[SI-LIST] Re: (no subject)
- From: "Sainath Nimmagadda" <gigabit@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 12 Mar 2002 12:53:37 -0800
This is about so-called lazy EM fields. If they accomplish propagation
with the least effort, they are that much more efficient. Not a semantic
point at all but laziness, in general, doesn't imply effectiveness. I am
afraid this might lead to misconception after repeated usage and someone
out there
wonders how come light travels so fast if fields are lazy.
If there is a reason (other than lowest energy or impedance)to call
fields as lazy, let us know.
Thanks,
Sainath
---------Included Message----------
>Date: Mon, 11 Mar 2002 23:41:29 -0800
>From: "scott" <scott@xxxxxxxxxxxxxxxx>
>Reply-To: <scott@xxxxxxxxxxxxxxxx>
>To: <chris.cheng@xxxxxxxxxxxx>
>Cc: <si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: (no subject)
>
>
>Chris and I are in agreement here. EM fields are lazy. They always
take
>the path
>of lowest energy, which is also the path of lowest impedance. If a
>capacitive path
>is lower impedance than an inductive path, image currents will prefer
>the capacitive
>path. The interplane capacitance often takes the majority of image
>current mismatches.
>The trade-off is that the additional interplane current also causes
>increased noise.
>
>These issues occur at the boundary where the signal transitions from
one
>reference to
>another. Power will be injected into the interplane capacitance and
>some impedance
>mismatch will be seen at the boundary. At sufficiently high enough
>frequencies, planar
>resonances can be excited. But for DDR operating frequencies, this is
>not a major issue.
>There will be some edge rate degradation as the signal crosses the
>transition boundary.
>
>regards,
>
>scott
>
>--
>Scott McMorrow
>Principal Engineer
>SiQual Interconnect Engineering
>18735 SW Boones Ferry Road
>Tualatin, OR 97062-3090
>(503) 885-1231
>http://www.siqual.com
>
>
>Chris Cheng wrote:
>
>>Here we go again. Unless you are dealing with 100ps edges (unlikely if
you
>>have 2.5V signals), the image current on the 5V plane will return
through
>>the plane capacitance between the 5V and gnd pair that sandwich the
>>stripline. Discrete decoupling caps or even planes on the other side
of the
>>5V plane will not have low enough impedance to make a difference. For
normal
>>SSTL DDR buses, the plane capacitance should have low enough impedance
for
>>the image current from the 5V plane to return through the gnd plane.
No big
>>issue.
>>
>
>
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