Testing for Near Field Radiated Emissions in the Time Domain

Figure 1. Triggering on a time-domain signal
likely to be coincident with radiated emissions
helps to locate where in the channel di/dt occurs.

There are three, principal root causes of the common currents that lead to radiated emissions in electronic devices:

1. Return path discontinuities

2. Physical structures that are not tightly coupled to the return plane

3. Ground loops causing common currents in cables

We’ll briefly demonstrate a bench top test for finding sources of near field radiated emissions caused by return path discontinuities using a real-time oscilloscope in the time domain.

Why the time domain? Although EMC compliance testing is done in the frequency domain, in the time domain we can see the signatures of near field emissions in a way that yields information about the root causes of those emissions. It is a type of pre-compliance EMC testing that can be easily done in your lab, without the expense of an anechoic chamber.

To demonstrate a few interconnect structures that cause near field emissions, we used the following equipment:

A real-time oscilloscope (we used 12-bit WavePro HD)
A signal generator
A current probe (we used a CP30A clamp-on style Hall-effect current probe)
A magnetic loop “sniffer” probe (or a 10x passive probe looped to create a “sniffer”)
The DUT

The key to testing in the time domain is to trigger the oscilloscope on some signal that has the potential for influencing radiated emissions, such as a switching signal. If we can trigger on the switching signal, we can find near field emissions that are synchronous with that signal. The time domain is so useful in helping us pinpoint exactly when we have emissions, when we have the di/dt in a channel.

Figure 2. Test board mimicking DUT with
discontinuity caused by gap in the return plane.

The DUT in our examples (Figure 2) is a 4-layer test board that was designed by Dr. Eric Bogatin and his students at University of Colorado, Boulder to demonstrate the effects of ground bounce on radiated emissions.

There are three sections. Each section contains two microstrip traces. One trace runs over a uniform wide plane, the other runs over a gap. This is a classic example of a return path discontinuity. When a signal is sent down the trace that crosses the gap, the return current has to snake back around the gap, forming a loop between the signal and return. We have increased the inductance in the path and are going to have a different voltage on the right side of the board than on the left side of the board. As a result, we're going to see radiated emissions from the loop antenna created around the gap by the return current, and from the patch antenna created by the differing voltages on each side of the board.

Our signal (generated by a function generator) is a 10 kHz square wave with a 9 ns edge.

The signal passing down the microstrip is shorted at the far end of the line so that the current is going to go back through the return plane and the gap in the return plane.

In order to have a signal to trigger on that tells us when the current is passing through the gap, we clamp a current probe onto the (short) loop to measure the current. In our test, about 100 mA of peak-peak current passes through the loop as a square wave (the yellow C1 trace).

The sniffer probe is connected into oscilloscope C2 using an SMA cable.

When do we expect to read radiated emissions from this structure? Answer: at the switching edges of the current signal, which are synchronous with the return current looping around the gap. It's the di/dt that causes the radiated emissions. With a constant current, we have magnetic field, but it is not changing, not radiating.

For the best view, we zoom in so that we see only the C1 trigger edge on screen, C1 vertical sensitivity at 20 mA/div and C2, the near field probe, at 50 mV/div. As we move our sniffer probe around the gap in the microstrip, we can see in the pink C2 trace some large magnetic fields that were picked up, inducing a voltage in the sniffer loop (Figure 3). Moving the sniffer probe around the board shows that the emissions are very localized to the discontinuity.

Figure 3. Set up for near field emissions test (right) and results showing resonance (left).

There is also a resonance in the C2 trace. Why do we have that resonance there? If you’ve watched Dr. Bogatin’s webinar on What Every Oscilloscope User Needs to Know About Transmission Lines, you will immediately recognize what look like reflections in a cable.

Let's consider the structure that we have. The low-impedance source loop goes down the cable and induces the di/dt in the vicinity of the gap. The near field emissions induce the voltage in that coil. That voltage moves down the transmission line into the oscilloscope, which is set for 1 M𝝮 impedance, so the signal goes from the 50 𝝮 impedance of the cable to the 1 M𝝮 impedance of the oscilloscope. It's going to reflect, come back, see the low impedance of the short at the other end, change sign, and bounce back and forth. What we're seeing here is ringing in the cable caused by introducing that low impedance voltage source. We're not really seeing the near field emissions. Or rather, we're seeing near field emissions exciting a ringing in our cable. That's why terminating the cable is so important.

When we set the termination to 50 𝝮, the resonance is removed, and what we see are the near field emissions from the structure itself (Figure 1).

Watch Dr. Eric Bogatin test radiated emissions from other board structures in the on-demand webinar, Pre-compliance EMC Testing with a Real-time Oscilloscope.