Codan Radio Communications CASC165A Manual De Usuario

Cascade Transceiver

Cascade System Modules Operation Guide

OG-CASC-SYS-MOD-1-0-0P

TRANSMITTER THEORY OF OPERATION

The transmitter portion of the CASCADE transceiver is a linearized amplifier capable of the following 

modulation schemes:

•  Narrowband Analog FM (12.5kHz channel)
•  P25 Phase 1 (C4FM)
•  LSM (CQPSK)
•  P25 Phase 2 (π/4-HDQPSK)

To facilitate these different modulation schemes, the transmitter is comprised of the following 

sections:

•  Baseband processing
•  Cascaded local oscillator synthesis
•  RF amplification
•  Cartesian feedback linearization

The baseband processing converts the digital data into analog I/Q signals to drive the RF chain. The I 

and Q signals contain the modulation information that is used by the Cartesian feedback linearization 

to modulate the cascaded local oscillators and generate the final RF output.

Two high performance RF synthesizers are used to generate the local oscillator for the transmitter. 

These synthesizers are cascaded together to allow for easy integer boundary spur steering. The first 

oscillator is used as a tunable reference for the second oscillator which generates the RF LO at twice 

the RF output frequency.

The output of the RF LO is differential; the signal is kept at a high level to maintain the high phase 

noise performance of the synthesizer and is then attenuated to a level that is acceptable to that of the 

Cartesian feedback linearizer.

Cartesian Loop

The Cartesian Loop is an analog linearization technique. Analog I and Q signals are used to modulate 

a local oscillator to generate the RF output. A portion of this output power is fed back into the 

CMX998 (Cartesian feedback loop transmitter IC) and downmixed to baseband. 

This baseband signal consists of the original transmitted signal plus any non linearities associated 

with the external circuitry. The baseband signal from the feedback port is subtracted from the original 

input signal to get an inversion of the non linearities which is then added to the original input signal to 

compensate for the non-linearities in the external circuitry.