Transmitter Circuits - Icom IC-F620 Service Manual

4-1-5 AF AMPLIFIER CIRCUIT (MAIN unit)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
The AF signals from the FM IF IC (IC1, pin 9) are amplified
at the AF amplifier section of the compander IC (IC14, pins
5, 4) and are then applied to the high-pass filter circuit
(IC21b).
The high-pass filter characteristics are controlled by the
FSW signal from the LCD driver IC (FRONT unit; IC1, pin 6).
When FSW signal is high, the cut-off frequency is shifted
higher to remove CTCSS or DTCS signals.
The filtered AF signals from the high-pass filter (IC21b,
pin 7) are applied to the de-emphasis section of compander
IC (IC14, pin 3) with frequency characteristics of –6
dB/octave, and are then passed through the low-pass filter,
high-pass filter, expander sections of compander IC (IC14).
The output signal from IC14 (pin 38) is applied to the elec-
tronic volume controller (IC6, pin 1).
The output AF signals from the electronic volume controller
(IC6, pin 2) are applied to the AF amplifier (IC18) and AF
power amplifier (IC8) to drive the speaker.
4-1-6 RECEIVER MUTE CIRCUITS (MAIN unit)
•
NOISE SQUELCH
The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.
Some noise components in the AF signals from the FM IF IC
(IC1, pin 9) are passed through the level controller (IC6, pins
24, 23). The level controlled signals are applied to the active
filter section in the FM IF IC (IC1, pin 8). Noise components
about 10 kHz are amplified and output from pin 7.
The filtered signals are converted into the pulse-type signals
at the noise detector section and output from pin 13 (NOIS).
The NOIS signal from the FM IF IC is applied to the CPU
(IC20, pin 37). The CPU then analyzes the noise condition
and controls the AF mute signal via "AFON" line (IC20, pin
18) to the AF regulator (Q39, Q40, D31).
• CTCSS AND DTCS
The tone squelch circuit detects AF 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 AF signals from the FM IF IC (IC1, pin 9)
passes through the low-pass filter (IC5) to remove AF
(voice) signals and is applied to the CTCSS or DTCS
decoder inside the CPU (IC20, pin 46) via the "CDEC" line
to control the AF mute switch.
4-2 TRANSMITTER CIRCUIT
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN unit)
The microphone amplifier circuit amplifies audio signals
within +6 dB/octave pre-emphasis characteristics from the
microphone to a level needed for the modulation circuit.
The AF signals (MIC) from the FRONT unit via J2 (pin 1) are
passed through the level controller (IC6, pins 9, 10) to the
microphone amplifier circuit.
The AF signals from the level controller (IC6) are applied to
the microphone amplifier section of compander IC (IC14, pin
12). The amplified signals are passed through the compres-
sor, low-pass filter and high-pass filter sections of IC14.
The filtered AF signals are amplified at the buffer amplifier
(Q21) and pre-emphasized with +6dB/octave at the pre-
emphasis circuit (R122, C187), and are then applied to the
IDC amplifier section of IC14 (pin 8).
The amplified AF signals are passed through the limitter
amplifier, low-pass filter and smoothing filter sections of
IC14 after being passed through the AF mute switch inside
of IC14.
The output signals from pin 6 are passed through the ana-
log switch (IC15), splatter filter (IC21d) and applied to the
level controller (IC6, pins 21, 22). The deviation level con-
trolled signals are then applied to modulation circuit as the
"MOD" signal.
The narrow/wide switch (Q22) is connected to the input of
the splatter filter (IC21d) and switched by the "NWC" signal
coming from the CPU (IC20, pin 19). When "NWC" is at a
high level, the narrow/wide switch (Q22) shifts the filter cut-
off frequency for narrow deviation selection.
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The AF signals from the level controller (IC6, pin 22) change
the reactance of varactor diode (D18) to modulate the oscil-
lated signal at the TX VCO circuit (Q14, D17). The modulat-
ed VCO signal is amplified at the buffer amplifiers (Q11,
Q10) and is then applied to the drive amplifier circuit via the
T/R switch (D14).
The CTCSS/DTCS signals from the CPU (IC20, pins 89–91)
are passed through the low-pass filter (Q37), level controller
(IC6, pins 12, 11) and mixer (IC21a), and are then applied to
the VCO circuit via the splatter filter (IC21d).
4-2-3 DRIVE AMPLIFIER CIRCUIT (MAIN unit)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to the level needed at the power amplifier.
The RF signal from the buffer amplifier (Q10) passes
through the T/R switch (D14) and is amplified at the YGR
(Q9) and pre-drive (Q8) amplifiers. The amplified signal is
applied to the power amplifier circuit.
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