[SI-LIST] FW: Diff. Pairs

  • From: "Paul Ikeda" <pikeda@xxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 7 Oct 2003 17:32:06 -0700

Eric,

I am also familiar with the proximity effect but I have always related =
it to higher losses due to increased skin effect. However now that I =
think about it I can see that it would also suffer from less dielectric =
loss since air is generally less lossy than dielectric and since =
dielectric loss is related to the delay not distance than Ravender's =
measurement confirms that there would be less loss due to dielectric =
loss. I know that as frequency increases the dielectric loss increases =
faster than the skin effect loss and becomes more dominant around 1 GHz =
so my question is, when comparing tightly coupled vs. loosely coupled =
micro strip do we always see more or less net loss or is it dependent on =
frequency and geometry. Has anyone done any research in this area?

Thanks in advance,
Paul

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Eric Bogatin
Sent: Tuesday, October 07, 2003 5:03 AM
To: Si-List
Cc: eric bogatin
Subject: [SI-LIST] Diff. Pairs


I have to agree with Jeremy's analysis of Ravender's measurement.
There is a difference in the velocity of a differential signal as the
coupling increases due to the larger percentage of fringe fields in
the lower dielectric constant air.

We describe this effect in terms of the odd mode velocity and the even
mode velocity of a diff pair. If the dielectric material is uniformly
distributed through the diff pair, so that where ever there are
electric fields, there is the same dielectric constant, there will be
no difference between the velocity of the signal traveling in the odd
mode (a differential signal) and the velocity of the signal traveling
in the even mode (common signal). This means that in a uniform
stripline, the velocity of the two modes is the same, while in
microstrip, the odd mode will generally be faster than the even mode.
This is not a rise time effect, it is a velocity effect.

The tighter the coupling, the bigger the difference in the odd and
even mode velocities, in an inhomogeneous transmission line.

When there is an inhomogeneous dielectric, a signal in the odd mode
will travel faster than a signal in the even mode. If you send in a
signal that has equal components of common and diff signal, the diff
component will reach the end of the line before the common component.
This is the real origin of far end cross talk.

We can think of far end cross talk as either the difference between
inductively and capacitively coupled current at the far end between
two, single ended transmission lines with coupling, or as the result
of the difference in speed between the odd and even modes in a diff
pair.

The details of this connection are in chapter 11 of my book.

--eric

********************************************************************
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Msg: #7 in digest
From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Diff. Pairs
Date: Mon, 6 Oct 2003 18:40:18 -0700

Steve,
Ravinder's edges propagate faster in the 1st case because tighter
spacing
between microstrip lines results in a greater portion of the field
propagating in air above/between the traces vs in the fiberglass under
the
traces.  The exact amount of speedup depends on the details of his
trace
geometry, soldermask geometry, and dielectric constants of the prepreg
and
soldermask.

Outside of the special case of a mixed dielectric (as above), changes
in
coupling will not affect velocity one way or another (I mean "pure"
propagation velocity here, not measured delay--see below).   However,
if you
do not hold impedance constant, increasing coupling with make the
impedance
will go down, which will result in "slower" edges throughout the
system (in
V/ns) due to reduced signal amplitude (and vice versa, less
coupling ->
higher Z -> "faster" edges due to larger signal swing).

Maybe not everyone agrees that there should be quotes around slower
and
faster in the sentence above.  When I think about signals propagating
on a
transmission line, I prefer to keep the "delay" effects separate from
the
"amplitude" affects, even though they may both affect the measured
delay.
For example there is one effect of changing coupling even in a uniform
dielectric while keeping Z constant; skin effect losses change due to
the
proximity effect.  This will slightly affect the measured delay (if we
measure at 50% of the transition) because it changes the waveshape at
the
receiving end of the line, but I don't consider it as changing the
velocity
because if we measure at the earliest recognizable point on the
transition,
it does not create any extra delay.

There are 2nd order effects (variation of Er with frequency) that can
complicate this mental separation of delay and amplitude, but I find
it does
a nice job of clarifying the 1st order effects (impedance changes and
attenuation).  I'm interested to hear any comments people have on it's
usefulness or things I should watch out for.

best regards,
Jeremy



|>--/\/\/--((((((((()--|>

Jeremy Plunkett
Signal Integrity Engineer
Broadcom Corp
www.serverworks.com

|>--/\/\/--((((((((()--|>


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Friday, October 03, 2003 4:40 PM
To: Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff. Pairs


Ravinder, something seems very wrong with the physics here.  I don't
know
if it is round-off error in your simulation, but Lenz' law agrees with
Lee.  Differential coupling resists any change.  The more tightly you
couple the two nets in a diff pair, the more it slows down the
transitions.  To cause a speed-up, they would have to switch in the
same
direction.  If we could manufacture a machine that accelerated
transitions
in opposite directions we could solve the world's energy needs.

Regards,


Steve.

At 01:12 PM 10/3/2003 -0700, Ravinder.Ajmani@xxxxxxxx wrote:
>One obvious benefit is the reduction in EMI because of reduced loop
area.
>I have verified this through simulation.  However, I am not able to
>understand the phenomenon of edge degradation due to close coupling.
I
>ran some quick simulations on six inch long differential Microstrip
nets
>under the following conditions:
>Case 1: Trace width 9 mils, separation 4 mils, differential impedance
99.3
>ohms
>Case 2: Trace width 12.5 mils, separation 40 mils, differential
impedance
>98.8 ohms
>
>The driver had a rise time of 250 ps.  The only difference I observed
>between the two waveforms was that in the first case the propagation
time
>was 63 ps less than the second case.  This is understandable since
the
>signals in the two branches of differential net have opposite
polarity,
>the coupling effect will speed them up.
>
>Next I tried the same simulation for Stripline nets.  In this case,
there
>was practically no difference between two waveforms (less than 5 ps
>difference in propagation delay).
>
>Am I missing something here.
>
>Regards, Ravinder
>Server PCB and Flex Development
>Hitachi Global Storage Technologies
>
>Email: Ravinder.Ajmani@xxxxxxxx

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