[SI-LIST] Re: Mode conversion
- From: Ben Rothchild <benrothchild@xxxxxxxxx>
- To: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 25 Feb 2002 03:05:04 -0800 (PST)
Jim,
Thank you for the inputs. I have just cut
a part of reply here below.
> One beauty of the differential transmission line is
> that the differential
> signal is very loosely coupled to the common-mode
> signal. What I mean by
> that is that you can introduce significant imbalance
> and produce a lot of
> common-mode, but you will find that the differential
> signal is only slightly
> effected. I can show you some evidence of this, if
> you are interested.
>
I would be more than happy to receive the material
you have mentioned above. However are you refering
to common mode range of the differential receiver
which allows considerable common mode signal on the
inputs and yet recover the differential signal
or
Is your explanation here limited to the differential
transmission line part of it.
Regards,
Ben
--- "Knighten, Jim L"
<JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> Ben,
>
> Your first question is an interesting one. If you
> have the situation where
> you have two differential transmission lines with
> the same differential mode
> impedances, but with different common-mode
> impedances, then you ask if when
> they are cascaded, does this mismatch in common-mode
> impedances create the
> conversion of differential signals to common-mode
> signals? This is a
> hypothetical and somewhat idealized situation. The
> answer is no, a
> common-mode impedance mismatch does not create
> imbalance, hence it does not
> cause mode conversion. But, you will find that the
> common-mode signal that
> exists on the first line (and it always exists in
> real life) will see an
> impedance mismatch at the junction of the two
> transmission lines and will
> undergo a reflection. The reflection may increase
> the common-mode noise
> characteristics of your system of transmission
> lines, but is unlikely to
> effect your differential signal. In other words,
> the common-mode reflection
> may be more of an EMI issue than a signal integrity
> issue.
>
> One beauty of the differential transmission line is
> that the differential
> signal is very loosely coupled to the common-mode
> signal. What I mean by
> that is that you can introduce significant imbalance
> and produce a lot of
> common-mode, but you will find that the differential
> signal is only slightly
> effected. I can show you some evidence of this, if
> you are interested.
>
> Mode Conversion in a connector: As Julian Ferry
> pointed out in another
> posting, there are "environmental" features in a
> connector that can be
> different in one side of the line vs. the other and
> cause the imbalance that
> results in mode conversion. A difference in path
> length through the
> connector is only one of several ways that this can
> happen.
>
> Along this line, you should realize that two
> parallel traces of equal length
> on a PCB built from common FR-4 material can
> experience imbalance. Fiber
> glass board material is inhomogeneous because the
> glass weave is not the
> same all over the board. So, you find that the
> electrical properties of the
> board can be a little different in the vicinity of
> one trace vs. the other.
> This gives you an inherent mode conversion mechanism
> in a real board that
> would appear to be very well balanced in terms of
> trace lengths. This
> phenomenon becomes more important as frequency
> increases.
>
> Hope this is helpful,
>
> Jim
>
>
>
> Jim Knighten, Ph.D.
> Teradata, a Division of NCR http://www.ncr.com
> 17095 Via Del Campo
> San Diego, CA 92127
> USA
> Tel: 858-485-2537
> Fax: 858-485-3788
> jim.knighten@xxxxxxx
>
> -----Original Message-----
> From: Ben Rothchild [mailto:benrothchild@xxxxxxxxx]
>
> Sent: Thursday, February 21, 2002 1:31 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Mode conversion
>
>
> Jim, Doug, Ken, Mick,
>
> Thanks for your explanation on Mode Conversion.
>
> And to all Si Experts.
>
> Is there a possibility of mode conversion when
> a differential signal passes from one set of
> ideal(equal length etc) differential lines with
> Zodd-1 and Zeven-1 impedances to another set
> of ideal differential lines with Zodd-2 and Zeven-2
> impedances where Zodd-1 = Zodd-2.
>
> Second is there a possibility of mode conversion
> when passing through a connector other than due to
> differences of pin lengths. Ofcourse coupling
> in the connector could be different than
> the transmisssion line and its effect on mode
> conversion will I suppose be the same explanation
> as for the case of two sets of differential lines
> mentioned above.
>
> Any insights/explanations
>
> Ben
>
> > A simple explanation that I've always liked is a
> > diff line with unequal
> > "lengths" (physical and/or mode induced)causes the
> > currents to have
> > different phases or arrival times, and therefore
> the
> > currents do not cancel
> > completely, i.e., are not exactly 180 phase from
> > each other. Due to the
> > high impedance of the receiver a very small amount
> > of current can develop a
> > large common mode voltage across the receiver
> input,
> > producing common mode
> > radiation. Microstrips have more issues than
> > striplines due to
> > inhomogeneous permittivties and surface effects.
> I
> > haven't found right
> > angle "corner" capacitance to have any measureable
> > effect ....yet. If
> > meander sections are tightly coupled, you could
> have
> > issues even at slow
> > speeds. If you lay them out correctly they pose
> no
> > problem.
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
> > Doug McKean
> > Sent: Tuesday, February 19, 2002 4:20 PM
> > To: Signal Integrity Group
> > Subject: [SI-LIST] Re: Mode conversion
> >
> >
> >
> > In terms of EMI, the full name for "mode
> conversion"
> > is
> > "differential-to-common mode noise conversion".
> > It's
> > when differential mode noise gets converted to
> > common
> > mode noise. The exact mechanics of this escapes
> me,
> > but does happen to varying degrees with just about
> > any differential buss layout. The worse the
> layout,
> > the
> > more common mode noise produced. For instance,
> > put lots of equal to or greater than 90 degree
> bends
> > in
> > a differential buss layout between points A and B
> > and
> > you will get lots more common mode noise as
> opposed
> > to
> > a straight buss configuration between the same two
> > points.
> > Not sure if it's some odd/even mode phenonmenon or
> > some additive crosstalk thing. I'm sure someone
> here
> > would know. - Doug McKean
> >
> >
> >
>
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