Automatic calibration of the quadrature balance within a cartesian amplifier

A Cartesian amplifier system comprising:

• an input for an input signal to be amplified;
• a power amplifier, having an output, for amplifying the input signal;
• means for resolving said input signal into "real" and "imaginary" components;
• a Cartesian loop feedback circuitry around the power amplifier having a pair of channels for processing "real" and "imaginary" quadrature signal components, means for summing said processed real and imaginary signals, and means to drive the power amplifier with one or more feedback signals derived from the summed real and imaginary signals from the respective outputs of forward paths of those channels,
• wherein the amplifier system is operative in a first, normal mode to amplify the input signal and a second, calibration mode to apply test signals to drive the Cartesian loop in place of the input signal;
• wherein the amplifier system comprises means for measuring the response of the Cartesian loop to the test signals and for calculating and setting a value or values of at least one predistortion factor, and means for measuring the response of the Cartesian loop by measuring the signal strength at the output of the power amplifier while the system is operated in the second, calibration mode;
• and wherein
• the Cartesian loop comprises, or is associated with, a signal pre-processor for pre-distorting resolved real and/or imaginary components of the input signal in accordance with the value or values of the at least one predistortion factor.