Studies On Calibration Method And FFT Analysis In Power Measurement Of Pulse-modulated RF Signal

Atmospheric pressure discharges have a wide range of potential applications in fields such as material surface processing and biomedicine because they do not require expensive vacuum system, and generate chemically active groups under lower gas temperature. Compared with atmospheric pressure corona and arc discharges, atmospheric pressure glow discharges (APGDs) are advantageous due to their processing being both relatively spatial-uniform and time efficient. However, APGDs tend to transit into arc discharges. RF APGDs are ignited and maintained at relatively lower voltage, as a result, uniform RF glow discharges are easier to be realized. When a dielectric barrier layer, which has a negative feedback effect, is inserted between the electrodes, the RF APGD can be operated in the more intense a-mode. The pulse modulated atmospheric pressure RF discharge can reduce gas temperature to suppress thermal instability and thus further extend operation region of uniform RF discharges. In the low pressure pulse modulated RF plasmas, the time-varying powers were calculated from the measurements with separated voltage probe and current probe. The study of the pulse modulated atmospheric pressure RF discharge power has not yet been reported. Thus, it will be a significant work to study the power measurement of pulse modulated RF.This paper reviews the method of RF power measurement, including the measure of using power meter and the measurement of using voltage and current probe. When the signal was measured by the voltage and current probes, we can calculate RF power by integration method or phase shift method. Only the voltage and current probes can be used when measure the pulsed RF power, and by comparing We have completed the measurement of pulse modulated atmospheric pressure RF discharge power by using MKS V/I probe and digital oscilloscope. MKS V/I probe output voltage and current signals are sampled by a digital oscilloscope, voltage amplitude, current amplitude and the phase between voltage and current are obtained by the FFT analysis. From the principle of FFT, FFT method cannot be simply used in the processing of non-stationary amplitude modulation signal, the frequency, amplitude and phase of the estimation error can achieve a reduction by modification.During the pulse modulated RF discharge power measurement, L.J. Overzet and F. Y. Leong-Rousey found that the phase angle between current and voltage during the turn-on deviated from the corresponding value of flat area and due primarily to time delay between the probes and trigger source. Our further numerical simulation study results show that AM signal has no effect on the phase. By the analysis of numerical simulation results, we found that AM,which pulse modulation generated, has effect on the FFT phase estimate value, and then influences FFT power, impedance estimate values, as we will discuss in this paper. The analysis shows that the main factor influencing the calibration coefficient is the phase between voltage and current, non-stationary amplitude modulation voltage, current signals processing by FFT method may give rise to problem, the real absorbed power, and discharge equivalent resistance obtained are credible only at zero reactive power points.