Column: EMC
but it is more accurate to call it a “dΦ/dt” loop. It’s important to note that the output
of the square magnetic field loop is a voltage. This assumes that the other end of the coaxial cable is connected to either a spectrum analyser or an oscilloscope with a 50Ω impedance. The mutual inductance between
conductors is typically lower than the inductance of either individual conductor. As a result, the output of a magnetic field loop represents a lower limit for the voltage per unit length across the inductance of a current- carrying conductor. Therefore, a magnetic field loop can be represented by the model shown in Figure 2. The simulation involves an 8cm magnetic- field loop with approximately 2cm-long conductors on each side. The mutual inductance is assigned a value of 10nH, while the self-inductance of the loop is 70nH. The 50Ω impedance of a spectrum analyser or an oscilloscope forms an L-C filter in conjunction with the loop’s inductance. This L-R configuration establishes a cut-off frequency as shown in the frequency response. Observing the graph, the voltage
response of the magnetic-field loop remains relatively flat until reaching the
The mutual inductance between
conductors is
typically lower than the
inductance of either individual conductor
cut-off frequency, which in this case is about 100MHz. Beyond this frequency, the sensitivity of the loop begins to decline at a rate of -20dB/decade. So the loop is capable of accurately measuring voltages up to at least 100MHz, making it a valuable tool for voltage measurements within this range. The current response of the magnetic-
field loop, shown in green in Figure 2, suggests that it can be used for measuring or estimating high-frequency currents, even those of which frequency components extend beyond the cutoff frequency of the magnetic-field loop.
Positioning There are two methods for positioning a magnetic-field loop over a PCB. When placed parallel to the PCB, as shown in Figure 3a, it captures the changing magnetic field using the entire loop area. This technique is known as “sniffing” and is used to identify the “hot” area on the PCB, which is the region with maximum changing magnetic field. The magnetic-field loop can be connected to either a spectrum analyser or an oscilloscope with a 50Ω impedance. During this process, the “hot” area is recognised when the scope or spectrum analyser displays the maximum values. On the other hand, when the
magnetic-field loop is positioned perpendicular to the PCB, as shown in Figure 3b, it measures the induced voltage on a specific track or trace on the PCB. The reason for placing the loop perpendicularly is to minimise the induced voltage on the side wires of the loop, ensuring a more accurate measurement. In this setup, it is recommended to connect the magnetic-field loop to a high-bandwidth oscilloscope as the measurement is in the time domain, requiring the ability to capture fast voltage transients.
Figure 3: Positioning a magnetic-field loop
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