What does the piezoelectric effect have to do with oscilloscopes? If you follow any of the electrical engineering YouTube channels, you’re likely familiar with Dave Jones & the EEVBlog. His latest video caught my eye “EEVBlog #983 – A Shocking Oscilloscope Problem”. Now, this made me stop in my tracks. Not because he’s highlighting an oscilloscope “problem,” but because after waiting for 982 videos, Dave thought this topic was finally worth using the word “shocking” as a pun. I don’t know about you, but if I had made 982 videos, I’d probably have played that card already. Although, our Keysight Oscilloscopes YouTube channel just broke 250 videos and we haven’t done it yet, so you never know.
Anyways, what could be such a big deal? As it turns out the topic is actually, well (sigh) shocking. Who knew that simply bumping an oscilloscope the wrong way could cause mystery signals to appear on the screen? What makes this happen? It occurs because the ceramic capacitors in the oscilloscope’s acquisition system act as a piezoelectric material. Whether you are using a cheap oscilloscope or a high end oscilloscope, the piezoelectric effect is something to be aware of.
How does piezoelectricity work?
Piezoelectric materials are crystalline substances that produce an electric potential when subjected to mechanical stress. Think about a crystal lattice. In general, a material’s molecules form into crystals because that is its most stable state. The molecular charges are arranged in an electrically neutral arrangement. Essentially, the positive and negative charges are all at a happy equilibrium. But as soon as an external physical pressure distorts the crystal structure, there will be an imbalance of charge. Take Fig 01 (GIF) for example. In a normal, non-compressed state the 2D lattice is at equilibrium. But as it’s compressed, the positive and negative charges “squish” out to opposite ends and create a potential across the structure. Basically, the lattice stops being an electrically neutral structure and has a charge distribution.
Alternatively, you can apply a voltage to a crystalline structure and it will physically change the shape of the crystal – the “reverse piezoelectric effect.” This is especially useful if you want to generate or sense physical time-varying waves.
The piezoelectric effect and oscilloscopes
What does the piezoelectric effect have to do with oscilloscopes? Try this and see for yourself:
- Grab a standard 10:1 passive probe and connect it to your oscilloscope
- Zoom in vertically on your signal to a small voltage per division setting
- Set your trigger level slightly above your baseline signal
- Remove the probe’s grabber hat & tap the exposed probe tip on a hard surface
- Don’t panic and always carry a towel
You should then see a signal show up on your screen. Remember, you may have to put your oscilloscope into “Normal” trigger mode to keep the signal onscreen. Alternatively, you may be able to forgo the probe all together and simply tap on a bare BNC or even the top of the chassis (like in Figure 2). Now, don’t panic, this is a behavior that every scope in existence exhibits. It’s worth noting that I had to smack the oscilloscope pretty stinking hard to get this strong of an effect.
A hand-numbingly hard slap demonstrates the piezoelectric effect on the Keysight InfiniiVision 1000 X-Series
A signal is showing up on the oscilloscope because designers use ceramic capacitors in both probes and in oscilloscope acquisition boards. Ceramic is a piezoelectric material, and the vibrations caused by physical force you exert on your probe and/or scope cause the capacitors to physically expand and contract slightly. This expansion and contraction creates an electric potential in the capacitors. Because these capacitors are part of the oscilloscopes acquisition system, that potential shows up on the oscilloscope screen. “So…” you ask me once you’re done hyperventilating, “have all of my measurements been bogus up to this point? Can I minimize this effect? Is this something I should worry about?”
No, yes, and probably not.
Unless you are working in the middle of a city-destroying earthquake or on the back of a kangaroo, you probably don’t have anything to worry about. Keysight oscilloscopes all go through extensive environmental and stress testing, including drop tests (up to 30 g’s of force!) and time on a vibration table (Fig 3). So, for Keysight oscilloscopes you can be confident that every-day vibrations won’t affect your measurements (but I can’t speak for other manufacturer’s testing procedures). If you are extra concerned about this or work on the back of a kangaroo, try using an equipment cart or table that has built-in suspension.
An Infiniivision 1000 X-Series oscilloscope being drop-tested. Just because it’s an inexpensive oscilloscope doesn’t mean it’s not rugged!
Clearly, under the right circumstances, you can visibly observe the piezoelectric effect on your oscilloscopes. However, in my years at Keysight, I have not seen a single instance of this ever effecting an engineer’s measurements. To borrow the words of Mike from Mike’sElectricStuff:
Patient to Doctor: “Doc, it hurts when I do this!”
Doctor to Patient: “Don’t do that!”