Pulse Width Modulation (PWM) is commonly used in switched mode power supplies and industrial control systems. It has the advantage of efficiently driving switched mode devices at a fixed frequency. Teledyne LeCroy’s measurement parameter based track function can demodulate the PWM signal and extract the underlying modulation signal, making it possible to assess correct tracking and linearity in PWM regulators/controllers. Figure 1 shows an example of a PWM measurement using the track function.
The top trace in Figure 1 (Ch1) contains 2ms of an acquired PWM waveform. Trace F1 is the Track function of the width parameter (P2) and shows the cycle by cycle variation of pulse width with time. The underlying triangle modulation is clearly evident. The frequency parameters in the table beneath the waveform read the frequency of the modulation (P5) as 1.0kHz and the carrier frequency (P1) as 66 kHz. The minimum and maximum values of the width parameter indicate a width range of 4.62 to 10.49 µs. This is shown in both the minimum and maximum values in the statistics of the width parameter P2 as well as in the Maximum (P3) and Minimum (P4) parameters of the track function F1. Knowing the extreme values, like the maximum width, allows setting the width trigger to trigger on the maximum width and synchronizing the PWM modulating signal with the scope acquisition.
The histicon of the width parameter, shown beneath the parameter readout, is useful for spotting defects in the modulation waveform. Clipping, limiting, cross-over distortion, and asymmetry are easily detectable in this miniature histogram. For oscilloscopes which support histogramming the histicon can be expanded into a full histogram display as shown in Figure 2.
The histogram in Figure 2 shows that the values of the width parameter are uniformly distributed. The modulation waveform is a stepped triangle wave. This is shown in the histogram by the discrete values of pulse width. The histogram parameter, Peaks (P6), reads the number of peaks in the histogram which is equal to the number of discrete steps (33 in this case).
The use of the track function based on the cycle-to-cycle measurement of pulse width demodulates a PWM signals and extracts the modulation waveform for further analysis.