A communication unit includes: a quadrature transmitter having analog transmit filter(s) for filtering a first quadrature test signal. An analog feedback loopback path selectively first routes the filtered quadrature first test signal to a quadrature receiver. The quadrature receiver has: at least one analog receive filter for further filtering the filtered quadrature first test signal; and a quadrature receive baseband circuit arranged to receive and decode the further filtered quadrature first test signal. The quadrature transmitter is arranged to receive a second quadrature test signal and the analog feedback loopback path selectively routes a filtered quadrature second test signal to the quadrature receiver via a second route such that the quadrature receive baseband circuit is arranged to determine a frequency-dependent quadrature imbalance of at least one component in the transmitter/receiver based on the decoded further filtered first quadrature test signal and the decoded further filtered second quadrature test signal.