EMI Lab (EC-306) - Important Questions

Explain the purpose and working principle of a Line Impedance Stabilization Network (LISN). Describe the typical test setup for measuring conducted emissions from a DUT (Device Under Test) using a LISN and a spectrum analyzer/EMI receiver. Include the role of the LISN, measurement port connections, grounding considerations, and how the measured voltage is related to the conducted emission limits.

Draw and explain the standard test setup for radiated emission measurements in an Open Area Test Site (OATS) or an anechoic chamber at measurement distance $R$. Explain the choice and placement of antennas, antenna polarization switching, measurement distances (e.g. 3 m, 10 m), site calibration or substitution antenna method, and how to interpret the measured field strength relative to regulatory limits.

Define Shielding Effectiveness (SE). Derive the expression for SE in decibels and explain the contributions of reflection loss and absorption loss. Use the field ratio definition and show the SE formula as

$$\text{SE (dB)} = 20\log_{10}\left(\frac{E_{\text{incident}}}{E_{\text{transmitted}}}\right).$$

Describe practical factors that affect SE of an enclosure.

Explain the difference between common-mode and differential-mode currents in a two-conductor power or signal cable. Describe how each mode contributes to conducted and radiated emissions and outline practical measurement techniques to separate and measure common-mode and differential-mode currents using current probes and a LISN.

Explain the significance of Resolution Bandwidth (RBW), Video Bandwidth (VBW), and detector types (peak, average, RMS) on a spectrum analyzer/EMI receiver when measuring emissions. Show how to convert a measured power level in $\text{dBm}$ to an equivalent RMS voltage across a $50\,\Omega$ system by giving the relevant formulas:

$$P(\text{W}) = 10^{\left(\frac{P_{\text{dBm}}}{10}\right)} \times 10^{-3}$$

and

$$V_{\text{rms}} = \sqrt{R \cdot P(\text{W})}$$

Apply these formulas to convert $0\,\text{dBm}$ into $\mu\text{V}_{\text{rms}}$ across $50\,\Omega$.

Compare an Anechoic Chamber and an Open Area Test Site (OATS) for radiated emission testing. Explain the advantages and limitations of each, and describe how site attenuation and calibration are performed for a chamber or OATS to ensure valid radiated emission measurements.

Describe a laboratory method to measure Shielding Effectiveness (SE) of a planar sample using a TEM cell or a coaxial transmission line test fixture. Explain how transmitted and incident signals are measured, and show how the measured S-parameters (e.g. $S_{21}$) can be used to compute the SE in dB. Provide the relation between $S_{21}$ magnitude and SE.

Design a single-stage LC low-pass EMI filter for a power line where the desired -3 dB cutoff frequency is $f_{c}=100\,\text{kHz}$. Assuming a source/load impedance of $50\,\Omega$, calculate suitable values of $L$ and $C$ for a simple series L and shunt C topology. Use the cutoff frequency relation

$$f_{c} = \frac{1}{2\pi\sqrt{\left( L C \right)}}$$

and state any practical considerations (component Q, parasitics, and placement) affecting EMI performance in real circuits.

Outline the basic ESD (Electrostatic Discharge) testing procedures used in EMC labs. Distinguish between contact discharge and air discharge methods, list typical test voltages used for human-body model ESD testing, and describe precautions and setup required to protect the DUT and measurement equipment.

Explain the principle and use of near-field probes in EMI debugging. Describe typical probe types (magnetic loop, electric-field), how to scan a PCB to locate hot-spots, and how probe orientation and probe-to-circuit distance affect measured signals.