Motorola ASTRO Digital Spectra, Instruction Manual

Page: 43 / 114

Theory of Operation: Audio Interface Board
6881086C22-B 21
Audio Interface
Board
General Description
The TRN7391C Audio Interface board (AIB) serves as the central interface for
all internal and external components of the ASTRO Spectra/Spectra Plus
Consolette. All signals are routed through the AIB where processing may
occur. Most audio paths incorporate some processing, while most control and
data signals are passed unaltered.
Multiple transmit and receive audio paths provide for numerous configurations
of the ASTRO Spectra/Spectra Plus Consolette. All audio signals passing
through the audio interface board are baseband signals in the range of
300-3000 Hz.
Most data and control signals are provided by 0-5V protected CMOS logic
circuitry. PTT* and MONITOR* are provided for external control of transmit and
receive functions. The ASTRO radio Vehicle Interface Ports (VIPS) are also
available at Accessory Connector 2 (W7 models only).
Detailed Description
Receive Audio Paths
Two sources from the ASTRO radio are used for receive audio. Detect audio
(DET_AUD) is located on J1 pin 17, and filtered audio (FILT_AUD) is located
on J1 pins 11 and 36. Both DET_AUD and FILT_AUD are at a fixed level
(approx. 300 mVrms); however, DET_AUD has a DC voltage level associated
with it, while FILT_AUD has no DC voltage. FILT_AUD is also present at
Accessory Connector 2 pin 4 and Accessory Connector 3 pin 4.
Detect audio (DET_AUD) passes to the Audio Interface board via J1 pin 17 at
a level of 300 mVrms, relative to a 1.0 kHz tone at 60% FSD. It is then amplified
by U102 pins 8, 9, and 10 and the level set by potentiometer R111. The level
should be set to attain 1.0 Vrms between RX+ and RX– (Accessory Connector
2 pins 1 and 16, respectively). The audio then passes through an audio
shaping filter (U102 pins 12, 13, and 14), which has unity gain for frequencies
below 3000 Hz. The output of the audio shaping filter (U102 pin 14) is split into
opposing phases by U102 pins 1, 2, 3, 5, 6, and 7 to provide a balanced output.
Q103 and Q104 provide a final amplification stage. The impedance between
RX+ and RX– can be set to 600
Ω
or 900
Ω
using S101-4 and 5. See Table 1
on page 9.
When the radio is in an idle state, the DET_AUD line of the ASTRO Digital
transceiver is muted using the U107 op-amp circuit in combination with
multiplexor U106. U107 compares the voltage level of the SPKR_HI line (U107
pin 3) to a fixed DC voltage level of 4V (U107 pin 2). When the mobile radio
routes received audio to the speaker, the SPKR_HI line measures
approximately 6 Vdc. This causes the output of U107 to activate transistor
Q101 connected to control input A of the two-input multiplexor (U106 pin 11).
When Q101 is active, U106 allows DET_AUD to be passed to the receive
audio circuitry. When Q101 is inactive, the receive audio path is muted.
Receive audio is also routed from U106 gate 1 to buffer amplifier U101 op-
amp 4 (MOB_RX). The output of this buffer drives the TAPE_OUT line at
approximately 190 mVrms, which is used for external recording purposes.