VARIABLE-FREQUENCY COMPLEX DEMODULATION TECHNIQUE FOR EXTRACTING AMPLITUDE AND PHASE INFORMATION

We present a variable-frequency variant of the complex demodulation te chnique. Complex demodulation is the digital equivalent of heterodyne detection, in which the input signal is multiplied by a carrier oscill ation {exp(-i omega(m)t)} of modulation frequency omega(m) and then lo w-pass filtered. The time-dependent amplitude and relative phase can t hen be computed from the filtered demodulate. This technique fails whe n the frequency of oscillation differs from the modulation frequency b y more than the cutoff frequency of the low-pass filter. We modified t he standard technique by computing iteratively a time-dependent modula tion frequency {omega(m)(t)} that tracks the signal frequency to withi n +/- 0.1% of the Nyquist frequency. This small difference frequency a llows the use of very narrow filter bandwidths for the study of narrow -band oscillations with improved signal-to-noise ratio. We apply this analysis tool to Mirnov oscillations measured by a magnetic probe and show the time variation of the amplitude of the fundamental and harmon ic frequencies during a plasma current ramp down. Future work will inv olve the application of this technique to ECE measurements. (C) 1997 A merican Institute of Physics.