Transmit Audio Circuits; Mic Input Path - Motorola GM-950 Manual

Transmit Audio Circuits

4.0 Transmit Audio Circuits
Refer to Figure 3.1 for reference for the following sections.
4.1

Mic Input Path

The radio supports two distinct microphone paths known as internal (from Control Head) and
external mic (from accessory connector J0400-2) and an auxiliary path (FLAT TX AUDIO). 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 at U0201-A7 (external mic) and U0201-
B8 (internal mic). Following the internal mic path; the microphone is plugged into the radio control
head and is connected to the controller board via J101-16.
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.
MIC
CONTROL HEAD
CONNECTOR
EXT MIC
FLAT TX
AUDIO
ACCESSORY
CONNECTOR
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.
5C.3-10
J0101
16
J0400
2
5
Figure 3.1 Transmit Audio Paths
J0103-3
A6
MIC AMP OUT
MIC
B8
IN
MIC
IN
EXT
MIC
IN
A7
LS SUMMER
D7
SPLATTER
AUX
TX IN
HS SUMMER
PRE EMP OUT
C8
IN
OPTION
BOARD
OUT
J0103-1
C7
TX IN
FILTERS AND
PREEMPHASIS
ASFIC U0201
FILTER
VCO
ATN
H8
LIMITER
ATTENUATOR
LIM IN
E8
Theory of Operation
MOD IN
TO
RF
SECTION
(SYNTHESIZER)
GEPD 5426-1