Motorola SSETM 5000 service manual Mode of Operation, Receiving

2-2 Theory of Operation: Mode of Operation

2.2Mode of Operation

This section provides an overview of the radio’s receive and transmit operation in the analog mode.

2.2.1Receiving

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).

RF Input RX Front End

Harmonic

Filter

Remote

Port

Harmonic	Antenna	Preselector		Preselector	1st LO
Filter	Switch	Filter	LNA	Filter		1st

Mixer

XTAL	ABACUS III - RX Back End
Filter

	3
ADC	RX_SSI_ DATA
	to VOCON Board

LO CKO

MAEPF-27278-B

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.

November 11, 2004 6881094C12-A