A communication apparatus comprising: a transmitter,TX; a receiver, RX; and a phase-locked loop configured to: generate a TX local oscillator, LO, signal and a RX LO signal, and successively provide a first phase shift and a different second phase shift between the TX LO signal and the RX LO signal, wherein the TX is configured to: generate a reference signal, and obtain a TX radio frequency, RF, signal by quadrature mixing of the reference signal with the TX LO signal, wherein the communication apparatus further comprises a RF loopback path configured to output the TX RF signal to the RX as a RX RF signal, wherein the RX is configured to obtain a representation of the reference signal by quadrature mixing of the RX RF signal with the RX LO signal, and wherein the TX comprises an adaptive filter configured to: based on the reference signal and the representation of the reference signal in case of the first phase shift between the TX LO signal and the RX LO signal, obtain a first estimate of a combined TX/RX frequency-dependent In-phase and Quadrature, I/Q, imbalance in a time domain, based on the reference signal and the representation of the reference signal in case of the second phase shift between the TX LO signal and the RX LO signal, obtain a second estimate of the combined TX/RX frequency-dependent I/Q imbalance in the time domain, and based on the first and second estimates, determine a contribution made by each of the TX and the RX in the combined TX/RX frequency-dependent I/Q imbalance in the time domain. a transmitter,TX; a receiver, RX; and a phase-locked loop configured to: generate a TX local oscillator, LO, signal and a RX LO signal, and successively provide a first phase shift and a different second phase shift between the TX LO signal and the RX LO signal, generate a TX local oscillator, LO, signal and a RX LO signal, and successively provide a first phase shift and a different second phase shift between the TX LO signal and the RX LO signal, wherein the TX is configured to: generate a reference signal, and obtain a TX radio frequency, RF, signal by quadrature mixing of the reference signal with the TX LO signal, generate a reference signal, and obtain a TX radio frequency, RF, signal by quadrature mixing of the reference signal with the TX LO signal, wherein the communication apparatus further comprises a RF loopback path configured to output the TX RF signal to the RX as a RX RF signal, wherein the RX is configured to obtain a representation of the reference signal by quadrature mixing of the RX RF signal with the RX LO signal, and wherein the TX comprises an adaptive filter configured to: based on the reference signal and the representation of the reference signal in case of the first phase shift between the TX LO signal and the RX LO signal, obtain a first estimate of a combined TX/RX frequency-dependent In-phase and Quadrature, I/Q, imbalance in a time domain, based on the reference signal and the representation of the reference signal in case of the second phase shift between the TX LO signal and the RX LO signal, obtain a second estimate of the combined TX/RX frequency-dependent I/Q imbalance in the time domain, and based on the first and second estimates, determine a contribution made by each of the TX and the RX in the combined TX/RX frequency-dependent I/Q imbalance in the time domain. based on the reference signal and the representation of the reference signal in case of the first phase shift between the TX LO signal and the RX LO signal, obtain a first estimate of a combined TX/RX frequency-dependent In-phase and Quadrature, I/Q, imbalance in a time domain, based on the reference signal and the representation of the reference signal in case of the second phase shift between the TX LO signal and the RX LO signal, obtain a second estimate of the combined TX/RX frequency-dependent I/Q imbalance in the time domain, and based on the first and second estimates, determine a contribution made by each of the TX and the RX in the combined TX/RX frequency-dependent I/Q imbalance in the time domain.