Aliasing: Why Your Oscilloscope May Be Lying To You [E223]
Thanks to our Partners, Pico Technology and Autel
Matt answers listener emails about oscilloscope aliasing—what it is, whether all scopes can do it, and how it can trick you into diagnosing failures that aren’t real. Using a “pegboard and golf tees” mental model, he explains how a digital storage oscilloscope samples voltage, stores it in memory, and then reconstructs what you see on-screen. The key takeaway: aliasing isn’t magic, it’s math—specifically the relationship between sample rate, timebase, and memory buffer. He also explains why some scopes (especially Snap-on) behave differently than Pico-style workflows, and how misunderstanding that screen-to-buffer relationship can create fake-looking “dropouts.”
Who This Episode Is For
Anyone using a handheld/PC-based automotive DSO (Pico, Snap-on, Autel, etc.)
Techs chasing intermittent cutouts, crank/cam dropouts, injector events, CAN glitches
Anyone who has ever said: “The waveform looked wrong… but the fix didn’t fix it.”
Key Topics Covered
What aliasing is (in plain language): the scope fails to accurately reconstruct the waveform you’re testing.
Can all oscilloscopes alias? The spicy answer is yes, they all can—especially digital scopes—depending on setup and limitations.
Analog vs. digital (audio analogy): Digital sampling is like digital audio—there are “samples,” and reconstruction depends on how well you capture the real signal.
The “pegboard model” for DSO operation: Up/down holes = voltage levels (vertical resolution). Left/right holes = time positions (sample points in memory). The scope measures voltage, then “plants a peg” in memory and connects the dots.
Vertical resolution vs. time performance: 8-bit can look stair-steppy. 12/16-bit improves vertical accuracy. But most real-world failures come from time-domain limitations (sample rate + memory dynamics)
Sample rate vs. buffer size (why scopes “fall apart”): Put too little time on screen → not enough samples to define the signal. Put too much time on screen → scope rejects/skips samples because the buffer can’t hold it all. Either way: the displayed waveform can become fiction.
How aliasing creates “phantom dropouts”: Gaps that look like crank sensor dropouts or reluctor issues. Can send you straight into the diagnostic swamp
Why Pico changed the game: Early Pico automotive scopes stood out because they brought big memory buffers to real shop problems. Capture longer events accurately, then zoom in for detail
Snap-on screen/buffer behavior is different (and people get burned): Snap-on scope often shows a “window” into a buffer (buffer bar flying across). You don’t “zoom in like Pico”; you effectively set detail first, capture the event, then zoom out to find it and return to your detail level. Misunderstanding this is a common cause of “dropouts” that are really aliasing/misuse
The Big Takeaways
Aliasing can make a good tech chase a bad story.
The waveform on-screen is an interpretation, not a photograph.
Know your scope’s strengths: Some are built for speed, some for memory, some for both—but your settings decide your fate.
If you’re hunting an intermittent: Your success depends on matching: expected event speed, sample rate, memory depth, the scope’s display/buffer behavior.
Practical “In-the-Bay” Tips
If the trace shows perfectly suspicious gaps: question your timebase, question your effective sample rate, verify with a different capture strategy (less time on screen, more sample rate, different scope mode)
Don’t trust a dropout unless: it repeats consistently under the same conditions, and you can capture it without stretching timebase beyond what your scope can support.
Learn your platform’s workflow: Pico-style: capture longer, freeze, zoom in. Snap-on-style: capture detail first, trigger/freeze, then zoom out to locate the event
Mentioned / Referenced (People + Tools)
Pico Technology
Autel
Older scope references: Fluke 90-series, Tektronix
Training/class voices mentioned: Harvey Chan, John Thornton, Scott Manna, Tim Iezzi, Ira Waldman, Scott Shotten
Thanks to our Partner, Pico Technology
Are you chasing elusive automotive problems? Pico Technology empowers you to see what's really happening. Their PicoScope oscilloscopes transform your diagnostic capabilities. Pinpoint faults in sensors, wiring, and components with unmatched accuracy. Visit PicoAuto.com and revolutionize your diagnostics today!
Thanks to our Partner, Autel
From drivability diagnostics and TPMS service to ADAS and advanced safety systems, Autel helps technicians follow OEM procedures and repair with confidence. Learn more at Autel.com
Contact Information
The Automotive Repair Podcast Network: https://automotiverepairpodcastnetwork.com/
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