Motorola pro7100 Manuel de service Page 23

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Theory of Operation 2-9
2.3.2 Transmit Audio Circuits
Refer to Figure 2-3 for the descriptions that follow.
Figure 2-3. Transmit Audio Paths
2.3.3 Microphone Input Path
The radio supports two microphone input paths. One from the control head external microphone
accessory connector J0501, pin 2, and one from the microphone auxiliary path (FLAT TX AUDIO) via
accessory connector J0501, pin 5. The microphones require a DC biasing voltage provided by a
resistive network.
The two microphone audio input paths enter the ASFIC CMP at U0221, pin 48 (external microphone)
and U0221, pin 46 (auxiliary microphone).
The microphone is plugged into the radio control head which is connected to the controller board via
J0401, pin 9. The signal is then routed via R0409 and line INT MIC to R0205. Resistors R0201 and
R0202 provide 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.
Capacitor C0204 provides dc blocking. The audio signal at U0221, pin 46 (TP0221) is 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 5, then it is
routed via line EXT MIC to resistor R0206. Resistors R0201 and R0204 provide a 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 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. Capacitor C0254 provides dc blocking.
MIC
IN
MOD IN
TO
RF
SECTION
(SYNTHESIZER)
36
44
33
40
J0501
ACCESSORY
CONNECTOR
J0401
CONTROL HEAD
CONNECTOR
MIC
EXT MIC
FLAT TX
AUDIO
42
32
5
48
46
9
2
IN
OUT
OPTION
BOARD
FILTERS AND
PREEMPHASIS
HS SUMMER
SPLATTER
FILTER
LS SUMMER
LIMITER
ATTENUATOR
VCO
ATN
TX RTN
TX SND
MIC
INT
AUX
TX
ASFIC_CMP
U0221
TP0221
TP0222
MIC
EXT
J0451
J0551
18
FLAT
TX RTN
EXPANSION BOARD
31
IN
39
OUT
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