Precise Measurement of Complex Coefficient Filters by Using Square Waves†

Usually, the frequency characteristics of the complex filter can be examined by applying a pair of quadrature sinusoidal waves ( and ) to a FUT (filter under test). While this method is recognized for its accuracy, it requires two sinusoidal test signals with tightly matched amplitudes and a strict phase shift over a broad frequency range. This becomes particularly challenging at higher frequencies. To address this issue, this paper proposes a new measurement method for the complex filter. Instead of sinusoidal waves, the proposed method adopts square waves which can be easily generated by using a digital divider as the test signals to the FUT. The output signals are observed using a spectrum analyzer. Since the Fourier‐series expansion of a square wave is decomposed into its fundamental and harmonic components, the frequency response is measured from the fundamental component using a spectrum analyzer equipped with a high‐resolution bandpass filter. This paper describes the proposed square‐wave method from theoretical aspects. Through experiment, the validity of the proposed method is confirmed. © 2025 The Author(s). IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.