Setting Up Your Oscilloscope for EFT Testing

Figure 1: The typical EFT test signal consists of
multiple exponential pulses arranged as pulse bursts.

A third type of electromagnetic compatibility (EMC) testing deals with how devices respond to electrical fast transients (EFT). EFTs are a series of fast, high frequency pulses, often occurring in bursts. These transient events are the result of electrical arcing. EFT pulse bursts occur when a power connection is made or broken, equipment is powered down or circuit breakers are switched. They also occur when inductive loads such as relays, switch contactors or heavy-duty motors produce bursts of narrow high-frequency transients on the power distribution system when de-energized.

The typical compound waveform used for EFT testing is shown in Figure 1.

Figure 2: Acquisition of two EFT bursts at
1.25 GS/s, zoomed to show several timing epochs.

The pulse waveform for EFT testing is defined by a risetime of 5 ns and a pulse width of 50 ns. These pulses are combined into bursts of 5 to 50 pulses spaced at from 10 to 100 µs (10 to 100 kHz). The bursts are typically spaced as much as 300 ms apart. Figure 2 shows an EFT test signal, with two EFT bursts captured by an oscilloscope on channel 2 sampled at 1.25 GS/s.

The two EFT pulse burst are shown in the upper left-hand trace (the two pink blocks). A series of zoom-on-zoom traces are opened to show several timing epochs. Trace Z2 (second from top left) shows a single burst. The zooms keep expanding the horizontal scale until finally at Z8 (bottom right) we see a single EFT pulse.

Following are six, important things to do to make sure you get the best EFT test measurements from your oscilloscope.

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.

Getting The Most Out Of Your Oscilloscope: Math Functions

Figure 1: Parameter math functions
provide a way to create custom
parameters

Parameter math functions are an important part of an oscilloscope's analysis capabilities. Using parameter math, you can create custom parameters based on simple arithmetic relationships between existing parameters. It allows you to add, subtract, multiply, divide, or rescale parameters (Figure 1).

Getting The Most Out Of Your Oscilloscope: Tracks and Trends

Figure 1: The track math function
shows how data changes over time

Our discussion of cursors and parameters leads us neatly into the topics of tracks and trends, which are both means of extending parameter measurement into a more analytical direction. We can do this using math functions that are part of your Teledyne LeCroy oscilloscope's toolset.

Getting The Most Out Of Your Oscilloscope: Cursors and Parameters

Figure 1: Cursors (top) and parameter
measurements (bottom) are both
powerful tools in their own right

Oscilloscopes give us a wealth of tools with which to view, measure, and analyze the performance of a circuit. Broadly speaking, two classes of such tools are cursors and parameter measurements. Taken on their own, both classes afford the user a great deal of capability. Put them together, though, and you can really start to gain deep insights into what's going on with a waveform.

The Periodic Table of Oscilloscope Tools: Measure

Figure 1: Measure
tools are at the heart
of an oscilloscope's
utility

An oscilloscope is only as good as the tools it provides to users for acquiring, viewing, measuring, analyzing, and documenting waveforms. We present an overview of our deep collection of oscilloscope tools in our Periodic Table of Oscilloscope Tools, and in prior Test Happens posts, we've surveyed the Capture and View categories. Today we'll break down the Measure section of the Table.

Reducing Errors in Switch-Mode Power Supply Measurements

Figure 1: Skew between voltage and current probes

results in power measurement errors

Almost all portable electronic devices, and lots of non-portables, come with switch-mode power supplies. These range from common "wall warts" to the larger brick-sized supplies that power a laptop. We've taken a look at the typical measurements one might make on a switching power supply and at why single-ended measurement techniques should take a back seat to differential approaches. Now, let's see what steps we can take to ensure that our measurements on power supplies are accurate.

Use Trend Functions To Take Power's Pulse

When troubleshooting or debugging almost anything, the first step is usually to check what's going into the device or circuit and what's coming out on the other side. So it is with power supplies, which leads us to power line monitoring and trend functions.