Inject voltage pulses and troubleshoot
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2008
Engineers often need to test systems for immunity to voltage pulses in cables, but how do you reliably inject those pulses? In the Webcast “Inject pulses into circuits and test for EMI immunity,” consultant Doug Smith explains how to use current probes to inject voltage pulses into cables. Smith, who is an expert on high-frequency measurements and electromagnetic compatibility (EMC), explains that by taking advantage of inductive coupling, you will be able to simulate conducted electromagnetic interference (EMI) in cables.
Fast pulses generate electric fields that can couple into cables and turn into current. Smith uses a pulse generator to inject up to 400 V into a cable. Because the pulses have a short duration—just tens of nanoseconds—the average power in the current probes isn't enough to damage them.
 Current probes let you inject pulses into cables for conductive EMI testing. |
Before you can inject a voltage pulse into a system, you should measure it and the current it induces. In the Webcast, Smith shows how to induce a current into a circuit. He uses two current probes, one to induce a current in a wire (figure) and a second to measure the current. The wire, passing through both current probes, connects between an oscilloscope probe's tip and ground. A voltage pulse from a pulse generator produces a current in the wire. The second current probe converts the current into a corresponding voltage for the oscilloscope to display. The induced current, when injected into a system, lets you test a system's immunity to conducted EMI.
In the Webcast, Smith also shows how to use two current probes and a spectrum analyzer to find the resonant frequency of a cable. Finding the resonant frequency of a wire is important, because the current induced by an injected voltage will peak at the cable's resonant frequency.
To see the details of how Smith sets up and performs the measurements, view the archived Webcast at www.tmworld.com/webcasts.
