Transmitter Circuits - Icom ID-800H Service Manual

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.
• AGC CIRCUIT
D19–D21,
Q13,
D525
Q16
AGC
AM
DET
AM/FM
AF signal
switch
IC5
Narrow
IC10
IC9
FI1
FI2
Wide
FM IF IC
IC8
Q18
• 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).
1st
D8
mixer
RX
SW
Q32 Q38
D9
RX
SW
Q39
Q35
D10
RX
IF
SW
Q36
IC12
Q37
4 - 4
Antenna
RF
RF
RF RF
Q40
RF
RF
Q41