Frequency deviation measuring circuit system for square-wave-FM signal

A new circuit system for measuring the modulation index of the square-wave frequency modulation signal is presented. The system is used for optical fiber communications to evaluate the overlapping which might occur at sideband frequencies around only harmonics of the carrier even if no overlapping has occurred at sideband frequencies around the fundamental of the carrier. Experiments were carried out for a voltage controlled oscillator operating at 2 MHz. An IF signal at 200 kHz was generated by using a local oscillator operating at 1.8 MHz. The frequency deviation was measured at the fundamental (200 kHz) and its harmonic frequencies (600 kHz and 1000 kHz) to precisely measure the modulation index at an IF signal frequency of 200 kHz