CFBL
The world is going digital. The need for increased data speeds and bandwidth efficiency is ever increasing, driving the move to more complex linear modulation schemes.

Linear (non-constant envelope) modulation schemes, ie. modulation schemes that have significant amplitude content, are a key problem area for the designer. The Tx Power Amplifier (PA) must be inherently linear which implies very high power consumption, not ideal for portable systems; or a Tx linearization scheme must be implemented.

There are a number of methods available to achieve the required transmitter linearization. These methods include: Adaptive Pre-Distortion, Feed-Forward Correction and Cartesian Feed-Back Loop. All have numerous implementation difficulties, are complex to integrate and some are subject to IPR issues.

This is where the Cartesian Feed-Back Loop (CFBL) provides the optimum solution. A CFBL optimises the efficiency and linearity of Tx PA for linear modulation schemes such as: QPSK, CQPSK, 8PSK, pi/4 DQPSK, pi/8 DQPSK, QAM and ODFM.
The CFBL was first developed in the early 1980s and is now established as the optimum solution in high efficiency linear transmitters.