NI 5105 - Measure PWM Waveform Duty Cycle

[toddber]

Will the Positive Duty Cycle scalar measurement in the NI-SCOPE software calculate duty cycle correctly on a PWM (pulse-width modulated) waveform?  I have a 400Hz signal that is pulse-width modulated with a 40Hz signal that I need to measure the duty cycle for.  Since this is not a normal, continuous waveform (like a sine wave or square wave) I don't know if the NI-SCOPE software will correctly calculate the Duty Cycle.  Has anyone tried this with success?  I am looking at using the NI-5105 because it has 8 channels, but am open to using other hardware and just multiplex my signals to the digitizer.

[DFGray]

The positive duty cycle scalar measurement should correctly calculate duty cycle on your pulse.  By default, the measurement finds the base and top of your waveform using a histogram of the Y values (there will be peaks in the histogram at the base and top values).  Duty cycle is calculated from the 50% level crossings between these two values.  You must have at least one complete cycle and only the first cycle is measured.

If that doesn't work for you, there are a plethora of parameters you can change (except for which pulse you measure - it is always the first).  For example, you can set the upper, lower, and level crossing values to specific % or voltage levels.  See the documentation and examples for details.

If you need to measure the duty cycle of multiple pulses in the same pulse train, there is a LabVIEW function (Pulse Measurements.vi) which allows you to do this.  It uses the same algorithms as the internal NI-SCOPE routines.

In either case, the measurement is independent of the hardware.  Any NI-SCOPE device will work.  If you have any other questions, let us know.