I believe contributors to this thread have been citing the following paper (unnamed until now):
"Article for Printed Circuit Design, By Lee W. Ritchey, 3Com Corporation 'Differential Signaling Doesn't Require Differential Impedance Or, How to Design a Differential Signaling Circuit' " I feel compelled to report that even though the author presents some noteworthy viewpoints about the practical aspects of differential logic transmission circuits, some of his assertions are in error, therefore his ideas must be applied with caution. In the fifth paragraph, the author presents the premise, ".................[differential signaling provides] immunity from common mode noise coupled into the pair by outside noise sources. ...........because any magnetic noise field couples equally into both wires resulting in a common mode signal." This statement is fundamentally WRONG. The pair in question forms a loop that is in effect a single turn pickup coil with the drivers serving as a virtual center tap. An emf is generated at the single turn coil terminals by varying magnetic flux fields cut by the plane of the coil. This is a differential voltage, NOT a common mode voltage. The greater the coil area, the more flux lines pass through the plane of the coil, consequently, the larger the emf. The shape of the loop doesn't matter; only the area. Any standard physics text or electromagnetics reference will verify this. The following sentence goes on to say, "[This] is of [no] real benefit when the wires are routed over the planes of a PCB." As a consequence of the initial wrong premise, this statement is also wrong. First of all, magnetic fields penetrate copper planes, with the degree of penetration dependent on the frequency. Therefore, trace loops adjacent to the planes will pick up these fields, the degree also dependent on the frequency. Finally, a trace loop voltage (emf) is a function of the area of that loop. Since a differential pair is effectively a loop, minimizing the pair separation minimizes the area, hence the voltage pickup as well. Clearly, in a magnetic noise environment, the differential pair spacing cannot be ignored. Once again, be careful. Regards, Fred A Rupinski ----- Original Message ----- From: "Brad Velander" <[EMAIL PROTECTED]> To: "'Protel EDA Forum'" <[EMAIL PROTECTED]> Sent: Monday, September 16, 2002 8:47 PM Subject: Re: [PEDA] Autorouter and Differential Pairs. > I knew the article and had read it thoroughly in the past several times. > ...................................he has done so much trying to characterize, > quantize or just plain dispel some of the old wives tales that plague this > industry. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[email protected] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
