Motorola PM400 Service Manual download pdf (Page 176)

Transmit Signalling Circuits 2-17

8.0 Transmit Signalling Circuits

Figure 2-8 Transmit Signalling Path

From a hardware point of view, there are 3 types of signaling:

• Sub-audible data (PL / DPL / Connect Tone) that gets summed with transmit voice or

signaling,

• DTMF data for telephone communication in trunked and conventional systems, and

• Audible signaling including MDC and high-speed trunking.

Note: All three types are supported by the hardware while the radio software determines which

signaling type is available.

8.1 Sub-Audio Data (PL/DPL)

Sub-audible data implies signaling whose bandwidth is below 300 Hz. 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 300 Hz, so these tones are never heard in the actual system.

Only one type of sub-audible data can be generated by U504 (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 U504-pin 18 at twelve times the desired data rate.

For example, for a PL frequency of 103 Hz, the frequency of the square wave would be 1236 Hz.

This drives a tone generator inside U504 which generates 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 U504-pin 40 (MOD IN), where it is sent to

the RF board as previously described for transmit audio. A trunking connect tone would be

generated in the same manner as a PL tone.

MOD IN

TO RF

SECTION

(SYNTHESIZER)

HIGH SPEED

CLOCK IN

(HSIO)

LOW SPEED

CLOCK IN

(LSIO)

ASFIC_CMP U504

MICRO

CONTROLLER

U403

SUMMER

5-3-2 STATE

ENCODER

DTMF

ENCODER

SPLATTER

FILTER

ENCODER

SUMMER

ATTENUATOR

SPI

BUS

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Motorola PM400 Service Manual Page 176

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