EMC Question of the Week: August 2, 2021

Trapezoidal waveform represented in the time and frequency domains

On average, how does reducing the duty cycle from 50% to 25% while maintaining the same period and transition times affect the amplitude of the harmonics of a pulse train above 1/(πtr)?

  1. They increase
  2. They decrease
  3. They stay the same
  4. They shift in frequency


The best answer is “c.” Reducing the duty cycle by a factor of 2, reduces the average power in the signal by a factor of 2 (3 dB). However, as long as the transition time is still short relative to the "on" time, τ, this power reduction occurs exclusively in the lower harmonics. The amplitude of the higher harmonics is dominated by the transition time relative to the period. 

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