Transmitter Circuits - Icom IC-F33GT Service Manual

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).
SQUELCH CIRCUITS
• NOISE SQUELCH
The noise squelch cuts off the RX AF line to mute the
AF output signals when no RF signals are received.
Extracting noise components (approx. 30 kHz signal) in the
demodulated AF signals, the squelch circuit turns the AF
power amplifier and AF switches ON and OFF.
The squelch circuits consist of the threshold level adjuster,
noise filter and noise detector.
FM-demodulated AF signals from the FM IF detector IC are
adjusted its level (=squelch threshold level) by DAC (D/A
converter), then passed through the noise filter to extruct
the noise components. The noise components are rectified
by noise detector to produce DC voltage corresponding to
the noise level, then applied to the CPU and compared with
the reference voltage preset in the CPU to control AF power
amplifier and AF switches ON and OFF.
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.
• AF CIRCUIT (TX and RX)
"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
• TONE SQUELCH
The tone squelch connects the RX AF line to emit the AF
output signals only when receiving a signal which consists
of matched tone frequency preset in the CPU.
Detecting CTCSS/DTCS signal in the demodulated AF
signals, the tone squelch circuit turns the AF power amplifier
and AF switches ON and OFF.
FM-demodulated AF signals from the FM IF detector IC are
passed through the tone filter to remove unwanted auible
siganls. then applied to the CPU and compared with the
tone frequency/code preset in the CPU to control AF power
amplifier and AF switches ON and OFF.
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.

5-2 TRANSMITTER CIRCUITS

TX AF CIRCUIT
The TX AF circuit consists of microphone amplifi er, ALC and
AF fi lter. ALC (Automatic Level Controller) is an amplifi er that
reduces its gain automatically to prevent over deviation. The
AF fi lter cuts off the signals except voice signals (3 kHz or
higher and 300 Hz or lower).
The AF signals (MIC) from the microphone (MC1) are
applied to the amplifier (IC23, pins 6, 7). The amplified sig-
nals are amplified again at the microphone amplifier section
of the base band IC (IC14, pins 3). The amplified signals
are passed through or bypass the compressor, scrambler
sections of IC14, and are then passed through the high-
pass, limiter amplifier, splatter filter sections of IC14.
The filtered AF signals from the base band IC (pin 6) are
applied to the FM/PM switch (IC13, pins 6, 7), and pass
through the low-pass filter (IC6, pins 1, 2). The filtered sig-
nals are applied to the D/A converter (IC12, pin 4). The out-
put signals from the D/A converter (IC12, pin 3) are applied
to the modulation circuit (D12).
AF signals range of 300 Hz to 3 kHz
IC23
volume
LPF
IC23
AMP
D12
to TX VCO circuit
(Q16, D10, D13, D501)
4 - 2
AF
IC15
AF
Speaker
AMP
Microphone