68P81083C20-D December 6, 2004
Controller Section Theory of Operation: Audio and Signalling Circuits 7-19
AUX TX IN2 is special in that it can “change” input impedance. The “voltage mode” signal to U0400
is 9.3 V if the source for the auxiliary transmit audio from J0403 is a current source. The “voltage
mode” signal is 0 V if the auxiliary transmit source is a voltage source. The difference being R0219 is
bypassed by the transmission gate or not. Typically current source mode will apply for SB9600
based devices.
C0217 sets the lower frequency (high pass) signal of approximately 1 Hz. The processing of the
signal on the auxiliary transmit path depends on how the ASFIC is configured. It can bypass filters,
pre-emphasis, limiter, and/or splatter filter.
TX Secure Audio (AUX TX IN1 and AUX TX IN3)
The audio follows the normal transmit audio processing until it emerges from the ASFIC pre-
emphasis out pin (U0200-C8), which is fed to the secure board residing at either option connector
J0401-7 / J0403-7. The Secure board contains circuitry to amplify, digitize, encrypt, and filter the
audio. The encrypted signal is then fed back from J0401-14 / J0408-14 to the AUX TX buffer through
R0217 or R0218, and then to the ASFIC AUX TX input (U0200-D7). The signal level at this pin
should be about 1 Vpp. The signal is then routed through the AUX TX path in the ASFIC (which
bypasses everything before including the ASFIC splatter filter) and summed into the main
modulation path. After the summer, it runs through the modulation attenuator and then to the AUDIO
MOD port U0200-H8.
Transmit Signalling Circuits
(Refer to Table 7-3 for reference for the following sections)
There are four types of transmit data:
1. Sub-audible data (PL/DPL/Connect Tone) that gets summed with transmit voice or signalling,
2. High speed (3600 baud) data for trunking control channel communication
3. DTMF data for telephone communication in trunked and conventional systems, and
4. MDC data for use in Motorola proprietary MDC systems. Select 5 and MPT-1327 signalling
can be supported by the MDC signaling hardware.
Sub-audible Data (PL/DPL)
Sub-audible data implies signalling 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 U0200 at any one time. The process is as
follows, using the SPI BUS, the µP programs the ASFIC (U0200) 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 PL CLK U0200-C3 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 U0200 which generates a staircase approximation to a PL sine
wave or DPL data pattern. This internal waveform is then lowpass filtered and summed with voice or
data. The resulting summed waveform then appears on U0200-H8 (AUDIO MOD), 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.
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