Motorola SSETM 5000 - Manual (Page 24)

November 11, 2004 6881094C12-A
2-2 Theory of Operation: Mode of Operation
2.2 Mode of Operation
This section provides an overview of the radio’s receive and transmit operation in the analog mode.
2.2.1 Receiving
When the radio is receiving ( see Figure 2-2 ), the signal travels from the antenna connector to the
transceiver 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).
Figure 2-2. Receiver Block Diagram
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 VOCON board.
On the VOCON board, 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.
RF Input
RX Front End
Harmonic
Filter
ADC
LO CKO
RX_SSI_ DATA
to VOCON Board
XTAL
Filter
ABACUS III - RX Back End
Antenna
Switch LNA
Preselector
Filter
Preselector
Filter 1st
Mixer
1st LO
3
MAEPF-27278-B
Harmonic
Filter
Remote
Port