Conducted Emissions Testing
What is Conducted Emissions Testing?
Your device creates electromagnetic energy and a certain portion of it will be conducted onto the power supply cord.
In order to restrict the amount of interference your device can couple back onto a power supply, test labs measure these emissions (usually from 150 kHz – 30 MHz, and verify that they comply with specified limits.
This helps to ensure that the local power supply remains relatively ‘clean’ and nearby devices won’t be affected by your device.
Conducted Emissions Test Setup
Above, you can see a diagram of a typical conducted emissions test setup, according to ANSI C63.4. The LISN (or LISNs) sit on the ground, while your product sits on a table (or remains floor standing if the equipment is large).
The RF port of a LISN connects directly to a spectrum analyzer (or via a transient limiter to prevent damage from voltage spikes).
Conducted Emissions Applicability
Conducted emissions testing is usually performed on devices that connect to an AC power supply. That is regardless of whether you’re using a pre-certified AC-DC power supply adapter.
For some standards, there area also limits placed on devices that operate from a DC power supply.
Conducted Emission Limits
Below you can see the FCC’s conducted emission limits for class A and class B devices. You’ll notice that class B limits (for domestic environments) are more strict, probably because the number of devices expected in a household is large, and users want to be sure that devices won’t interfere with each other through conducted noise on the AC power supply.