Transmit Signalling Circuits - Motorola MCS 2000 Service Instructions Manual

Auxiliary TX Audio
Paths
TX Secure Audio
(AUX TX IN1 and
AUX TX IN3)
Transmit
Signalling Circuits
capacitor between ASFIC pre-emphasis out U0200-C8 and ASFIC limiter in
U0200-E8 AC couples the signal between ASFIC blocks and prevents the DC
bias at the ASFIC output U0200-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,
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 U0200-H8 AUDIO MOD, at which point it is routed to the RF
section.
There are three auxiliary transmit audio inputs that are routed to the auxiliary
transmit path in the ASFIC. These are AUX TX IN1, AUX TX IN2, and AUX TX
IN3. The 3 paths, only one of which can be active at a time, are buffered by
U0202.
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.
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.
(Refer to Figure 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.
Controller Section Theory of Operation 7-19