[SI-LIST] Re: Common Mode vs. Even Mode
- From: "Loyer, Jeff W" <jeff.w.loyer@xxxxxxxxx>
- To: "'degonzal@xxxxxxxxxxxxxxx'" <degonzal@xxxxxxxxxxxxxxx>,"Loyer, Jeff W" <jeff.w.loyer@xxxxxxxxx>,"Mirmak, Michael" <michael.mirmak@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 16 May 2002 12:52:38 -0700
Absolutely right, thanks for catching this.
Jeff Loyer
-----Original Message-----
From: Dean Gonzales [mailto:degonzal@xxxxxxxxxxxxxxx]
Sent: Thursday, May 16, 2002 11:18 AM
To: jeff.w.loyer@xxxxxxxxx; Mirmak, Michael; si-list@xxxxxxxxxxxxx
Cc: Muranyi, Arpad; eric@xxxxxxxxxx; doug@xxxxxxxxxx
Subject: RE: [SI-LIST] Re: Common Mode vs. Even Mode
That is almost correct, however there is no such a thing as a "Differential
Mode"...only Odd Mode or Even Mode. For the case of symmetrical differential
pairs, Differential Driving corresponds to the Odd Mode voltage
pattern...Common Driving corresponds to the Even Mode voltage pattern.
Shout-out goes to Dr. Bogatin for making this point crystal clear. :)
Regards,
Dean Gonzales
ServerWorks.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On
Behalf Of Loyer, Jeff W
Sent: Thursday, May 16, 2002 10:05 AM
To: Mirmak, Michael; si-list@xxxxxxxxxxxxx
Cc: Muranyi, Arpad; 'eric@xxxxxxxxxx'; 'doug@xxxxxxxxxx'
Subject: [SI-LIST] Re: Common Mode vs. Even Mode
Here's my perception of how the terms are (or should be) used.
Strong warning: Mode in these contexts are completely different animals than
the "Mode" in TEM (Transverse ElectroMagnetic) mode.
Note: in the following I use the term "impedance a trace sees" instead of
the more verbose (but accurate) "impedance a signal sees as it travels down
a trace" (ok, so "see" is not really accurate).
Differential Mode: Driving two coupled traces with opposite polarity
signals. This can be either on purpose or accidentally (often unavoidably).
Common (or Even) Mode: Driving two coupled traces with the same polarity
signals. I believe this is always done accidentally (albeit unavoidably); I
don't know of any instance where you would intentionally do this.
Single-ended Impedance: impedance a trace sees when all other coupled traces
are left dormant.
Odd-mode impedance: impedance a trace sees when a coupled trace (or traces)
is (are) driven in the differential mode. This is most important in 2
instances:
1) where the coupled traces constitute a differential pair. This is the
impedance the traces would ideally see, if all their signal remains in the
odd mode. For ideal termination of the differential pair, this must be
considered, such as on page 66, 67 of the Hall/Hall/McCall book.
2) where the undesired effect of a coupled trace (or traces) needs to be
considered. I.E., it's known that adjacent traces in a bus will be
transitioning in the opposite direction, and some coupling is unavoidable.
If the coupling is significant (strong coupling and/or for a substantial
distance), you must consider this effect - your characteristic impedance
will be lowered and some S.I. degradation is inevitable.
Differential Impedance: this is the sum of the odd-mode impedances for 2
coupled traces (twice a single trace's odd-mode impedance for symmetrical
traces). This may be important if the coupled traces constitute a
differential pair, for selection of your termination resistor value,
depending on your termination scheme. (Often differential pairs are
terminated by putting a single resistor equal to the differential impedance
between the two traces).
Even-mode impedance: impedance a trace sees when a coupled trace (or traces)
is (are) driven in the common mode. This is most important in 2 instances:
1) where the coupled traces constitute a differential pair. This is the
impedance the traces will see, if any of the signal transitions from the
odd-mode to the even-mode, due to delay differences, for instance. For
ideal termination of the differential pair, this must be considered, such as
on page 66, 67 of the Hall/Hall/McCall book.
2) where the undesired effect of a coupled trace (or traces) needs to be
considered. I.E., it's known that adjacent traces in a bus will be
transitioning in the opposite direction, and some coupling is unavoidable.
If the coupling is significant (strong coupling and/or for a substantial
distance), you must consider this effect - your characteristic impedance
will be raised and some S.I. degradation is inevitable.
Common Impedance: I've found no use for this term, however it's defined.
-----Original Message-----
From: Mirmak, Michael [mailto:michael.mirmak@xxxxxxxxx]
Sent: Thursday, May 16, 2002 8:51 AM
To: si-list@xxxxxxxxxxxxx
Cc: Muranyi, Arpad; 'eric@xxxxxxxxxx'; 'doug@xxxxxxxxxx'
Subject: [SI-LIST] Common Mode vs. Even Mode
In the current literature, the equations used to analyze a differential
system's traces are fairly consistent: differential impedance is equal to
twice the odd mode impedance, where the odd mode is the difference between
the single-ended and coupling impedances, etc.
One point of difference is the definition of common mode impedance. Several
sources, including Douglas Brooks' 1998 Printed Circuit Design article,
state that the common mode impedance is equal to one-half the even mode
impedance. Similar statements can be found in industry specifications, such
as those for Serial ATA.
On the other hand, Eric Bogatin has explicitly stated in his differential
training materials ("Differential Impedance Finally Made Simple", 2000) that
common mode impedance is equal to the even mode impedance; yet, on this
forum, he has recently stated (in an SI-LIST e-mail, Feb. 25, 2002) that
common mode impedance is one-half of even mode impedance. The "common mode
Z = even mode Z" definition is also given in a 1999 Printed Circuit Design
article by Kaufer and Crisafulli.
What is the "correct" definition of common mode impedance? I suspect that
there is a referencing or perspective discrepancy here, but I would like to
hear a formal explanation, preferably from the above experts or others on
the list.
Thanks!
- Michael Mirmak
Intel Corp.
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