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: generate a transmitter compensation filter at least by (i) delivering, to the transmitter, complex multi-sine signals for transmission over a first plurality of observed frequencies within a predetermined baseband frequency range, (ii) estimating a first plurality of frequency responses that compensate for in-phase and quadrature (IQ) imbalance at the first plurality of observed frequencies within the predetermined baseband frequency range, and (iii) determining, using the first plurality of frequency responses and by fitting the first plurality of frequency responses to a polynomial function, a transmitter polynomial surface, and compensate, using the transmitter compensation filter, at least one of the signals to be transmitted by the transmitter to reduce IQ imbalance in the transmitted signals, including using the transmitter polynomial surface to calculate a frequency response that reduces the IQ imbalance in the transmitted signals. 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: generate a transmitter compensation filter at least by (i) delivering, to the transmitter, complex multi-sine signals for transmission over a first plurality of observed frequencies within a predetermined baseband frequency range, (ii) estimating a first plurality of frequency responses that compensate for in-phase and quadrature (IQ) imbalance at the first plurality of observed frequencies within the predetermined baseband frequency range, and (iii) determining, using the first plurality of frequency responses and by fitting the first plurality of frequency responses to a polynomial function, a transmitter polynomial surface, and compensate, using the transmitter compensation filter, at least one of the signals to be transmitted by the transmitter to reduce IQ imbalance in the transmitted signals, including using the transmitter polynomial surface to calculate a frequency response that reduces the IQ imbalance in the transmitted signals. generate a transmitter compensation filter at least by (i) delivering, to the transmitter, complex multi-sine signals for transmission over a first plurality of observed frequencies within a predetermined baseband frequency range, (ii) estimating a first plurality of frequency responses that compensate for in-phase and quadrature (IQ) imbalance at the first plurality of observed frequencies within the predetermined baseband frequency range, and (iii) determining, using the first plurality of frequency responses and by fitting the first plurality of frequency responses to a polynomial function, a transmitter polynomial surface, and compensate, using the transmitter compensation filter, at least one of the signals to be transmitted by the transmitter to reduce IQ imbalance in the transmitted signals, including using the transmitter polynomial surface to calculate a frequency response that reduces the IQ imbalance in the transmitted signals. <EOS>