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 resolve accurately voltage-current phase differences even when the difference is very close to $90^{\circ}$. The latter has been a major issue with V-I probes when used with miniature RF plasma devices like 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 $90^{\circ}$. It turns out, however, that when V-I probes are used with such devices without joint calibration, these frequently yield phase shifts over $90^{\circ}$. Also, since power absorption is proportional to the resistive part of the impedance it becomes very sensitive to the phase difference when it is close to $\approx90^{\circ}$. Thus, it is important to be able to resolve the phases accurately. Post-calibration, V-I probes would be indispensable for the electrical characterization of APPJs for determining average RF power $P_{av}$, plasma impedance $Z_p$, etc.Typical post-calibration V-I data yields, $Zp \approx 93.6 - j 1139 {\Omega} (81.5 - j 1173 {\Omega})$ at $P_{av} \approx 9.8 W$ $(\approx7.7 W)$ for helium (argon) gas.