EMC Question of the Week: October 1, 2018

For a digital clock signal that resembles a 3.3 V_pp square wave, the amplitude of the first harmonic

1. is approximately 2 volts
2. is less than 0.2 volts
3. may be more than 20 volts
4. depends on the frequency of the clock signal

Answer

The best answer is "a". For a square wave, a trapezoidal wave, or anything reasonably resembling a square wave; the amplitude of the first harmonic is approximately 2/π times the signal voltage. The third harmonic will be 1/3rd the amplitude of the first harmonic and the fifth harmonic will be 1/5th the amplitude of the first harmonic. 

Generally, a worst-case envelope of all harmonics will have an amplitude at the nth harmonic no greater than 1/n times the amplitude of the first harmonic. However, ringing in the waveform is capable of causing peaks that exceed this envelop at harmonics near the ringing frequency.

Slowing the transition times of the clock signal causes the worst-case envelope at the upper harmonics to start falling off at rate equal to 1/n². Controlling the transition times of digital signals is a very effective way of reducing the power in harmonics at about 10 times the clock frequency and higher. 

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