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Modulation Analysis - PWM
Use Track And Timing Functions To Analyze PWM Signals
LAB
in PDF format
Pulse Width Modulation
(PWM) is commonly used in power supplies and industrial control systems.
It has the advantage of efficiently driving switched mode devices at a fixed frequency.
LeCroy’s Jitter and Timing Analysis 2 (JTA2) analysis option
contains a number of functions and parameters to extract the underlying modulation signal,
making it possible to assess correct tracking and linearity in
PWM regulators/controllers.
The top trace in Figure 1 (Ch2) contains 50 ms of an acquired PWM waveform. Trace F3 is
the Track function of the width@level parameter and shows cycle by cycle pulse width
vs. time. The underlying ramp modulation is clearly evident.
Figure 1 Analyzing a PWM waveform using the track of the width@level parameter areas.
The frequency parameters in the
table beneath the waveform read
the frequency of the modulation as 50 Hz and the carrier frequency
as 24.99 kHz. The minimum and maximum values of the width parameter indicate a
width range of 923 ns to 16.8 µs.
The histicon of the width at level
parameter, shown beneath the
parameters, is useful for spotting defects in the modulation waveform.
Clipping, limiting, crossover distortion, and asymmetry are easily detectable in this
miniature histogram. The scope can also display the histogram function, as shown in Figure 2.
Figure 2 Studying the variation of pulse width using analog persistence
The JTA2 option also includes
duty cycle, frequency and period functions to characterize other
aspects of the waveform variation. It also contains related parameters
such as duty cycle at level, frequency at level, and period at level. Each of these parameters
makes cycle-by-cycle measurements of the signal, which can be further analyzed
using histogram or trend functions. They can also be used as operands in waveform math operations.
It is possible to perform basic arithmetic on these functions or to perform more advanced
math, such as integration
or differentiation. Users who prefer a more traditional view of time varying data
can use the color-graded analog persistence displays to qualitatively
assess circuit operation. This type of analysis is shown in
Figure 2.
Analog persistence uses color or
intensity to indicate the frequency
of occurrence of events on the display. In Figure 2, it shows over 723,000 pulses superimposed
over each other. The yellow/green areas indicate that pulse edges occur more often in
these areas than in the blue tinted
area. This information matches the histogram display, which indicates
increased population at periodic intervals through the range of pulse width values.
There is no lack of tools for the
measurement and analysis of PWM based control systems in LeCroy oscilloscopes. JTA
combined with other math options, long acquisition memory, and fast processors offer cost effective,
time saving measurement solutions to users.
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