Setting Up Your Oscilloscope for Surge Testing

Figure 1: A typical EMC surge test waveform with
1.2 µs rise time and 50 µs half amplitude time.
Zoom trace Z1 shows the details of the rise time.
Click any image to expand it.

Electrical surges result from phenomena like lightning strikes and switching transients.  Electronic devices are subjected to simulated surges to confirm that they continue to operate properly following a surge, just as they are tested against electrostatic discharge. 

Surge pulses are similar to ESD pulses in that they have a very fast rise time, but the fall time is much, much slower. Figure 1 shows a typical surge pulse waveform. Surge testing involves similar measurements to those made for  ESD pulse testing—such as rise time, pulse width and max—but surge testing also requires some additional measurements, such as area under the pulse curve and transmitted charge. 

Figure 2: To verify the surge generator waveform, the
oscilloscope is connected to the generator through an attenuator.

As with ESD pulse tests, oscilloscopes are primarily used to “test the tester,” confirming the output of the surge generator. So, before a surge generator is hooked up to the device under test, it's required to hook it up to an oscilloscope and conduct a series of measurements to make sure the surge pulse is within specification. Figure 2 shows a typical surge test setup.

Following are three, important things to do to make sure you get the best surge measurements from your oscilloscope.

Setting Up Your Oscilloscope for ESD Pulse Testing

Figure 1: An ESD calibration test setup. 
The ESD gun discharges its waveform
into a properly attenuated current target. 

Electrostatic discharge (ESD) pulse tests are a type of conducted immunity testing done to confirm that a device can withstand a sudden transient electrostatic discharge. It is done by using an ESD gun to shoot a pulse of the required voltage at a device while testing that the DUT continues to operate properly. The ESD pulse shape simulates a person, carrying a static charge, touching a device. When their fingertip first touches the device, there is a leading edge with a high peak and fast decay, often visible in the real world as a spark flying from the fingertip. This is followed by a second edge due to the charge in the rest of the human body that propagates toward the fingertip with a time delay.

Oscilloscopes are most often used to “test the tester” in ESD pulse test setups, confirming that the pulse from the ESD gun is the right shape and meets the requirements of the standard to which the device is being tested. A typical calibration test setup is shown in Figure 1. The pulse from the ESD gun is fired directly into a current shunt target connected to the oscilloscope through an attenuator required to keep the signal within the limits of the oscilloscope’s 50 Ω input, which is used for this testing. Then, key parameters of the ESD pulse are measured per one of several standards, such as IEC 61000-4-2.  

ESD standards require a range of measurements. The most common are the initial edge 10% to 90% rise time, peak amplitude, pulse width, amplitude and current levels at specified times from the initial edge (e.g., T1 and T2), and time to half value. 

Following are four, important things to do to make sure you get the best ESD pulse measurements from your oscilloscope.

Why IEEE's Pulse Definitions and ESD Pulses Don't Mix

Figure 1: The IEEE's pulse definitions, which don't fit
the bill for measuring ESD pulses

The IEEE's pulse definitions, found in the organization's Std 181-2011 that covers transitions, pulses, and related waveforms, set the bar for how pulse measurements are determined. These definitions, which are in the DNA of all oscilloscopes, are just the thing for measuring repetitive pulses such as clock signals but not so much for ESD/EMC measurement requirements. In this post, we'll discuss why that is and what you should do differently for measuring ESD pulses.

Making EMC/ESD Pulse Measurements

Figure 1: Oscilloscopes are used for
testing in the green-shaded boxes

There are many circumstances in which electromagnetic compatibility (EMC) and electrostatic discharge (ESD) testing are a fact of life. Many countries have adopted IEC international standards that dictate certain levels of immunity, as have the automotive, medical, military, and aerospace industries. In this post, we'll begin looking at how oscilloscopes figure into tests for both radiated and conducted EMC/ESD immunity.