Abstract: An apparatus and method for local oscillator calibration compensates for filter passband variation in a mixer circuit, such as a receiver circuit. The receiver includes at least a mixer circuit and a filter coupled to the output of the mixer. During operation, the mixer mixes an RF input signal with a first local oscillator (LO) signal to frequency translate a selected channel in the RF input signal into the passband of the filter. During a calibration mode, the RF input signal is disabled, and the first LO signal is injected into the filter input by leaking the first LO signal through the mixer circuit. The frequency of the LO signal is then swept over a frequency bandwidth that is sufficiently wide so that the actual passband is detected by measuring the signal amplitude at the output of the bandpass filter, thereby determining any variation in the passband of the filter from the expected passband.