EMC Consutancy & Design solutions

1. Introduction – why a small change can make great differences in the plot Did this happen to you. You go to the testing lab. you do a full RE scan and the scan looks great. Then you Rotate the EUT 90° Lift a cable Move the EUT by 20 cm Change cable routing And the emissions plot changes. This is not randomness. It is physics. Radiated EMC measurements are strongly influenced by how the product couples to its environment. I want to discuss this today, so, next time, when you

When I debug EMC problems on a DC/DC converter, I use a very simple test to understand whether the dominant mechanism is Differential Mode (DM)  or Common Mode (CM) . The test is simple: I increase the load and observe what happens to the emissions. If: The peak frequencies stay the same, But the amplitude increases with load, then the behaviour is consistent with differential-mode emissions . This is aligned with what TI explains in their EMI material: Differential-mode nois

Introduction In switch‑mode DC‑DC converters, not all currents are equal from an EMC, signal integrity, or reliability perspective. A critical distinction must be made between low di/dt current paths  and high di/dt current paths . Misidentifying these paths is one of the most common root causes of excessive EMI, poor efficiency, unstable switching behaviour, and failed compliance tests. This article focuses on: How to correctly identify high di/dt and low di/dt current loops

Optocouplers are widely used in isolated power supplies to transfer feedback information across an isolation barrier. In schematics, they appear as a clean and robust solution: no galvanic connection, clear separation between primary and secondary, and good noise immunity. In practice, optocouplers are often one of the main entry points for switching noise  into the control loop. They are commoinly used in isulated AC/DC, DC/DC and PoE applications. This article explains why

This is one of the most common layout mistakes I see during EMC reviews. The schematic is correct.The common-mode choke is correctly placed at the interface. But the PCB layout shows a solid ground plane directly underneath the choke . Imagine a GND plane below the chocke. The intention is usually good: keep the ground plane continuous .Unfortunately, at high frequency, this decision often defeats the common-mode choke entirely. The Hidden Assumption: Inductors Are Not Ideal

Introduction When I develop a new product, I usually already have a good idea which EMC test is more likely to cause problems. Based on my experience, I decide which test to run first . That may seems excessive, but believe me, you may save a lot of time and money by choosing in which orders you do your test. What I mean is simple: if I believe conducted emissions (CE)  will be the harder test, I start with CE. f I expect radiated emissions (RE)  to be the main challenge, I s

Pre-compliance vs full compliance (and why it matters more than you think) Look, I'm just going to say it: Radiated Emissions (RE) and Conducted Emissions (CE) are the two EMC tests that'll tell you almost everything you need to know about your product. Think of them like getting an X-ray. Once you see those test results, the weak spots in your design just jump out at you. I've been doing this long enough that I can usually predict what's going to cause problems later just by

How This Article Started I was reviewing a design for a customer a while back, and they had this active capacitor circuit being used as an EMI filter. My customer looked at the schematic and asked me, "Francesco, what is this?" I couldn't help myself – I smiled and said, "That's a 1 Farad capacitor." We both had a good laugh. But that moment stuck with me, because it's actually a perfect way to understand what active EMI filters do. They can behave like impossibly large passi

In around 90% of CE (Conducted Emission)  issues on the mains input, the solution is simple: just add a common-mode (CM) filter .Most of the time, I use Würth Elektronik  filters — they’re reliable and predictable. However, every once in a while, something strange happens.Instead of reducing  the conducted emissions, the filter actually makes them worse  — sometimes by several dB between 1 MHz and 30 MHz .Let’s see why this happens and how to fix it. The Typical Setup In a st

If you are an electronic designer engineer you may want to read this article.... Surge and transient immunity testing often reveals weaknesses that are hard to predict during design. In one of my recent EMC test sessions, I encountered a case that perfectly illustrates why component-level surge ratings  can be misleading — and why system-level surge design  is what really matters. The Setup The test involved surge testing on an input port connected through cables longer than

Introduction Varistors (typically MOVs) are widely used on mains lines for surge / transient protection. Under normal conditions they are passive, but when high‐voltage transients (switching surges, lightning induced, line over­voltage) occur, the varistor conducts, clamps the voltage and absorbs energy. Over time the repeated stress (especially large impulses) causes the varistor’s characteristics (e.g., breakdown/clamp voltage) to degrade. Eventually the device may fail (ei

Today I want to talk about how apparently short  PCB tracks can still create big EMC problems —especially when they carry a clock in the 20–30 MHz  range. A very common setup You’ve got a clock source (XO or MCU clock out) driving something like a USB/Ethernet transceiver or an SPI flash. The driver is low-impedance (≈ 20 Ω), the track length is, say, 25 cm , and the receiver load is around 10–20 pF . Let’s pick a concrete example: 25 MHz  clock. If you only look at the funda

What I’m trying to show you today is counterintuitive. If you’re failing radiated emissions around 200–230 MHz  by just 3–4 dB , there’s...

Scope:  Why some products show broadband  radiated emissions from 30 MHz–1 GHz , why issues often persist up to 400 MHz–1 GHz  even on...

Scope:  How to read narrowband  features on the RE plot (100–1000 MHz is where they often pop up) and trace them back to clocks,...

Goal:  Use the radiated emissions (RE) plot to quickly identify the likely noise source and prove it with simple, low‑risk tests. 1)...

High-speed differential pairs, such as those found on HDMI lanes are more than just tightly coupled traces. At multi-gigabit speeds, the...

Thermal-relief “spokes” help solderability, but they add inductance  and cut slots  in your reference plane—exactly where high-frequency...

Here is an interesting conundrum I faced some years ago whilst I was working on a product subjected to heavy fields of over 50 V/m during...

Quick Recap of Part 1 In Part 1, we explored how a simple change in antenna orientation  can help you tell whether a radiated emission is...