[SI-LIST] Re: Mode conversion
- From: Julian Ferry <julian.ferry@xxxxxxxxxx>
- To: "'benrothchild@xxxxxxxxx'" <benrothchild@xxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Thu, 21 Feb 2002 14:02:52 -0500
Mode conversion can most definitely occur in a connector for reasons other
than differences in path length. Anything that causes a difference in the
impedance of one terminal of a pair, as compared the impedance of the other
terminal of the same pair, can lead to problems. Basically, the shorthand
term for such differences is "balance", which has been mentioned here
previously. If the terminals are perfectly balanced, the impedance of each
will be identical. This impedance can be referenced to ground, or it could
be referenced to any other entity or entities.
Its not unusual to find imbalances at the outer edge or corner of a multi
row connector. That is, where a terminal pin might have only one
"neighbor", as opposed to the two neighbors that the internal terminals
have. Imbalances can also be generated where one terminal of a pair is more
tightly coupled to ground (maybe a ground pin or a shield) than the other
terminal of the pair.
Julian Ferry
High Speed Engineering Manager
Samtec, Inc.
-----Original Message-----
From: Ben Rothchild [mailto:benrothchild@xxxxxxxxx]
Sent: Thursday, February 21, 2002 4:31 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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