Icom IC-F51 - Manual (Page 8)

4 - 2
4-1-5 AF AMPLIFIER CIRCUIT
(MAIN AND FRONT UNITS)
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 (IC1, pin 9) are amplified
at the AF amplifier section of the base band IC (IC10, pin 5)
and are then applied to the low-pass filter section of it.
The filtered signals passes through the high-pass filter to
suppress unwanted harmonic components. The signals
pass through (or bypass) scrambler and expander sections,
and are then applied to (or bypass) the scrambler IC (IC14)
via the analog switch (IC13). The signals are amplified at the
amplifier section of the base band IC (IC10), and pass
through the AF mute switch (IC406) and low-pass filter
(IC403). The filtered signals pass through the AF volume,
and are then applied to the AF power amplifier (IC405) to
drive the speaker.
4-1-6 RECEIVE MUTE CIRCUITS
(MAIN AND FRONT UNITS)
• NOISE SQUELCH
A 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 D/A converter (IC6, pin
1). The 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
(FRONT unit; IC401, pin 41). Then the CPU analyzes the
noise condition and controls the AF mute signal via “AFON”
line from expander IC (FRONT unit; IC410, pin 7) to the AF
power controller (FRONT unit; Q401, Q402).
• 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 “DET” AF signals from the FM IF IC (IC1, pin
9) passes through the low-pass filter (IC5, pin 5) to remove
AF (voice) signals, and are then applied to the amplifier
(MAIN unit; IC5, pin 10). The amplified signals are applied to
the CTCSS or DTCS decoder inside of the CPU (FRONT;
IC401, pin 44) via the “CDEC” line. The CPU outputs AF
mute control signal, and is then applied to the I/O expander
IC (IC410). The IC outputs AF mute circuit (IC406) and AF
power supply circuits (Q401, Q402) control signals via the
“AFON” line.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(FRONT AND MAIN UNITS)
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.
This transceiver employs the base band IC which is com-
posed of microphone amplifier, compressor, scrambler, lim-
iter, splatter filter, MSK modulator, etc. at the microphone
amplifier section.
The AF signals (MIC) from the microphone (MC401) are
passed through the microphone mute switch (IC406, pins 2,
1), and are then applied to the amplifier (IC407, pins 2, 6).
The amplified signals pass through (or bypass) the scram-
bler IC (IC14) via the analog switch (IC13), and are then
applied to the microphone amplifier section of the base band
IC (MAIN unit; IC10, pins 3, 4). The amplified signals are
passed through or bypass the compressor, scrambler sec-
tions of IC10 (MAIN unit), and are then passed through the
high-pass, limiter amplifier, splatter filter sections of IC10
(MAIN unit).
The filtered AF signals are applied to the FM/PM switch
(MAIN unit; IC11, pin 6), and pass through the low-pass fil-
ter (MAIN unit; IC5, pin 1). The amplified signals are applied
to the D/A converter (MAIN unit; IC6, pin 4)
The output signals from the D/A converter (MAIN unit; IC6,
pin 3) are applied to the modulation circuit (MAIN unit; D18).
AF mute
(IC406)
Base band IC
(IC10)
"DET" AF signal
from FM IF IC (IC1, pin 9)
5
20
Scrambler IC
(IC14)
LPF
AF
volume
AF
IC405
Speaker
(SP1)
IC403
18 18
Analog switch
(IC13)
8
11
10
9
19
15
• AF AMPLIFIER CIRCUIT