Transmit Signalling Circuits
Theory of Operation 4-11
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.
Figure 4.2
Transmit Signalling Paths
G1
C3
G2
H8
MOD IN
TO RF
SECTION
(SYNTHESIZER)
GEPD 5433
6
7
5
HIGH SPEED
CLOCK IN
LOW SPEED
CLOCK
DTMF
CLOCK
ASFIC U0201
MICRO
CONTROLLER
U0101
HS
SUMMER
5-3-2 STATE
ENCODER
DTMF
ENCODER
SPLATTER
FILTER
PL
ENCODER
LS
SUMMER
ATTENUATOR
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