Dear Eric, Thanks for the clearance. I would like to share some different thoughts. They may not be quite correct. Actually, if we notice that "mode" is typically a field concept (pattern) and "voltage" is a circuit concept (based on integral of fields under certain conditions), it is not difficult to distinguish them. Voltage can be in both in time domain and frequency domain, mode only in frequency domain. Although there are some authors using the same word "mode" to describe the decomposition of voltage vectors, the word appears as "differential mode" and common mode", not as "even and odd modes". Obviously, we do not talk about "ground plane" in optics or waveguide theory since the quasi-static concept is invalid theoretically and practically. But "mode" still survives. There are even higher order modes !! Even we talk so much about current (electrons) flows in electronics, the fact is that no single electron can physically migrate from one end of a wire to the other. In stead, electromagnetic field propagates. Another question is what are the essences of differential signals ? It is the relation between two signals sent and received by the same pair of driver and receiver. Same magnitude and out of phase ? Cancelling return currents ? Looks like, differential signals are described in both time and frequency domain too. If you look at all the digital design books, differential signals are described as two step pulses with opposite signs. However, in analogy design, they are defined as two signals with the same magnitude and out of phase (like full wave analysis). Of course, it can be easily shown that they are consistent by doing Fourier Transformation. However, if differential signals are related simply to (even or odd) mode (as we did), both must be in frequency domain. Now the discussion we are having is about the propagation (transimission) of differential signals using transimission lines. The purpose is to transmit differential signals from the driver to the receiver. In principle, the status in the middle of the transmission is not important if the output keeps the differential relations. Although we can not prove that using symmetric lines is the only solution, it is the simplest nominal solution and the easiest and probably the best practice. Finally, when we say differential mode and common mode, do we mean the local (static) (on one plane) relation ? It has nothing to do with the propagation. Regards, Mick -----Original Message----- From: Eric Bogatin [mailto:eric@xxxxxxxxxxxx] Sent: Monday, February 25, 2002 8:06 PM To: si-list@xxxxxxxxxxxxx; doug@xxxxxxxxxx Cc: garyo@xxxxxxxxxxxx; eric@xxxxxxxxxxxx Subject: [SI-LIST] Even mode, common mode, and mode conversion Doug and others- you have hit upon one of the most common sources of confusion in the industry, about odd and even and common and differential. The problem is, most folks use these terms incorrectly, and the error has propagated so much, it has turned into fact. This isn't even one of those myths, it's just a blatant error. In a nutshell, odd and even modes, and any modes in general, are special voltage patterns that propagate undistorted down a pair of transmission lines. For example, in a pair of microstrip traces, if you send a +1 v on one line and a 0v signal on the other, the actual voltage on the two lines will change, as the signals move down the line. The 0v line will see a growing negative signal as the far end cross talk builds up and the +1v signal will drop and distort as it looses energy to the quiet line. This voltage pattern is not a mode. It is just a particular driven voltage pattern. There is nothing special about it. However, there are two special voltage patterns that you can impose on the lines which will not change as the signals propagate down the lines. If you put a +1v on each line, wrt the return plane below, there will be no voltage difference between the two signal lines and the voltage pattern will continue undistorted. The other voltage pattern is a +1v and a -1v applied to the two lines, wrt the return plane. These special voltage patterns are called modes. We typically call the +1v, -1v pattern the odd mode and the +1v and +1 v the even mode. The odd mode impedance is the impedance of one line when the pair is driven in the odd mode. The even mode impedance is the impedance of one line when the pair is driven in the even mode. As the coupling between the lines increases, the odd mode impedance gets smaller, since the line has a higher current needed to drive a signal in the odd mode, as there is more current between the lines due to the coupling. The even mode impedance increases as the coupling increases since one line "shields" some of the capacitance to the plane of the other line. We can drive the pair of lines anyway we want. When we drive the signals with a differential signal, and we have symmetric lines, we happen to drive them in the odd mode. There is no such thing as a differential mode. There is odd mode and there is driving a signal differentially. Many of us have gotten in the bad habit of saying "differential" mode when we really mean odd mode. Differential impedance refers to the impedance the difference signal sees as it propagates down the differential pair of lines. What makes it very confusing for most folks is that if you are brainwashed into thinking of differential mode and odd mode, then its hard to understand what the difference between the odd mode impedance and the differential impedance is. In fact, the differential impedance is just 2 x the odd mode impedance. It is the voltage of the difference signal divided by the current going into one trace, when there is a differential signal between the two traces. The common impedance is the impedance the common signal sees propagating down the differential pair of lines. The common impedance is actually 1/2 the even mode characteristic impedance. If these are topics you are interested in, we cover them in great detail in our signal integrity training classes. The issue of odd and even mode impedance and differential and common driving is covered in detail in our GTL122 SI 101 class. Check our web site for details: www.gigatest.com --eric ******************************************* Dr. Eric Bogatin GigaTest labs www.gigatest.com ******************************************* From: "Doug Brooks" <doug@xxxxxxxxxx> To: <si-list@xxxxxxxxxxxxx> Sent: Monday, February 25, 2002 9:31 AM Subject: [SI-LIST] Even mode, common mode, and mode conversion > > Gurus, > > I've been following the thread on mode conversion and suspect I am not the > only one with this question. Can someone explain, in layman's terms: > > The distinction between even mode and common mode? > between odd mode and differential MODE (not to be confused with > differential traces) > > And does the existence of differential traces add any complications to > these distinctions? > > Thanks for bringing the dummies along with you! > > Doug Brooks ------------------------------------------------------------------ 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 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 ------------------------------------------------------------------ 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 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