Pre Compliance Methods for Emission Testing – Part 2

Continued From Part 1…

GTEM Cell

• GTEM Cell: GTEM Cells are like mini semi-anechoic chambers. GTEM cells can be used both for emission testing as well as radiated immunity testing. For this pre-compliance option, you would need a GTEM along with a spectrum receiver/analyzer. The equipment under test is placed inside the Gigahertz Transverse Electromagnetic (GTEM) Cell, between a resistive load and antenna. Though the GTEMs vary in size from small (50 cm) to a few meters, but the size of the EUT is restricted to a cubic area only within the GTEM. Due to the small dimensions, it is very easy to generate a large electrical field of 30V/m or higher using only small power-amps than one would need in a Semi-anechoic chamber. GTEM cells are not only easy to use and cost effective, but they also save a lot of time since one can do 8hrs work of emission testing in less than 1hr only with an automated GTEM. However, the results from a GTEM should be taken with care because they can vary a great deal from a chamber or OATS.
• Cable Current Clamps: This can be a good and an inexpensive method to sanity check whether your cabling is able to meet the radiated emissions requirement at a test lab. For this pre-compliance option, two equipment i.e. current probes and spectrum receiver/analyzer are required. By measuring the radio frequency common mode currents present on each of the cables connecting to your product, you can easily judge the approximation to the far field electric field strength that is measured at an EMC Testing Lab due to those cables.

Conducted Emissions Pre-Compliance

Regulatory bodies have prescribed limits for the amount of noise that electronic equipment connecting to a public AC power source can inject back onto the power grid.  

Line Impedance Stabilization Network (LISN)

An EMC Testing Lab uses Line Impedance Stabilization Network (LISN) to test these emissions along with a spectrum analyzer. Other equipment like CDNs may also be required depending on the setup.

A LISN is a low pass filter strategically placed between the EUT and a DC or AC power source to provide an RF noise measurement port which connects to an oscilloscope or a spectrum analyzer, and to create known impedance. It separates the unwanted RF signals from the power source. Commercial LISNs are reasonably inexpensive ranging between $1k – $5k.

Annex A of CISPR 16-1-12 or section 4.8 of C63.4 gives circuit diagrams for a few different types of LISNs that you can also create on your own. But since you’ll be dealing with a large amount of energy stored in capacitors and main voltages, this route can be really dangerous.

ITC India is an NABL accredited testing laboratory providing EMC Testing Services along with many other testing services as per the client’s requirement. Contact b.vig@itcindia.org to get your TESTING QUOTE TODAY or visit our website https://itcindia.org/emc-emi-testing/ to know more about EMC Testing.