Mail*Link(r) SMTP FWD>DES: ODD/EVEN Mode Impedances
John and other Technet's
I haven't seen a lot (or an abundance) of postings yet on being "odd or
even". So I hope the following stimulates some additonal correspondance.
John <[log in to unmask]> asked:
>Could anybody please explain the "odd mode" the "even mode"and the
>"common mode" impedances, as in Differential Impedance. What they
>are, the benefits and drawbacks?
Once again, ol'Ralphie is subjecting you to a long response to a few
relatively short questions.
Well -- here's an attempt to cover most of the request, and I hope this
provides you with a satisfactory response.
ODD/EVEN
The "odd/even mode" of operation of transmission lines is related to operating
frequencies and physical (electrical) spacing. The key is the distribution of
the electrical and magnetic fields. First I'll attempt to describe the
odd/even mode of operation then "differential"
For convenience, odd/even mode of operation will be limited to a microstrip
transmission line. In an "even-mode" of electrical operation the electrical
and magnetic fields are symetrical around the signal conductor.
EVEN-MODE
In the "even-mode" of electrical signal operation, the electrical fields are
between the signal conductor and the signal reference conductors (signal
ground planes) as shown in Figure 1. The primary electrical fields are
between the signal "s" conductors and the signal reference ground plane "g".
Note that the electrical field of both signal conductors is "+" with respect
to signal ground. Note that in the "signal ground (return) plane" the "-"
signs represent the signal return in the ground plane. The magnetic fields
are in the same orientation, some of the magnetic field for each signal
conductor will be local around the signal conductor; and some will be more
global, and will magnetically link up (add) with adjacent magnetic fields of
the same orientation. This magnetic coupling is not desirable (except for
couplers and transformers) and is termed "backward/reverse crosstalk" in
traditonal crosstalk analysis.
mmmmmmmmmmmmmmmmmmmmmmmmmmmmm
m + + + + + + m
m mmmmmmm mmmmmmm m
m___m__sssss__m_________m__sssss__m___m_______
m mmmmmmm mmmmmmm m
m +++++++++ +++++++++ m
mmmmmmmmmmmmmmmmmmmmmmmmmmmmm
+ + + + + + + + + +
g-ggg-ggg-ggg-ggg-ggg-ggg-ggg-ggg-ggg-gggggggggg
gggggggggggggggggggggggggggggggggggggggggggggggg
Figure 1, even-mode electric "+" & "-" (dash/minus), magnetic (m) fields, "s"
signal conductor, _____ (underscore) surface of the base material, and "g"
signal ground plane.
ODD-MODE
In the "odd-mode" of electrical signal operation, some/all of one of the
electrical signal conductor is signal referenced (return/ground) in the signal
ground plane AND AN THE ADJECENT (one or more) SIGNAL CONDUCTOR(S), see Figure
2. In the odd-mode of signal operation, one signal conductor has an
electrical field that is positive with respect to a signal ground plane and
the other signal conductor has a negative electrical field with respect to the
same signal ground plane. The "odd-mode" can be a differential transmission
line, if the electrical signal is applied between the two "signal conductors".
In other words, assume the left "signal" conductor is "the signal conductor"
and the right signal conductor is the "signal return"; what this is
electrically like is the electrical power supply system, in that one conductor
is the source of power and the other conductor it's return.
ee
mm>>>mm e e mm<<<mm
m + + + em me - - - m
m mmm>mmme m m emmm<mmm m
m___m__sssss__m___m___ _m___m__sssss__m___m_______
m mmm<mmme m m emmm>mmm m
m + + + + + em me --------- m
mm<<<mme e e mm>>>mm
+ + + + + ee - - - - -
g-ggg-ggg-ggg-ggg-ggggggg+ggg+ggg+ggg+ggg+gggggggggg
gggggggggggggggggggggggggggggggggggggggggggggggggggg
Figure 2, odd-mode electric "+" & "-" (dash/minus), differential electrical
field "e", magnetic "m" fields, "s" signal conductor, _____ (underscore)
surface of the base material, and "g" signal ground plane. This could be a
"co-planar differential transmission line".
(;-), isn't Figure 2 a mess!!!!, now to try attempt to explane the fig.
The electrical field on the left transmission line is positive "+" with
respect to ground and the transmission line on the right. Lets assume also,
that the magnetic field around the left transmission line is in a clockwise
orientation (the >> and << arrows) due to the direction of the current flow in
the conductor. In the signal conductor on the right, the electrical field on
the conductor is negative "-" with respect to ground and the other signal
conductor, the current flow is reversed (in comparison to the current in the
other signal conductor), and this current establishes a magnetic field in
reverse direction (counter- clockwise) with respect to the other conductors
magnetic field. There is another electrical field established between the
signal conductors (represented by the circle of "e"s.
At some high frequency(ies) the electrical spacings between conductors may be
such that the will have more of an electrical tendency to travel as
differential signals rather than as a microstrip or stripline; not by design,
rather by default; and this could really impact (upset) you desired circuit
performance.
KEY FEATURES/CONCERNS:
In even-mode electrical operation, in Figure 1, the magnetic fields will add,
this causes magnetic cross-talk between the electrical signals -- traditional
stuff.
In the odd-mode electrical operation shown in Figure 2, the magnetic fields
are in oppositon to each other, the currents are balanced (equal) and in
opposite directions. Therefore, any signal transitions in electrical signal
currents are coupled as an "aiding" current in the other signal conductor due
to reverse inductive coupling in the other conductor, in essence, to
effectively "enhance" the signal. In contrast, the inductive coupled current
(or induced voltage) in the even-mode operation, Figure 1 conditions is in
opposition (reversed) to the other signal conductor.
Figure 2 can be a differential transmission line if (as mentioned) one of the
microstrip transmission line's signal conductor is "the signal" and the other
microstrip transmission line is the "signal return".
Why differential transmission lines? It can improve signal integrity for EMC
(electromagnetic compatibility) reasons (improve noise immunity), as any
coupled or induced "noise" (or garbage) is coupled to both conductors with the
same polarity (current or voltage) and if the signal receiver is a "true
differential amplifier", this "common mode" signal errors will cancel each
other.
"Differential Impedance" -- is the electrical impedance between the two signal
conductors. (between the left and right microstrip "signal conductors" in
figure 2.)
The differential transmission lines do not have to be "co-planar", they can be
opposite (stacked) with respect to each other as in Figure 3.
________________________ outer layer or signal reference ground plane
ssssssssss signal / signal return
ssssssssss signal return / signal
_______________________ outer layer or signal reference ground plane
Figure 3, cross-section of a "stacked" differential transmission line
Differential Transmission line enhancements -- twist the differential signal
conductors to reduce the "loop" and improve noise rejection, as shown in
Figure 4
sssss\ /ggggg\ /sssss\ /ggggg
X X X --- etc
ggggg/ \sssss/ \ggggg/ \sssss
Figure 4. A "twisted" differential transmisson line to improve noise
reduction.
Comment - I wonder how far up the wall this would drive a printed board
designer or CAD system for a "32-bit wide" bus design. (;-) But hey, when
signal integrity and product performance is at stake! who knows????
Both figure 3 and figure 4's differential transmission lines can be shielded
by surrounding the differential signal conductors with ground planes, grounded
conductors and plated-through holes around the differential pair.
References: Any "good" EMC text/handbook, look for "single-ended",
"common-mode", "differential" signals. Also, most "analog" semiconductor
manufacturers have "applications" handbooks/manuals. Most of these analog
applications have tutorial sections on "instrumentation" and "differential"
amplifiers.
Ralph Hersey
e-mail: [log in to unmask]
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