An apparatus includes a receiver chain includes an input node and a transmitter chain comprising a tap and output nodes. An auxiliary transmitter chain comprises an auxiliary input node coupled to the tap node, an adaptive filter unit and a signal output coupled to the input node. The filter unit includes an interpolation processor, and an adaptive filter and a coefficient processor coupled to the receiver chain and the adaptive filter. The transmitter chain generates a first waveform comprising a plurality of frequency-separated signals. Contemporaneously, the auxiliary chain applies a second waveform comprising another plurality of frequency-separated signals at the input node. The receiver chain receives a composite waveform comprising a waveform coupled from the output node and the second waveform. The first waveform lacks signals required for determination of coefficients of the filter whose frequencies coincide with the another plurality of signals of the second waveform.