Digital Mode Of Operation; Figure 3-2. Receiver Block Diagram - Motorola Astro XTS 4000 Service Manual

3-2
3.2

Digital Mode of Operation

This section provides an overview of the digital mode receive and transmit theory of operation.
3.2.1 Receiving
When the radio is receiving
Main board, passing through the antenna switch and the receiver front end. The signal is then
filtered, amplified, and mixed with the first local-oscillator signal, generated by the voltage-controlled
oscillator (VCO).
RF Input
Harmonic
Filter
XTAL
Filter
The resulting intermediate frequency (IF) signal is fed to the IF circuitry, where it is again filtered and
passed to the Abacus III digital back-end IC. In the digital back-end IC, the IF signal is mixed with the
second local oscillator to create the second IF at 2.25 MHz. In the back-end IC, a bandpass, sigma-
delta, analog-to-digital converter then decodes the second IF signal, and outputs, on the radio's
serial synchronous interface (SSI) bus, digital audio to the digital signal processor (DSP).
The dual-core processor's digital-signal processor (DSP) digitally filters the PCM audio. The DSP
decodes the information in the signal and identifies the appropriate destination for it.
• For a voice signal, the DSP will route the digital voice data to the CODEC inside the audio and
power supply support IC, for conversion to an analog signal. The CODEC will then present the
signal to the receive audio pre-amplifier, then to the audio power amplifier, which drives the
speaker.
• For signaling information, the DSP will decode the message and pass it internally to the
microcontrol unit of the dual-core processor.
(See Figure 3–2), the signal comes from the antenna connector to the
RX Front End
Tuneable
Pre-Selector
Antenna
Filter
Switch
ABACUS III - RX Back End
LO
Figure 3–2. Receiver Block Diagram
Basic Theory of Operation: Digital Mode of Operation
Tuneable
Post-Selector
LNA Filter
ADC
CKO
1st LO
1st
Mixer
3
RX_SSI_ DATA
to VOCON Section
MAEPF-27278-A