Choose your domain

Most engineers are comfortable with oscilloscopes, but fewer are comfortable with spectrum analyzers. Courtesy of Yokogawa.

EMI consultant Daryl Gerke runs into situations where a circuit change that greatly affects the frequency domain shows little change in the time domain. In one case, a spectrum analyzer revealed excessive EMI in a product, but its designer and management were reluctant to alter the switching power supply's clock signal to improve emissions. "Show me on a scope that you haven't affected efficiency," the designer told Gerke. Gerke added a ferrite to the power supply's switching signal, which filtered the high-frequency harmonics that caused excessive emissions. (Most of the power is contained in the fundamental frequency and low harmonics.)

The designer in Gerke's story felt more comfortable working in the time domain than in the frequency domain. The same holds true for many digital designers, said signal-integrity consultant Dr. Howard Johnson. "EMI engineers should put their spectrum analyzer on a cart and wheel it to the digital design lab," he said. "Digital designers don't usually think about a circuit's frequency spectrum. They could learn from EMI specialists who use spectrum analyzers every day." Johnson also noted that digital designers often resist slowing their signal edges because they fear increased jitter. With a better understanding of frequency-domain issues, designers can predict when to apply EMI filtering without affecting signal quality.

Doug Smith, a consultant in high-frequency measurements, often works on both EMI and signal-integrity issues. Smith said, "When you slow a rising edge in a digital data stream, look at an eye diagram to make sure that you haven't added excessive jitter." He also noted, "Many signals are faster than they need to be. A low-frequency clock with a fast rise time can cause more EMI problems than a signal with a higher fundamental frequency because of harmonics."

Smith said that all engineers should be comfortable with a scope and with a spectrum analyzer, but he added that you can learn a great deal about a circuit's integrity and EMI performance with just an oscilloscope. "Look at your signals. If you see repetitive noise riding on a signal, then check its frequency. It just might correspond to a power supply's switching frequency or to another signal."

Smith noted that too few designers take the time to see why a circuit works. Spend an hour or two with an oscilloscope, spectrum analyzer, or both to look at critical parameters such as jitter and noise. Measure common-mode currents, even when your design works properly. You'll have a better understanding of how your design works.