The first thing to do is to make sure that the oscilloscope is on a clean power source. Make sure that it is plugged into a branch circuit different from the ESD test equipment.
Next, reduce the length of the ground lead. Even if you are using the shortest ground clip available with the scope, the lead will be too long. The loop area formed by the ground lead will act as an antenna. (As a test, you could place the scope probe by the test unit with the ground lead clipped to the probe tip. You will still see a significant signal on the "grounded" probe.) Remove the cap from the probe to expose the ground reference shaft. Take some hook-up wire and wrap a few tight turns around the ground barrel of the probe. Cut it off with enough length from the coil to solder to the board. Remove the coil and solder it to a good ground reference on the circuit board near the point that you want to measure. Take another short piece of hook-up wire and solder it to the signal that you do want to measure. Place the probe through the coil for the ground and wrap the signal wire around the probe tip. This should let you do some hands free measurement. This will keep the ground return loop as small as possible. The coaxial scope probe cable should block enough noise to prevent coupling along that path. You will still see some radiated noise getting into the probe, but you will get much more accurate measurements than you can with the standard scope probe ground leads. Ted Eckert Regulatory Compliance Engineer American Power Conversion Please respond to "Bailey, Jeff" <jbai...@sstech.on.ca> To: "'emc-pstc'" <emc-p...@ieee.org> cc: (bcc: Ted Eckert/SDD/NAM/APCC) From: "Bailey, Jeff" <jbai...@sstech.on.ca> on 12/22/99 12:56 PM Subject: ESD troubleshooting Hello all, first off I would like to thank all who responded to my inquiry about the EMC program offered by UMR, I am looking into enrolling further now. I am now looking for any pointers from those more experienced than I. I have a product that is giving me some grief when indirect ESD events applied to the HCP are applied at 4kV and above. I would like to be able to view the disturbances that are present on the PCB during the event so I can try to trace down where they are coming from and judge what attempts at fixing the problem actually provide any difference. Can any of you give me some pointers or rules of thumb on how to keep the interference from the event form coupling onto a set of scope leads? I can see lots of noise but cannot definitely say that it is present on the PCB or if it is just coupling into the leads. There is noise present visible when the scope probes are left unconnected but the trace is quite different from that seen with them connected to the PCB. (but of course there is no solid reference with the probes unconnected) I need to isolate the scope leads from the test so I can actually see what is getting on to the PCB, is this actually possible??? Thanks for any comments or help. Best Regards and best wishes for the holiday season to everyone!! :-) Jeff Bailey EMC Technologist SST - A Division of Woodhead Canada Ltd. Phone: (519) 725 5136 ext. 363 Fax: (519) 725 1515 Email: jbai...@sstech.on.ca Web: www.sstech.on.ca All comments contained in the message are my own and do not necessarily express the views of SST/Woodhead Canada Limited. --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
