EMC Question of the Week: October 19, 2026

time and frequency-domain representations of a sine wave

A 100-MHz sine wave with a peak-to-peak amplitude of 2 volts is measured with a 50-Ω EMI test receiver. A single narrow-band spike is observed at 100 MHz. What is the quasi-peak amplitude of that spike? 

  1. 23 dBm
  2. 19 dBm
  3. 13 dBm
  4. 10 dBm

Answer

The best answer is “d.” A 2-Vpp sinusoidal signal has a peak amplitude, 1 Vp, and an rms amplitude, 0.707 Vrms. An EMI test receiver always measures the rms amplitude. The power delivered to the 50-Ω input is (0.707)2/50 = 10 mW. Expressed in dBm, this is 10 log(10) = 10 dBm. Because the signal is continuous, the amplitude is the same whether the peak detector or the quasi-peak detector is used. 

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