|Figure 1: Oscilloscopes are used for|
testing in the green-shaded boxes
First, let's consider just what kind of EMC/ESD testing oscilloscopes are used for. The overall scope of EMC/ESD testing falls into four broad quadrants (Figure 1). Of those four, oscilloscopes come into play in the two shaded in green: radiated and conducted immunity. In the former case, we want to know if the equipment under test (EUT) can withstand EMC/ESD emissions in free space or via cables. In the latter, we're investigating whether the EUT is susceptible to transient pulses arising from switching or to surges.
|Figure 2: A diagram of the test setup for ESD calibration|
(left) and of an idealized IEC 61000-4-2 simulator waveform
Most ESD testing follows the IEC 61000-4 standard that must be met for products to earn the CE mark. Figure 2 shows a diagram from the specification showing the current shunt target and where the ESD gun should connect to it. At right in Figure 2 is a depiction of an idealized IEC 61000-4-2 pulse. Such a pulse has a very fast rise time followed by an exponential decay that may hit several bumps as it deteriorates.
Typically, rise and fall times for ESD pulses fall in the range of 0.7 to 1.0 ns. Measurement calls for the capture of a single pulse and to measure and verify its rise time (for positive-going pulses) or fall time (for negative-going pulses). For the most part, required oscilloscope bandwidths are 1, 2, or 4 GHz. However, some of the newer ESD standards are drawing bandwidths up into the 16 to 20 GHz range to accommodate very fast pulse rise times.
Next, we'll dig into IEEE standard pulse definitions and why they serve well for measuring clock signals but maybe not so well on ESD pulses.