My First Sharktooth Wave

measuring capacitance on an oscilloscope


In my regular YouTube wanderings W2AEW’s video “#90: Measure Capacitors and Inductors with an Oscilloscope” showed up. My current study has been related to building an Arduino based LC meter to measure Capacitors and Inductors, but that is a bit of a chicken and egg problem because you need to have precise accounting of the inductors and capacitors in the circuit for the LC meter because the measurements come from noting change when adding another capacitor or inductor to the circuit. (Yeah, you can purchase ones with a 1% tolerance, but I’m looking around for how much I can build and keep my costs down.) Also, I had a fun conversation in G+’s Ham Radio Homebrewing group where Sporadic-Z was encouraging me to try wire wound inductors and this peaked my interest.

So this method W2AEW is talking about requires a “Fast Edge Pulse Generator” circuit to generate a sharp square wave which is another tool I don’t have at this point, but I realized the Siglent oscilloscope has a 1khz square wave generator so I hooked that up to the breadboard to provide my power and throw together the capacitor test circuit. The first capacitors I grabbed to try this out on were big electrolytic capacitors. That was pointless and therefore a failure/learning opportunity. These big guys took the wave and made it a straight line, but eventually I realized the problem with that and went with some smaller capacitors I’d pulled out of something ages ago and I found success in the form of a sharktooth wave. I was able to find a few other similar capacitors and see variations. The pictures below are of the most aesthetically pleasing wave. I’m not sure what the values of the capacitors are at this point, but he provided his notes in the video on PDF. I thought that pretty awesome of him so printed out a copy and will try to do the math later.

So that wave isn’t in his suggested form. He recommended doing 8 squares per division high and I need to remember that when I go back for the math portion of this exercise. There were some other good hints about how to line things up to make measurements of stuff easier which is one of the reasons I intend to watch again. That seemed a skill worth collecting to make my life easier. In most videos they move things around without talking about why they are doing it. I appreciate W2AEW assuming I don’t know much.

I’m hoping I can get to trying out the inductor stuff tomorrow. If I can measure out a few of these capacitors maybe I can find the 10pF and 1nF ones that are suggested in the notes or understand the math enough to substitute something I do have. I’m also expecting fully understanding this will be good for understanding the LC meter I’m contemplating so when I have trouble with that I’ll have a clue about how to troubleshoot it.


If you’re “following along at home”, here is the video. I’d recommend subscribing to his channel if this interests you. His videos seem to be growing more interesting as I learn more about how this stuff works. He’s up to 230-some videos now. I enjoyed his more recent one on panel meters. The 6 video series on Capacitors and Inductors also looks good.