EMC Question of the Week: January 5, 2026
A switching power supply with a fixed duty cycle creates narrow impulses that appear at a repetition rate equal to the switching frequency. For measurements in the frequency range from 30 to 1000 MHz with a 120 kHz resolution bandwidth, which of these power supply switching frequencies are likely to produce average detector emissions that are significantly lower than the peak detector emissions?
- 20 kHz
- 2 MHz
- both of the above
- none of the above
Answer
The best answer is “a.” In peak detection mode, the noise from the 20-kHz power supply will appear to be broadband with ~5 harmonics in each 120-kHz slice of the spectrum. A resolution bandwidth of 120 kHz corresponds to a measurement dwell time of about 2.7 microseconds. Impulses occur every 50 microseconds. The recorded peak value will correspond to the energy conveyed by a single impulse occurring at the moment the analyzer sweeps through a particular frequency band. But that peak is only observed in less than one out of every ten sweeps. On average, the received power is well below the recorded peak value.
On the other hand, when the switching frequency is 2 MHz, there is an impulse every half microsecond. With a dwell time of 2.7 microseconds, multiple occurrences will be detected with every sweep. With a resolution bandwidth of 120 kHz, narrow-band harmonics will be observed appearing every 2 MHz. If the switching frequency is constant, the average detected amplitudes of these harmonics will be nearly equal to the peak amplitudes.
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