Hi Zhangkun, Istvan, and the group, I would like to modify Istvan's response somewhat. Ground currents flowing from the circuit, through the probe ground lead, and down the shield into the scope chassis typically (although not always) dominates everything else. It is just L(of the ground lead)di/di that creates a voltage across the probe from tip to ground lead attachment point to the probe. The probe reads this voltage like any other. This voltage is typically about 50 to 100 times larger than what leaks through the probe cable shield. Doug Istvan Novak wrote: > Dear Zhangkun, > > If you connect the signal and ground pins of a probe together and > connect both to a ground point on your system, the signal you see on the > oscilloscope could be a combination of several things: close-field > radiated noise picked up by the active probe head, radiated noise picked > up by the cable connecting the probe head to the scope, large > common-mode noise converted somewhere along the path due to saturation > of circuitry, etc., etc. The purpose of such baseline measurement is not > necessarily to identify the sources or these noise contributors one by > one, rather to establish their cumulative effect as a lower limit of the > measured values, below which you need to throw out the data. > > Best regards, > Istvan .......lots of lines deleted -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: doug@xxxxxxxxxx \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- ------------------------------------------------------------------ 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