EMC Question of the Week: July 6, 2026

A current waveform (square wave) and 4 possible measured waveforms

A small loop magnetic-field probe is placed on a circuit board trace. The trace carries a current with a waveform that looks like a 60-mA, 250-MHz square wave. The magnetic-field probe's loop inductance is 50 nH. When connected to a 50-Ω oscilloscope input, we measure a voltage waveform that looks like   

  1. the current waveform
  2. the current waveform with no DC component
  3. pulses corresponding to positive and negative transitions
  4. a 250-MHz sine wave

Answer

The best answer is “b.” The waveform of the magnetic field has the same shape as the current waveform (a 250-MHz square wave). The voltage picked up by a small loop probe is proportional to frequency as long as the inductive reactance of the probe is small relative to the resistance of the scope input. So, at low frequencies, the measured waveform would resemble the derivative of the magnetic-field's waveform. However, at high frequencies, where the inductive reactance of the loop is greater than the scope input impedance, the measured voltage is no longer a function of frequency. The boundary between the low-frequency and high-frequency behavior occurs when ωL = R (or, f = R/2πL). In this example, that boundary frequency is 

f= R 2πL = 50 Ω 2π 50× 10 −9  H =159 MHz.

Since the fundamental and all harmonics of a 250-MHz square wave are higher than this boundary frequency, the harmonic content is not distorted. The measured waveform looks just like the current waveform without the DC component.

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