Radiated vs Conducted Emissions: How to Decide What to Test First
- Francesco Poderico
- 2 days ago
- 4 min read
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 start with RE.
This choice comes almost entirely from experience. Over time, you begin to recognise that certain product families tend to have characteristic EMC “footprints”. For example:
Medical and laboratory equipment often struggles first with CE (no Y caps! )
EV chargers almost always deserve early CE attention (The pilot pin uses earth as return in a digital world)
Tablets, consumer electronics, and cable-rich digital products usually expose their weaknesses first in RE
This does not mean the other test is unimportant or optional. Both CE and RE are required for compliance. However, starting with the test that is most likely to fail first gives faster insight, clearer root causes, and a more efficient debug path.
On this page, I explain — in practical terms — why and how I choose whether to start with radiated or conducted emissions, and what types of products benefit most from each approach.Please note that I always start from one of these two tests; the decision is not whether to test, but where to begin.
When Radiated Emissions Tell You More Than Conducted Emissions
Radiated emissions testing provides a system-level view of how electromagnetic energy escapes the product. Because RE captures emissions from:
PCB structures ( 250 MHZ - 1 GHz)
Attached cables and harnesses (30 MHz -250 MHz)
Connector and shield terminations (30 MHz -250 MHz)
it often gives early visibility of problems that are related to the entire system.
In one of my previous posts, I explained how I divide the frequency range into three broad regions to help identify likely culprits.
That same way of thinking applies here: RE helps you quickly understand where energy is coupling into radiating structures, regardless of the original source.
In my experience, RE is often the most informative first diagnostic tool for products dominated by fast digital activity.
RE tends to be particularly useful when:
The product contains high-speed digital circuitry or multiple clocks
External I/O cables are present during normal operation
The enclosure design is still evolving
Emissions vary with operating mode or configuration
In practice, RE testing quickly highlights which parts of the system act as effective radiators, something CE alone cannot reveal. Do we have an unterminated heatsink? -> RE will tell you!
Examples where RE is usually the better starting point
Consumer electronics with high-speed I/O
Embedded computing platforms (tablets, phones, SBCs, etc.)
IoT devices with external sensor or data cables
In short: most electronics with a strong digital presence and attached cables, heatsinks, etc.
When Conducted Emissions Are the Better Starting Point
Conducted emissions testing focuses on noise that couples onto power and (some) I/O lines, typically measured using LISNs or equivalent networks. CE is often the better first step when:
The product is mains-powered or contains AC/DC or DC/DC converters
Failures are expected at lower frequencies (e.g. missing Y capacitor, poor LP filter, earth and GND connected together)
Cable routing (Ethernet, USB, etc.) near AC power cable.
Examples where CE usually deserves priority
Industrial controllers with SMPS and high-speed interfaces
Medical and laboratory equipment(for example, missing or constrained Y-capacitors often makes CE more challenging)
EV chargers(the pilot signal uses earth as a return path and can inject noise onto PE, often leading to CE issues)
In these cases, starting with RE can make it harder to distinguish between power-related noise and secondary radiation effects.
CE vs RE: Practical Decision Flowchart
Use this flowchart to decide which test to run first before you even go to the EMC testing facility.
START
|
|— Is the product mains-powered or has a noisy DC input?
| |
| |— YES →
| | • Are there SMPS, motor drives, or large converters?
| | |
| | |— YES → Start with CONDUCTED EMISSIONS (CE)
| | |
| | |— NO → Continue evaluation
| |
| |— NO →
|
|— Does the product normally operate with external I/O cables attached?
| |
| |— YES →
| | • Are there fast digital edges or multiple clocks?
| | |
| | |— YES → Start with RADIATED EMISSIONS (RE)
| | |
| | |— NO → Continue evaluation
| |
| |— NO →
|
|— Is the enclosure unfinished, plastic, or lightly shielded?
| |
| |— YES → Start with RADIATED EMISSIONS (RE)
| |
| |— NO →
|
|— Are early failures expected below ~30 MHz? (e.g. medical decice EVC)
| |
| |— YES → Start with CONDUCTED EMISSIONS (CE)
| |
| |— NO → Perform short CE + RE scans
|
END
So....
Radiated and conducted emissions are complementary diagnostic tools, not competing tests. Choosing which one to start with is a strategic decision that can significantly reduce EMC labs cost and debug time.
The right first test is the one that tells you where the energy is escaping, not simply the one listed first in the standard.
If your product has failed EMC testing and you are stuck, you can email me with:
• Product type (industrial / consumer / medical)
• Test that failed (RE, CE, RI, ESD, etc.)
• Frequency range of the issue (if known)
• Main power voltages involved (if known)
I will tell you whether this is something I typically help with and where the problem usually comes from.
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