[SI-LIST] Re: Mode conversion
- From: Ben Rothchild <benrothchild@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 21 Feb 2002 01:30:34 -0800 (PST)
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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