Analysis of the Effect of Differential-Mode and Common-Mode Impedance Matching on the Common-Mode Rejection Ratio of a Differential Electro-Optical Voltage Probe

A differential electro-optical voltage probe is characterized. The two main parts of the probe circuit are the attenuator circuit and the laser diode. The probe circuit is characterized by measuring S-parameters. The probe circuit has a stable common-mode rejection ratio (CMRR) up to 4 GHz, with a quick drop at higher frequencies. The probe circuit performance is simulated by combining the individual attenuator and laser characteristics connected in cascade. The differential-mode and common-mode signal propagation is analyzed in terms of voltage and impedance matching. The impedance matching between the attenuator and the laser, in combination with their individual characteristics, determines the CMRR frequency characteristic of the probe circuit. By suppressing the common-mode signal propagation, relative to the differential-mode signal, an electro-optical system with a CMRR higher than the nominal CMRR of the laser diode can be designed.

I. Introduction

Characteristics of a broadband differential electro-optical voltage probe are measured [1]. The probe circuit consists of two main parts, the attenuator circuit and the laser diode (Fig. 1). The attenuator is used to lower the input signal level in order not to overdrive the laser. The laser diode converts the measured radio frequency (RF) signal into an optical signal, which is transmitted to the photodetector. The laser is biased using a dedicated bias circuit. The probe circuit design is discussed in [2].