1. Introduction
RF signals in the frequency range beyond the microwave band is attracting much attention for next-generation wireless communications, so-called beyond 5G [1],[2]. An important milestone for utilizing the frequency region is to evaluate RF signal parameters with high accuracy [3]. The conventional evaluation method based on electric frequency down-conversion would have restrictions of the evaluation accuracy and the minimum detection sensitivity, due to performance of nonlinear electronic devices used for the downconversion. In order to avoid this issue, we have proposed an RF signal evaluation method [4] that uses microwave / millimeter-wave photonics that are also applied to RF signal filtering [5] and RF signal upconvertion into millimeter-wave region [6],[7]. However the proposed method requires a reference RF signal whose frequency is the same as that of the RF signal under test: i.e. a frequency multiplexer composed of a nonlinear electronic device is also required for evaluating the high-frequency RF signal. In this paper, we propose a method that enables us to evaluate a high-frequency signal by use of the beat originating from the harmonics of modulation light-wave, to adopt a low-frequency reference RF signal (source). Based on the model analysis, it has been shown that RF parameter dependence of the obtained low-frequency beat signal can be used in evaluation of the RF signal parameters.