Transmit Audio Circuits; Microphone Input Path - Motorola VHF2 Service Information

Transmit Audio Circuits

7.0 Transmit Audio Circuits
J2
MIC
CONTROL HEAD
CONNECTOR
P1
EXT MIC
FLAT TX
AUDIO
ACCESSORY
CONNECTOR
7.1

Microphone Input Path

The radio supports 2 distinct microphone paths known as internal (from control head J2-15) and
external mic (from accessory connector P1-2) and an auxiliary path (FLAT TX AUDIO, from
accessory connector P1-5). The microphones used for the radio require a DC biasing voltage
provided by a resistive network.
The two microphone audio input paths enter the ASFIC CMP at U504-pin 48 (external mic) and
U504-pin 46 (internal mic). The microphone is plugged into the radio control head and connected to
the audio DC via J2-pin 15. The signal is then routed via C5045 to MUX U509 that select between
two paths with different gain to support Low Cost Mic (Mic with out amplifier in it) and Standard Mic.
7.1.1 Low Cost Microphone
Hook Pin is shorted to Pin 1(9.3V) inside the Low Cost Mic, This routes 9.3V to R429, and creates
2.6V on MIC_SENSE (u.P U403-67) by Voltage Divider R429/R430. U403 senses this voltage and
sends command to ASFIC_CMP U504 to get GCB3 = '0'. The audio signal is routed from C5045 via
U509-5 (Z0), R5072, U507, R5026, C5091, R5014 via C5046 to U504- 46 int. mic (C5046 100nF
creates a159Hz pole with U504- 46 int mic impedance of 16Kohm).
U509
15
MUX
MIC
INT
46
35
2 48
U509
42
5
MUX
38
GCB5
FLAT TX
AUDIO MUTE
Figure 2-7 Transmit Audio Paths
24kOhms
44
TX SND
ASFIC_CMP
MIC
GCB3
IN
U504
LIMITER
MIC
EXT
HS SUMMER
SPLATTER
FILTER
AUX
TX
LS SUMMER
36
TX RTN
FILTERS AND
PREEMPHASIS
VCO
MOD IN
ATN
40
ATTENUATOR
2-15
TO
RF
SECTION
(SYNTHESIZER)