Transmit Audio - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

3-74
The audio power amplifier (U0204), is a DC-coupled-output, bridge-type amplifier. The gain is
internally fixed at 40 dB. Speaker audio leaves U0204 on pins 4 and 6. For dash-mount radios, the
audio is routed to the speaker via J0402, pins 19, 21, 23, and 25, and then to J2, pins 20 and 26. The
amplifier is biased to one-half of the A+ voltage. An audio isolation transformer must be used if
grounded test equipment, such as an audio analyzer or service monitor, is to be connected to the
speaker outputs.
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C a u t i o n
Normally, R0220 pulls up U0204, pin 8, to SW_B+ to enable the audio PA. When the radio is
squelched, the audio PA is muted by the microprocessor. Q0200 is enabled to provide approximately
4.5 V through a voltage divider (Q0200 and R0222) to U0204, pin 8, which mutes the audio PA.
When SW_B+ is turned off, the voltage from SW_A+ on U0204 falls below 2 V, placing the audio PA
in standby, which turns off the U0204 output transistors to the speaker.
voltages present at U0204, pin 8, during its various conditions.

3.8.11 Transmit Audio

The mobile microphone connects to the front of the control head through connector P104.
Microphone audio (MIC_HI) enters the main board via J0401, pin 4, and is routed to multiplexer
U0209 (refer to
for the microphone's internal circuitry.
The multiplexer allows TX modulation audio to be routed from one of 3 possible sources: mobile
microphone (MIC_HI), AUX_MIC (audio sourced from the J2 rear accessory connector in motorcycle
configuration), or AUX_TX (audio from a VRS). The resulting audio source is then sent to a two-
stage, programmable gain/attenuation circuit comprised of U0201 and U0202. The gain is
accomplished via a programmable digital potentiometer in the amplifier circuit. This gain is
adjustable via CPS and is programmed in ±3 dB steps. After passing through an anti-aliasing filter,
audio is sent to the input of the CODEC (U0200) where it is digitized via its internal A/D converter.
The digital audio data is then sent via the SAP SSI bus to the DSP at the Patriot IC. As with speaker
data samples, the DSP reads the microphone audio samples and processes, pre-emphasizes,
filters, and adds signaling information to this data. As with the received trunking data, low-speed
transmit data is processed by the MCU and returned to the DSP. For secure messages, the digital
signal data can be passed to the secure module prior to DSP processing and modulation. The DSP
transfers the data to and from the secure module through the SAP SSI transmit (TXD) and receive
(RXD) lines.
May 25, 2005
The speaker outputs must NOT be grounded in any way.
Table 3-16. PA Condition Voltages at U0204, Pin 8
Power Amplifier
Condition
Standby
Mute
Enabled
Figure 3-50 on page 3-75). Resistors R0200 and R0204 provide 9.6 Vdc bias voltage
Theory of Operation: Controller Section
Table 3-16
U0204 Pin 8
(Vdc)
0-2
3.3-6.4
8.5-17
shows the
6881096C74-B