Method and apparatus for suppressing local oscillator leakage in a wireless transmitter

Abstract

A direct conversion transmitter comprising two local oscillators, an in-phase and quadrature modulating signal and in-phase and quadrature modulators, adds a leakage detector and corrective algorithm with one or more digital to analog converters to generate a corrective signal. The corrective signal corrects for, minimizes, and suppresses local oscillator carrier leakage into the transmitted signal. In accordance with the invention, direct up transmitter carrier leakage is automatically measured at regular intervals during the transmit cycle. A DC correction, or a correction waveform as a function of time, is input to both the 'in-phase' and 'quadrature' modulators at the baseband input. The two correction signals are tuned according to a minimization algorithm to achieve the lowest possible carrier component in the spectral output of the transmitter at its ambient operating conditions and present power output level. As the ambient conditions, such as temperature, and transmitter power level changes, the two correcting input DC currents, or correction waveforms, are further tuned towards the optimal values for that operating condition. Optimization is accomplished by an adaptive feedback technique employed to achieve maximum carrier suppression.