Test instruments and methods for compensating iq imbalance

A test instrument comprising:

• a transmitter configured to transmit signals to a unit under test;
• a receiver configured to receive signals from the unit under test;
• and
• a controller configured to reduce in-phase and quadrature (IQ) imbalance of the signals to be transmitted and received, at least by:
• transmitting complex multi-sine signals over a first plurality of observed frequencies within a predetermined baseband frequency range,
• receiving the transmitted multi-sine signals over a second plurality of observed frequencies within the predetermined baseband frequency range, the second plurality of observed frequencies being offset from the first plurality of observed frequencies,
• estimating corresponding sets of frequency responses for the transmitted and received signals that compensate for IQ imbalance at the first and second pluralities of observed frequencies within the predetermined baseband frequency range,
• determining, using the sets of frequency responses, polynomial surfaces,
• approximating each of the polynomial surfaces with a corresponding one of transmitter and receiver multi-variable polynomials, and
• reducing the IQ imbalance of the signals to be transmitted and received based on frequency responses calculated using the transmitter and receiver multi-variable polynomials.