Transmit Signalling Circuits - Motorola GM328 Service Manual

1-14
2.3

Transmit Signalling Circuits

Refer to 1-4 for reference for the following sections.
MICRO
CONTROLLER
U0101
From a hardware point of view, there are 3 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.
2.3.1 Sub-audible Data (PL/DPL)
Sub-audible data implies signalling whose bandwidth is below 300Hz. PL and DPL waveforms are
used for conventional operation and connect tones for trunked voice channel operation. The
trunking connect tone is simply a PL tone at a higher deviation level than PL in a conventional
system. Although it is referred to as "sub-audible data", the actual frequency spectrum of these
waveforms may be as high as 250 Hz, which is audible to the human ear. However, the radio
receiver filters out any audio below 300Hz, so these tones are never heard in the actual system.
Only one type of sub-audible data can be generated by U0221 (ASFIC CMP) at any one time. The
process is as follows, using the SPI BUS, the µP programs the ASFIC CMP to set up the proper low-
speed data deviation and select the PL or DPL filters. The µP then generates a square wave which
strobes the ASFIC PL / DPL encode input LSIO U0221-18 at twelve times the desired data rate. For
example, for a PL frequency of 103Hz, the frequency of the square wave would be 1236Hz.
44
HIGH SPEED
19
CLOCK IN
(HSIO)
82
SPI
BUS
85
80 18 LOW SPEED
CLOCK IN
(LSIO)
Figure 1-4 Transmit Signalling Paths
Controller Board Audio and Signalling Circuits
HS
SUMMER
5-3-2 STATE
ENCODER
SPLATTER
FILTER
DTMF
ENCODER
ASFIC_CMP U0221
PL LS
ENCODER SUMMER
ATTENUATOR
TO RF
40
SECTION
MOD IN
(SYNTHESIZER)