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GTX LTR/Privacy Plus 800 MHz Mobile Service Manual
Transmit Signalling Circuits
See Figure 8-4 for reference for the following sections.
From a hardware point of view, there are three types of
signalling:
• Sub-audible data (PL/DPL/LST/Connect tone)
that gets summed with transmit voice or signal-
ling,
• DTMF data for telephone communication in
trunked and conventional systems, and
• Audible signalling High speed Trunking.
NOTE
The hardware supports all three types
while the radio software determines
which signalling type is available.
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
U201 (ASFIC) at any one time. The process is as fol-
lows, using the SPI BUS, the µP programs the ASFIC 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
MICRO
CONTROLLER
U101
June, 2000
ASFIC U201
G1
HIGH SPEED
5
CLOCK IN
7
G2
DTMF
CLOCK
LOW SPEED
6
C3
CLOCK
Figure 8-4. Transmit Signalling Paths
6880905Z99-O
Controller Audio & Signaling Circuits
PL CLK U201-C3 at twelve times the desired data rate.
For example, for a PL frequency of 103 Hz, the fre-
quency of the square wave would be 1236 Hz.
This drives a tone generator inside U201 which gener-
ates a staircase approximation to a PL sine wave or
DPL data pattern. This internal waveform is then low-
pass filtered and summed with voice or data. The
resulting summed waveform then appears on U201-
H8 (MOD IN), where it is sent to the RF board as pre-
viously described for transmit audio. A trunking con-
nect tone would be generated in the same manner as a
PL tone.
High Speed Data
High speed data refers to the 3600 baud data wave-
forms, known as Inbound Signalling Words (ISWs)
used in a trunking system for high speed communica-
tion between the central controller and the radio. To
generate an ISW, the µP first programs the ASFIC
(U201) to the proper filter and gain settings. It then
begins strobing U201-G1 (TRK CLK IN) with a pulse
when the data is supposed to change states. U201's 5-
3-2 State Encoder (which is in a 2-state mode) is then
fed to the post-limiter summer block and then the
splatter filter.
From that point it is routed through the modulation
attenuator and then out of the ASFIC to the RF board.
Microphone audio is muted during High Speed Data
signalling.
Dual Tone Multiple Frequency (DTMF) Data
DTMF data is a dual tone waveform used during
phone interconnect operation. It is the same type of
tones which are heard when using a "Touch Tone" tele-
phone.
HS
SUMMER
5-3-2 STATE
ENCODER
SPLATTER
FILTER
DTMF
ENCODER
LS
PL/DPL/LST
SUMMER
ENCODER
ATTENUATOR
Theory of Operation
H8
MOD IN
TO RF SECTION
(SYNTHESIZER)
8-11
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