How to Avoid Oscilloscope Aliasing Pitfalls for Accurate Measurements - Workbench Wednesdays 78

In this episode of Workbench Wednesdays, James dives into the world of oscilloscopes and the Nyquist theorem, shedding light on the often-misunderstood concept of aliasing. He begins by explaining the Nyquist-Shannon theorem, which states that to accurately reconstruct a signal, you need to sample it at least twice as fast as its fundamental frequency. However, James points out a crucial detail – the theorem requires sampling faster than two times the signal frequency, not exactly two times. 

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How to Avoid Oscilloscope Aliasing Pitfalls for Accurate Measurements - Workbench Wednesdays 78

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James demonstrates this with a sine wave example, showing how slight shifts in the sample points can lead to incorrect amplitude in the reconstructed waveform. He then introduces aliasing, where the sample rate is much slower than the signal, resulting in a fake, lower-frequency waveform. This is an important concept for makers and engineers to grasp when working with digital oscilloscopes. 

Moving on to practical examples, James illustrates how changing the sample rate affects measurements. He shows how a 100 MHz sine wave can appear as 20 MHz on the scope when the sample rate is insufficient, but is accurately represented when the sample rate is increased to 400 Msps. He emphasizes that understanding the sample rate is vital for accurate measurements and troubleshooting. 

James doesn't stop at sine waves. He explores more complex shapes, such as square waves, and demonstrates how they can lead to aliasing when the sample rate drops below the Nyquist frequency. He also briefly discusses FFTs on digital oscilloscopes, highlighting limitations in frequency span. 

To avoid aliasing, James delves into the oscilloscope's memory depth, explaining how it's connected to sample rate and time-based settings. He highlights how some oscilloscopes automatically adjust memory depth and sample rate to keep the screen filled. James also points out that interleaved ADCs may alter the sample rate when channels are enabled or disabled, underlining the importance of knowing the sample rate. 

In conclusion, James provides practical advice on determining the required sample rate for different signals. He recommends sampling at least 2.5 times faster than the fastest frequency in your signal, with square waves requiring additional considerations. He advises keeping an eye on the sample rate when adjusting the time base, as significant changes can indicate aliasing issues. Finally, he encourages viewers to seek help on the element14 Community for any oscilloscope-related questions and provides a list of his oscilloscope-related videos for further exploration. 

Bonus Content:

•  What does Bandwidth mean for Oscilloscopes? - Workbench Wednesdays 47 
•  Electronics Engineer Reviews KORG NTS-2 Oscilloscope Kit - Workbench Wednesdays 75 
•  Workbench Wednesdays 76: Checking Out Digilent's Analog Discovery 
•  PICOSCOPE 2204A - USB Oscilloscope - Review 
•  Bald Engineer’s Oscilloscope Videos  

Bill of Material:

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