We present a generic method for determining transfer function of a DUT using simple time-domain computations in place of conventional computationally intensive frequency domain methods. Our method uses a chirp stimulus and an average of absolute value to extract energy in different frequency bins. For an N0-size spectrum, this enables a reduction of computational complexity from N0*ln(N0) multiply-accumulates (using an FFT) to N0 accumulates. This makes it an attractive strategy for BIST. We present the theoretical basis, test case simulations, and a comparison with the FFT method.