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10 ways to maximise use of oscilloscopes

15 May 2014  | Arthur Pini

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Detection of frequency, phase, and pulse-width modulated signals

Many mid-range oscilloscopes offer a track or time-trend function that generates a waveform based on cycle-by-cycle variations of a measured timing parameter. The track function is time synchronous with the source waveform so that changes in frequency, width, or phase are easily correlated with the source waveform. This provides a way of demodulating FM (frequency modulated), PM (phase modulated), or PWM (pulse width modulated) signals. Figure 7 shows an example of demodulation of a phase modulated (PM) waveform using a track of the TIE (time interval error) parameter.

Figure 7

Figure 7: Using the track of the TIE parameter the instantaneous phase of each cycle of the PM waveform is plotted vs. time, thus demodulating the phase-modulated signal.

TIE is the time difference between a threshold crossing on a waveform and that threshold crossing's ideal position. It is essentially the instantaneous phase of the signal. A track of TIE therefore shows the cycle-to-cycle variation of the carriers phase and produces a waveform showing the phase variation time synchronous with the original modulated carrier. The vertical scale is in units of time and can be easily converted to phase by a simple rescale operation. In a similar manner, a track of the frequency parameter will show the modulation signal of an FM modulated carrier and a track of pulse width yields PWM demodulation.

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