Transmit Audio Circuits; Microphone Input Path; Figure 2-3. Transmit Audio Paths - Motorola CDM1550 LS+ Detailed Service Manual

Theory of Operation
2.3.2 Transmit Audio Circuits
Refer to Figure 2-3 for the descriptions that follow.
J0451
J0551
'1 8
31 IN
FLAT
TX RTN
.-------~;-----'3 2 l 9
()UT
~6~~~
MIC
EXT MIC
FLAT TX
AUDIO
EXPANSION BOARD
Jil IN
J0401
Figure 2-3. Transmit Audio Paths
2.3.3 Microphone Input Path
OUT
MOD IN TO
40 RF
sEonm;
(SYNTHESIZER)
2-9
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 de blocking. The audio signal at U0221, pin 46 (TP0221) is approximately
14mV for 1.5kHz deviation.
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
I
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 de blocking.