EMI Lab (EC-306) - Important Questions

Explain the construction, purpose and operation of a Line Impedance Stabilization Network (LISN). Include the role of the LISN in conducted emissions measurements and list typical specifications required for compliance testing.

Describe in detail the procedure to measure conducted emissions from a power line using a LISN and an EMI receiver/spectrum analyzer. Your answer should include test setup, reference planes, detector types (quasi-peak, peak, average), frequency range (typical 150 kHz to 30 MHz), recommended RBW/VBW settings and how to record and report results.

Define shielding effectiveness (SE). Derive the expression for SE in decibels in terms of incident field $E_{i}$ and transmitted field $E_{t}$ and explain the physical meaning of reflection loss, absorption loss and multiple-reflection correction.

Explain the transmission-type method for measuring shielding effectiveness in the laboratory. Draw and describe the experimental setup, list calibration steps and show how to compute SE from the measured transmitted and incident signals.

Explain the difference between common-mode and differential-mode currents in a two-conductor power lead. Describe practical measurement methods for each mode (including use of current probes and LISN data) and list at least three mitigation techniques for reducing common-mode currents.

Compare and contrast the use of a general-purpose spectrum analyzer and a dedicated EMI receiver for EMI measurements. Discuss the importance and operation of quasi-peak, peak and average detectors and the effect of RBW/VBW on measured amplitudes.

Describe the conducted immunity test setup using a coupling/decoupling network (CDN). Explain how the CDN is used to inject RF disturbance onto the power port, list standard test levels and frequencies, and summarize safety and calibration considerations.

Design a common-mode choke for suppression of conducted EMI at a target frequency $f_{0}$. Derive the relation between the required impedance $Z_{cm}$ at $f_{0}$ and the inductance $L_{cm}$. Use the relation for inductive reactance and show how to compute number of turns if the core geometry and permeability are given.

Explain the use of near-field probes (loop and E-field probes) for locating and measuring radiated emissions in the lab. Describe probe calibration, orientation, distance effects and how to interpret probe output qualitatively to find emission hotspots on a PCB or cable.

Describe how a clamp-on current probe is used to measure common-mode current on a cable. Show the relationship between the probe output voltage $V_{p}$ and the measured current $I_{m}$ given the probe transfer impedance $Z_{t}$ using a formula.

Explain how time-domain sampling and FFT are used in EMI analysis. State the Discrete Fourier Transform formula for a sequence $x(n)$ of length $N$ and explain spectral leakage, windowing and how they affect measured spectral amplitudes.

Discuss grounding and bonding techniques used in reducing EMI in laboratory measurements. Explain single-point and multi-point grounding, ground loops, and practical steps to minimize measurement errors caused by poor grounding.