This article presents the fundamentals of EMI and describes three simple pre-compliance tests that any engineer can implement in their lab to evaluate the electromagnetic behavior of a design.
1. EMI Fundamentals
EMI refers to any unwanted signal generated by a device that can interfere with the operation of other equipment. It is primarily classified as:
1.1 Radiated Emissions
These are electromagnetic signals emitted into space that can propagate without a physical conductor. They affect nearby systems and can cause communication or sensor failures.
1.2 Conducted Emissions
These are unwanted signals that travel along power or interface lines (AC or DC). They manifest as noise in the 150 kHz to 30 MHz range, according to the most common standards (CISPR/FCC).
1.3 Susceptibility or Immunity
This assesses how resistant a piece of equipment is to external radiated or conducted noise.
EMI problems typically originate from:
High-speed current switching,
poor PCB routing,
large return loops,
poor power decoupling, and
incorrect use of cables or shielding.
It is best practice to identify these problems before official certification.
2.3 Simple Pre-Compliance Tests
The following are three practical and cost-effective tests that allow you to identify EMI problems without the need for expensive laboratory equipment.
Test 1: Near-Field Probe Radiated Emission Scanning
Description
A near-field probe (H-loop or E-field) is used in conjunction with a spectrum analyzer to detect sources of electromagnetic noise directly on the PCB.
Objectives
Identify radiation hotspots on the board.
Locate problematic traces, inductors, or DC-DC converters.
Observe emission peaks in critical bands (30 MHz – 1 GHz).
Basic Procedure
Connect the probe to the spectrum analyzer (typical RBW: 100 kHz – 120 kHz).
Move the probe slowly across the PCB surface at a depth of 1–5 mm.
Record significant peaks and their location.
Compare:
Before and after layout modifications
; With different load configurations;
In different operating modes
Advantages:
Low cost (commercial or custom-printed probes).
Highly accurate for locating the source of the problem.
Test 2: Conducted Emission Measurement using LISN and Spectrum Analyzer.
Description:
Conducted emissions are evaluated by placing a LISN (Line Impedance Stabilization Network) between the device and the power supply.
Objectives
: Detect noise injected into the network or external cables.
Simulate controlled impedance conditions according to CISPR standards.
Measure peaks in the 150 kHz to 30 MHz range.
Basic Procedure
: Connect the DUT (Device Under Test) to the LISN.
Power the LISN from an isolated source.
Connect the LISN's measurement port to the spectrum analyzer.
Record noise levels, comparing them to pre-compliance reference limits.
Analyze the effect of EMI filters, ferrites, and layout changes.
Advantages:
Results close to those of a real test.
Allows evaluation of the effectiveness of LC and EMI filters.
Test 3: Pre-Compliance of Radiated Emissions using an Improvised Chamber or Broadband Antenna.
Description
: This test evaluates radiated emissions at a distance using a broadband antenna (log-periodic or biconical) and a spectrum analyzer.
It does not replace an anechoic chamber test but allows for the detection of trends.
Objectives:
Verify if the device exceeds typical radiation levels.
Compare configurations, distances, and orientations.
Evaluate firmware changes, operating modes, or frequency variations.
Basic Procedure:
Place the antenna 1–3 meters from the DUT.
Configure the spectrum analyzer:
Range: 30 MHz – 1 GHz or more.
Classic RBW: 120 kHz.
Measure in horizontal and vertical orientations.
Compare levels with unofficial reference curves (relaxed limits).
Advantages:
Very useful for detecting problems before certification.
It allows observation of harmonics and structural noise in the system.
3. Interpretation and Correction of Common Problems
High emissions at low frequencies (150 kHz – 10 MHz)
Switched converters
Insufficient LC filters
Long cables without ferrites
Emissions at intermediate frequencies (30 MHz – 300 MHz)
Long traces acting as antennas
Poorly managed returns
Gaps in reference planes
High emissions in GHz
High-speed digital signals
Shielding or connector failures
Jittering oscillators or PLLs
Typical corrective actions
Improve power decoupling (small, closely spaced ceramic capacitors).
Reduce return loops.
Add appropriate ferrites and EMI filters.
Ensure solid ground planes.
Redesign critical routing or control slew rate.
4. Conclusion
Performing EMI pre-compliance testing is one of the best strategies to reduce risks, costs, and delays in the certification process. With relatively simple tools—near-field probes, a LISN, and a broadband antenna—it's possible to identify radiated and conducted emissions problems early on.
These tests don't replace an accredited laboratory, but they do allow for design validation, the location of noise sources, and preparing the product to successfully pass official EMC tests.