Motorola MCX600E, Service Manual, Page: 48 / 105

Transmit Audio Circuits
4-10 Theory of Operation
From here the signal is routed to R0206. R0204 and R0205 provide the 9.3VDC bias and R0206
provides input protection for the CMOS amplifier input. R0205 and C0209 provide a 1kohm AC path
to ground that sets the input impedance for the microphone and determines the gain based on the
emitter resistor in the microphone’s amplifier circuit.
Figure 4.1 Transmit Audio Paths
Filter capacitor C0210 provides low-pass filtering to eliminate frequency components above 3 kHz,
and C0211 serves as a DC blocking capacitor. The audio signal at U0201-B8 should be
approximately 80mV for 1.5kHz or 3kHz of deviation with 12.5kHz or 25 kHz channel spacing.
The FLAT TX AUDIO signal from accessory connector J0400-5 is buffered by op-amp U0202-1 and
fed to the ASFIC U0201-D7 through C0205.
4.2 External Mic Path
The external microphone signal enters the radio on accessory connector J0400 pin 2 and connects
to the standard microphone input through R0413. Components R0414 - R0416, C0413, C0414,
C0417 are not used.
MIC
IN
MOD IN
TO
RF
SECTION
(SYNTHESIZER)
C7
A6
J0103-3
E8
C8
H8
J0400
ACCESSORY
CONNECTOR
J0101
CONTROL HEAD
CONNECTOR
MIC
EXT MIC
FLAT TX
AUDIO
D7
J0103-1
5
A7
B8
16
GEPD 5426-1
2
IN
OUT
OPTION
BOARD
FILTERS AND
PREEMPHASIS
LS SUMMER
SPLATTER
FILTER
HS SUMMER
LIMITER
ATTENUATOR
VCO
ATN
TX IN
MIC AMP OUT
MIC
IN
EXT
MIC
IN
AUX
TX IN
PRE EMP OUT LIM IN
ASFIC U0201
Figure 1a Transmit Audio Paths (Open Architecture Radios)
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