Theory of Operation
2.3.2 Transmit Audio Circuits
Refer to Figure 2-3 for the descriptions that follow.
J0451
J0551
'18
31 IN
FLAT
TX
RTN
.-------~;-----'
3
2l
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.
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