Doug Smith (www.dsmith.org or www.emcesd.com) has done work and written some articles about making scope measurements with paper-clip loops or a shorted scope probe like yours. Ideally, your probe isn't picking up any voltage from the board, but the probe loop makes it into a magnetic probe. However, since the scope itself is probably grounded (through the AC mains) too, connecting the shorted probe leads to "ground" or anything on your PC board causes a handful of interesting things to happen, including some rather large ground loops that you might not want to think about. In truth, these loops are probably there all the time even when you're probing "normal" signals, so some of the effects are things you ought to concern yourself with all the time anyway. I've heard someone suggest a way to measure "ground noise" on a PCB, is to float the scope (isolate it from AC mains ground), connect the scope probe pin to "ground" on the board, and the scope's "ground" lead to a DC power rail. In other words, connect it backwards between GND and PWR. (I don't understand the reasoning behind doing that, however.) Ask yourself what you want to measure. What is "ground noise", anyway? It makes no sense to even think about the "ground noise" at a single point in a circuit. Voltages are always relative to something! A single IC has no "ground noise" unless the IC has multiple GND pins. Presumably, what you want to check is how much this ground TP differs from that ground TP over there. If you think you are measuring "ground noise" on a circuit relative to your lab's system ground, think about the frequency range over which that assumption applies in a non-transmission line environment. Regards, Andy ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu