A detector device for calibrating a digital filter to replicate a transfer function of a signal processing apparatus, comprising: a first input to receive a first signal; a second input configured to receive a response signal of the signal processing apparatus to the first signal; a controllable FIR filter; a comparison-block to compare the phase and amplitude after a correction has been applied by the controllable FIR filter; a feedback loop; and an interpolation-block; wherein the at least one detector is configured to determine, at least, the feedback control signal at a first frequency and at a second frequency, and wherein the interpolation-block is configured to interpolate to determine calibration information for programming of the transfer function of said digital filter.