifier sec-
tion.
The AF signals from the FM IF IC (IC9, pin 9) are amplified
at the AF amplifier section in the base band IC (IC14, pin
23), and are then applied to the high-pass filter and low-
pass filter section of it.
The filtered signals pass through the high-pass filter to sup-
press unwanted harmonic components. The signals pass
through (or bypass) scrambler and expander sections. The
signals are amplified at the amplifier section in the base
band IC (IC14).
The output signals from IC14 (pin 20) pass through the low-
pass filter sector (IC23, pins 1, 2), and are then applied to
the AF amplifier (IC15, pin 8) via the AF volume (R315).
The power amplified AF signals are output from pin 10 and
applied to the internal speaker that is connected to J4 via [SP]
jack (J2).
• AF AND MIC AMPLIFIER CIRCUIT
"DET" AF signal
from FM IF IC (IC9, pin 9) 23
IC13
FM/PM switch
IC6
LPF
"CTCSS/DTCS" signal from
D/A conveter IC (IC12, pin 11)
"TONE" signal from CPU via low-pass
filter (IC22, pin 43)
20
Base band IC
(IC14)
7
3
IC12
4 3
FM mod.
D/A converter
4-1-6 SQUELCH CIRCUITS (MAIN UNIT)
• NOISE SQUELCH
A squelch circuit cuts out AF signals when no RF signals
are received. By detecting noise components in the AF sig-
nals, the squelch circuit switches the AF amplifier controller.
A portion of the AF signals from the FM IF IC (IC9, pin 9)
are passed through the D/A converter (IC12, pins 1, 2). The
signals are applied to the active filter section in the FM IF
IC (IC9, pin 8). The active filter section filters and amplifies
noise components. The amplified signals are converted into
the pulse-type signals at the noise detector section. The
detected signals output from pin 13 (NOIS) via the noise
comparator section.
The "NOIS" signal from the FM IF IC is applied to the CPU
(IC22, pin 75). Then the CPU analyzes the noise condition
and outputs AF mute control signal from pin 84 to control
the squelch switch (Q502) as the "MUTE" signal.
• CTCSS AND DTCS
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS or DTCS). 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 "DET" AF signals from the FM IF IC (IC9,
pin 9) pass through the low-pass filter (IC19, pin 5) to
remove AF (voice) signals, and are then applied to the
amplifier (IC19, pin 3). The amplified signals are applied to
the CTCSS or DTCS decoder in the CPU (IC22, pin 46) via
the "CDEC" line. The CPU outputs AF mute control signal
from pin 84 to control the squelch switch (Q502) as the
"MUTE" signal.
IC23
volume
LPF
IC23
AMP
D12
to TX VCO circuit
(Q16, D10, D13, D501)
4 - 2
AF
IC15
AF
Speaker
AMP
Microphone