Alias or not ?

[c1Ron]

The measurements shown are data from a Hall effect based current probe connected to an NI 9215 analog input on a cDAQ-9133 CPU. The current probe measures the DC current for a 3-phase AC motor drive inverter. The horizontal axis shows time in seconds and the vertical axis shows amperes.

The top plot shows data when using the onboard clock with a sampling frequency of 100 Hz as the sampling clock for the NI-9215 module.

The bottom plot shows data when using a TTL pulse running at 25 kHz, synchronous with the drive PWM, as the sampling clock for the NI-9215 module.

Note that there is an interval at the start and at the end of the data acquisition, where the PWM is not running in both plots. 

Also, note that the data in the lower plot samples on the rising edge of each 250'th pulse of the 25 kHz - resulting in 100 Hz sampling frequency on those measurements as well. 

When we don't synchronize the data acquisition, e.g. as in the top plot, then we consistently get this slow waveform on both current and voltage, even if sampling with various frequencies.

The next figure shows equivalent data, where the top plot shows data using the onboard clock with 5 Hz sampling time and the lower plot shows data using the 25 kHz TTL pulse from the drive, here sampling every 25'th sample - resulting in 1000 Hz sampling frequency

Is this waveform an alias of the PWM ?

If we were to sample the NI-9215 fast enough, then would we see the same waveform with a 25 kHz frequency ?

[popokhakan]

Hello there,

There is a lot going on, so let's unpack your inquiry. I'm going to ask you to answer several questions so we can proceed with the solution of your problem.

• What is your setup? Can you provide photos and schematics?
• How did you configure your hardware? Which program are you using? It seems to me the graphs are not from LabVIEW.
• What should the signal look like? What's its frequency? Please specify the signal you are acquiring.
• Why are you sampling at such low sample rates and comparing it with high rates?

Please answer the questions in as much detail as possible.