Novel high-impedance probe for multi-gigahertz signal measurement

This paper describes a high-impedance probe based on the electro-optic sampling (EOS) for multi-GHz signal measurement of on-board circuits. The probe head and detection optics are assembled into a compact probe module. The probe head consists of a metal needle tip and electro-optic material (EOM). The top of the shaft is bonded to the EOM. When the tip makes contact with the signal line of a circuit under test, the signals are measured by detecting the polarization change of laser pulses that propagate through the EOM. Experimental results show that the effective 3-dB down bandwidth of the EOS high-impedance probe is 5.5 GHz, which is 2 times higher than that of a conventional active probe. The minimum detectable voltage is <1 mV/√Hz. The input resistance and capacitance are >100 MΩ and <0.2 pF, respectively. The probe system has been successfully applied to measure multi-GHz signals of a communication IC mounted on a board