Issue link: https://iconnect007.uberflip.com/i/1535183
72 DESIGN007 MAGAZINE I MAY 2025 Figure 2: A 20-MHz square wave run through channel 1 output. Figure 3: A 20-MHz sine wave run through channel 1 output. Figure 4: A 20-MHz triangle wave run through channel 1 output. thing you might see from a board's clock signal. In Figure 2, we see a cable connected to the function gen- erator channel 1 output. at's t he default cable that came with the func- tion generator, with current being run through a 50 Ω resistor between the red and black leads. A current probe monitors the cur- rent flowing on the output cable, and a spectrum analyzer shows its frequency content from 10-85 MHz. From this simple 20-MHz square wave, you can see that we have a 60-MHz signal that's equally as strong as the 20-MHz signal. at's the third harmonic (20 x 3) of the 20-MHz signal. Square waves are notorious for generating harmonics based on their fundamental switching frequency—and the faster the switch- ing time, the stronger the harmonics will be and the higher in frequency they'll go. A 50% duty cycle square wave will generally produce only the odd harmonics; if you change the duty cycle, you'll start to see more contribu- tion from the even harmonics as well. You can see the difference between the square wave and a pure 20-MHz sine wave in Figure 3, where the 60-MHz peak has mostly disappeared. Slow Iet Down… e moral of this story is to consider the EMC tradeoffs being made when you select the transition speed for almost any switching operation in your system. To reduce EMC headaches, pick the slowest rise and fall time that will allow your system to meet its per- formance specs. Figure 4 shows the dramatic change you get from chang- ing those transition times. Here we've changed the square wave to a triangle wave, taking half the cycle time to rise and then fall.