Transmit Audio Circuits - Motorola 6864115B62-C Detailed Service Manual

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2.2

Transmit Audio Circuits

Refer to Figure 3-1 for reference for the following sections.
FLAT
TX RTN
EXPANSION BOARD
MIC
CONTROL HEAD
CONNECTOR
EXT MIC
FLAT TX
AUDIO
ACCESSORY
CONNECTOR
2.2.1 Mic/Data Input Path
The radio supports 2 distinct microphone paths known as internal (from controlhead) and external
mic (from accessory connector J0501-2) and an auxiliary path (FLAT TX AUDIO, from accessory
connector J0501-5). The microphones used for the radio require a DC biasing voltage provided by a
resistive network.
These two microphone audio input paths enter the ASFIC CMP at U0221-48 (external mic) and
U0221-46 (internal mic). Following the internal mic path; the microphone is plugged into the radio
controlhead and is connected to the controller board via J0401-9.
From here the signal is routed via R0409 and line INT MIC to R0205. R0201 and R0202 provide the
9.3VDC bias. Resistive divider R0205 / R0207 divide the input signal by 5.5 and provide input
protection for the CMOS amplifier input. R0202 and C0201 provide a 560 ohm 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.
C0204 serves as a DC blocking capacitor. The audio signal at U0221-46 (TP0221) should be
approximately 14mV for 1.5kHz or 3kHz of deviation with 12.5kHz or 25kHz channel spacing.
The external microphone signal enters the radio on accessory connector J0501 pin 2 and is routed
via line EXT MIC to R0206. R0203 and R0204 provide the 9.3VDC bias. Resistive divider R0206 /
R0208 divide the input signal by 5.5 and provide input protection for the CMOS amplifier input.
R0204 and C0202 provide a 560 ohm 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.
J0451
18
J0401
TP0221
9
J0501
TP0222
2
5
U0211-4
FROM
µP Pin3
Figure 3-1 Transmit Audio Paths
44
TX SND
MIC
INT
46
ASFIC_CMP
MIC
IN
U0221
LIMITER
MIC
EXT
48
HS SUMMER
42
SPLATTER
FILTER
AUX
TX
LS SUMMER
THEORY OF OPERATION
J0551
31 IN/OUT
OPTION
39
OUT
BOARD
33
IN
OUT
32
36
TX RTN
FILTERS AND
PREEMPHASIS
VCO
MOD IN
ATN
40
ATTENUATOR
TO
RF
SECTION
(SYNTHESIZER)