Icom ID-800H, Руководство по обслуживанию, Страница: 14 / 59

FM IF IC
IC8
AM/FM
switch
AGC
AF signal
D19–D21,
D525
Antenna
IF
RF
RF
RF RF
RF
RF
AM
DET
RX
SW
RX
SW
RX
SW
IC5
Q13,
Q16
Q18
D8
Q38
Q32
Q35
IC12
Q39
Q36
Q40
Q41 Q37
D9
D10
1st
mixer
Narrow
IC10
IC9
Wide
FI2
FI1
• AGC CIRCUIT
4 - 4
A portion of the AF signals from the AM/FM switch (IC5, pin
1) pass through the low-pass filter (Q3) to remove AF (voice)
signals, and are then applied to the CTCSS/DTCS decoder
inside the main CPU (IC505, pin 29) via the “DTCS_IN” line.
Then the main CPU analyzes the decoded tone signals and
output the AF mute signal as "AF_MUTE" from pin 55 to
the AF mute switch (Q512).
4-1-9 AGC CIRCUIT (MAIN UNIT)
The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain to keep the audio output at a constant level during
AM mode operation. The receiver gain is determined by the
voltage on the AGC line (Q13, pin 4).
The signal from the AM detector circuit (Q13) is detected at
the AGC detector (D19–D21). When receiving strong sig-
nals, the detected voltage increases and the AGC voltage
decreases via the AGC circuit. The AGC voltage is used for
the bias voltage of the receive switching PIN diodes to atten-
uate the received signals. Therefore, this transceiver keeps
the audio output at a constant level.
4-1-10 S-METER CIRCUIT (MAIN UNIT)
The S-meter circuit indicates the relative received signal
strength while receiving.
The S-meter signal from the FM IF IC (IC8, pin 12) is
applied to the main CPU (IC505, pin 40) via the “RSSI” sig-
nal line.
The S-meter signal is applied to the sub CPU and is then
displayed on the LCD.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals from
the microphone to a level needed at the modulation circuit.
The microphone amplifier circuit is commonly used for both
the VHF and UHF bands.
• AUDIO SIGNALS
The AF signals from the microphone are amplified at the
microphone amplifiers (Q505, IC506), and are then applied
to the microphone level controller (IC509, pins 1, 2). The
level adjusted AF signals
While operating DV mode, the level adjusted microphone AF
signals from the microphone level controller (IC509, pin 1)
are amplified at the ALC amplifier (IC513, pins 3, 5) via the
ALC switch (IC512, pins 1, 7) and are then applied to the
IDC amplifier (IC3A, pin 1, 3).
While operating in FM mode, the level adjusted microphone
AF signals from the mic level controller are applied to the
IDC amplifier (IC3A, pins, 1, 3) via the ALC switch (IC512,
pins, 1, 6).
The amplified signals from the IDC amplifier (IC3A, pin 1)
are passed through the low-pass filter (IC3D, pins 13, 14)
to suppress unwanted components and then applied to the
buffer amplifier (IC3C, pins 8, 9).
During DV mode operation, the buffer amplified signals from
the buffer amplifier (IC3C, pin 8) are applied to the CODEC
circuit as "DAFIN" signals via J507 (pin 5).
During FM mode operation, the buffer amplified signals from
the buffer amplifier (IC3C, pin 8) are applied to the D/A con-
verter (IC4, pins 13, 14) to adjust the modulation level. The
level adjusted signals from the D/A convertor (IC4, pin 13)
are applied to the VHF or UHF modulation circuits separate-
ly via the analog switch (IC517, pins 1, 6).
• PACKET DATA SIGNALS
The packet data signals from the 8-pin jack (J504, pin 1)
pass through the limiter circuit (D524) to limit the level.
When setting to 9600 bps mode, the signals pass through
the analog switch (IC509, pins 3, 4), and then pass through
the switch again (pins 8, 9).
The switched signals are amplified at the buffer amplifier
(IC3C, pins 8, 9), and are then passed through the D/A con-
verter (IC4, pins 13, 14).