A joint calibration technique for improving measurement accuracy of voltage and current probes during synchronous operation for RF-based plasma devices.

This paper presents a joint calibration scheme for voltage (V) and current (I) probes that helps accurately resolve the voltage-current phase differences even when the difference is very close to 90°. The latter has been a major issue with V-I probes when used with miniature RF plasma devices such as the atmospheric pressure plasma jet (APPJ). Since the impedance of such miniature devices is predominantly capacitive, the phase difference between the voltage and current signals is very nearly 90°. It turns out, however, that when V-I probes are used with such devices without joint calibration, these frequently yield phase shifts over 90°. Also, since the power absorption is proportional to the resistive part of the impedance, it becomes very sensitive to the phase difference when it is close to ≈90°. Thus, it is important to be able to accurately resolve the phases. Post-calibration, V-I probes would be indispensable for the electrical characterization of APPJs for determining the average RF power P, plasma impedance Z, etc. Typical post-calibration V-I data yield Z ≈ 93.6 - j 1139 Ω (81.5 - j 1173 Ω) at P ≈9.8W (≈7.7W) for helium (argon) gas.