Ptt Sensing And Tx Audio Processing; Transmit Signalling Circuits - Motorola MCX600E Service Manual

4.3

PTT Sensing and TX Audio Processing

Mic PTT coming from the Control Head via connector J101-4 is sensed by the µP U0101 pin 22. An
external PTT can be generated by grounding pin 3 on the accessory connector. When microphone
PTT or external PTT is sensed, the µP will always configure the ASFIC for the internal mic audio
path.
Inside the ASFIC, the MIC audio is filtered to eliminate frequency components outside the 300-
3000Hz voice band, pre-emphasized if pre-emphasis is enabled. The capacitor between ASFIC pre-
emphasis out U0201-C8 and ASFIC limiter in U0201-E8 AC couples the signal between ASFIC
blocks and prevents the DC bias at the ASFIC output U0201-H8 from shifting when the ASFIC
transmit circuits are powered up. The signal is then limited to prevent the transmitter from over
deviating. The limited MIC audio is then routed through a summer which, is used to add in signalling
data, and then to a splatter filter to eliminate high frequency spectral components that could be
generated by the limiter. The audio is then routed to two attenuators, which are tuned in the factory
or the field to set the proper amount of FM deviation. The TX audio emerges from the ASFIC at
U0201-H8 MOD IN, at which point it is routed to the RF section.
5.0

Transmit Signalling Circuits

Refer to Figure 4.2 for reference for the following sections. From a hardware point of view, there are
three types of signalling:
1. Sub-audible data (PL/DPL/Connect Tone) that gets summed with transmit voice or signalling,
2. DTMF data for telephone communication in trunked and conventional systems, and
3. Audible signalling including Select 5, MPT-1327, MDC, High speed Trunking.
NOTE: All three types are supported by the hardware while the radio software determines which
signalling type is available.
MICRO
CONTROLLER
U0101
Theory of Operation
HIGH SPEED
G1
5
CLOCK IN
7 G2 DTMF
CLOCK
ASFIC U0201
6
C3 LOW SPEED
CLOCK
Figure 4.2 Transmit Signalling Paths
HS
SUMMER
5-3-2 STATE
ENCODER
SPLATTER
FILTER
DTMF
ENCODER
PL LS
ENCODER SUMMER
ATTENUATOR
Transmit Signalling Circuits
TO RF
H8
SECTION
MOD IN
(SYNTHESIZER)
GEPD 5433
4-11