Quadrature modulator and calibration method

A method of calibrating a quadrature modulator, comprising:

• a) applying a first test tone signal to an in-phase modulation branch input of the modulator and a ninety degree phase-shifted version of the first test tone signal to a quadrature modulation branch input of the modulator;
• b) measuring the level of a local oscillator (LO) feed through in an output signal of the modulator and in response adjusting base band dc offset voltages to minimize the LO feed through;
• c) applying a second test tone signal to the in-phase modulation branch input of the Quadrature modulator and a ninety degree phase-shifted version of the second test tone signal to the quadrature modulation branch input;
• and
• d) measuring the level of an undesired upper sideband frequency component in the output signal and in response adjusting base band gains the in-phase and quadrature modulation branches and a LO phase error to minimize the undesired sideband.

A quadrature modulator, comprising:

• a) an in-phase modulation branch receiving as an input an analog in-phase base band signal, the in-phase modulation branch including a first dc offset adjustment circuit, a first base band gain adjustment circuit, and a first mixer;
• b) a quadrature modulation branch receiving as an input an analog quadrature based band signal, the quadrature modulation branch including a second dc offset adjustment circuit, a second base band gain adjustment circuit, and a second mixer;
• c) a local oscillator means for providing a local oscillator signal to the first mixer and a phase shifted version of the local oscillator signal to the second mixer,
• d) a summer for summing the outputs of the first and second mixers;
• e) envelope detection means for detecting an output signal of the modulator and providing a signal representative of the amplitude of the output signal of the quadrature modulator;
• f) band pass filter means for filtering the amplitude signal;
• and
• g) a log detector for measuring the strength of the filtered amplitude signal, the log detector providing a compensation signal for adjusting the phase shift of the local oscillator and the dc offsets and base band gains of the in-phase and quadrature base band signals.