Pre-compliance EMC Testing Using a Real-time Oscilloscope

Figure 1. Formula for calculating radiated power from electric
field measured at a given distance. Only a few nanowatts of
radiated power can cause a product to fail an EMC certification test.

When designing an electric circuit board, we always start with a schematic. All it tells us is the components in use, how they are connected, and what the functionality of the system is. The schematic tells us absolutely nothing about signal integrity, power integrity or electromagnetic interference (EMI). All the schematic tells us about is the connectivity.

Problems with signal integrity, power integrity and EMI all come to life when we turn that schematic into a physical implementation, because once we have connectivity established by the interconnects, the only thing interconnects are going to do is screw up our beautiful design. They're going introduce noise, and that noise is going to cause some combination of signal integrity, power integrity and EMI problems. The best we can do is to minimize its appearance and impact using best design practices.

In this series, we'll focus on design issues that affect EMI, and how you can use a real-time oscilloscope to find the root causes of EMI that negatively affect a product's electromagnetic compatibility (EMC).

 EMC refers to an electric/electronic device's ability to interact safely with other electronic devices (and living creatures) within its electromagnetic environment. Although the oscilloscope does not actually test for EMC, it can be an important tool for pre-compliance EMC testing in both time and frequency domains.

There are a number of certification tests for EMC. The one we'll discuss here, the United States Federal Communications Commission (FCC) Part 15 for Radiated Emissions, tests that a product meets the standard for acceptable far-field radiated emissions for Class A or Class B. The tests are very similar, with just a few small differences. 

Figure 2. FCC-style EMC certification test
in an anechoic chamber.

Fundamentally, the product under test is placed in an anechoic chamber so that there are no sources or reflections from the environment, only the radiated emissions from the product. The product is situated one meter above the floor, and an antenna is placed some distance away (Figure 2). 

For Class B, the more stringent test for consumer environments/products, the antenna is 3 meters, or about 10 feet, away. The antenna moves 180 degrees above and below, and 360 degrees around the product looking for the absolute worst case radiated emissions in all orientations as the product is operating normally, scanning with a 120 kilohertz bandwidth detector, and sweeping a wide range of frequencies. The test confirms that the peak radiated emissions from the product within that bandwidth across a wide spectrum are below the maximum acceptable levels required by the FCC. 

Table 1: FCC maximum allowed radiated emissions
for Class B products operating at various frequencies.

Table 1 shows a few of the maximum values at various frequency ranges for Class B products. Generally, if a product passes Class B, it will also pass Class A. The Class B criteria for passing at low frequencies, roughly 100 MHz and below, is about 100 microvolts per meter (µV/m). If a product radiates less than 100 µV/m electric field when measured from 3 meters away at frequencies of 100 MHz and below, then it can pass an FCC test. This is going to be our estimate of what is considered acceptable total radiated emissions for a consumer device.

One way to understand what that number means is to imagine a radio station, broadcasting 360 degrees in a spherical pattern (Figure 1). We're going to stand three meters away from this radio station to measure its electric field strength. If we have a radio source that's broadcasting isotropically, what is the power that must be radiating from that source in order to give that electric field strength at that distance? Keep in mind that an electric field is an amplitude, not a power. The power is calculated as the square of the electric field. 

Using the formula in Figure 1, where E is the electric field strength in V/m, R is the distance from the source to where the field is measured in meters, and P is total radiated power in Watts, if we measured 100 µV/m at 3 meters, the total radiated power in Watts is 10 nW. That's all. If a Class B device radiates more than a few nanowatts of power in a 360-degree orientation, it will fail an FCC test or other EMC tests required to ship products in the United States and other countries.

That's why it can be so difficult to pass a radiated emissions EMC test, because it doesn't take very much power radiating into that 120 kHz bandwidth of the FCC test to fail, and why there's so much engineering involved in EMC. Full FCC-style testing in an anechoic chamber is very expensive. No one wants to submit their product to such a test unless they are fairly confident it will pass, which is why pre-compliance EMC testing is so important for anyone designing an electronic product.

Watch Dr. Eric Bogatin demonstrate many bench top tests for radiated emissions in the on-demand webinar, Pre-compliance EMC Testing with a Real-time Oscilloscope.