Hi there, Good question, I knew that AC follows the path of least impedance, but had know idea at what frequency this effect would become apparent. Bonus question, same deal, as long as it's got the path of lowest impedance. But I have an extension to the bonus question. Assuming you have PWR-GND plane pairs in a multilayer stackup, is one inherently noisier than the other? ie is it preferable to route next to the GND plane....or does that depend? I guess a factor would be the coupling between the planes, but who is to say the GND will be less noisy than a PWR rail? Also when we have 3 or 4 plane pairs, but with each PWR plane being a different voltage, with different noise levels, how does this effect our "clean" GND rail? In the end we just do what the engineers ask and put sensitive signals next to GND planes, it sounds obvious and safe to route next to GND planes, but without further investigation I feel there is a lot of "ASS U ME ing" going on. Regards, Steve Steve Kingdon PCB Design Engineer Allied Telesyn Research >>> [log in to unmask] 03/12/04 07:54:54 >>> Return Current Path vs. Frequency -=-=-=- Easy question: Imagine a "U"-shaped trace above a solid ground plane, with a connection at each end. Inject a signal through the "U", and vary the frequency. At what frequency will you see the return current follow the shape of the U rather than the shortest path? -=-=-=- Bonus question: What if I replace the ground plane with a power plane? (I say it makes no difference. ) thanks, Jack (the "arguing with a new engineer" guy) --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 --------------------------------------------------------------------------------- NOTICE: This message contains privileged and confidential information intended only for the use of the addressee named above. If you are not the intended recipient of this message you are hereby notified that you must not disseminate, copy or take any action in reliance on it. If you have received this message in error please notify Allied Telesyn Research Ltd immediately. Any views expressed in this message are those of the individual sender, except where the sender has the authority to issue and specifically states them to be the views of Allied Telesyn Research. --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ---------------------------------------------------------------------------------