[SI-LIST] Re: lumped model vs distributed model
- From: "Clewell, Craig" <cclewell@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 30 Nov 2001 16:27:09 -0500
Jason,
You'll probably get 10,000 replies to this one, but since it's late on a
Friday and I haven't seen any of them yet I thought I might give you my
version...
You have to think about the speed your model is going to operate at and the
lengh. We need to pick some operating frequency so lets assume that you
need 3.5GHz bandwidth. If you were to use the "estimation" that the
BW=.35/Trise and need a bandwidth of 3.5GHz that means your rise time would
be about 100ps. Now lets look at your lump model. I'll exagerate the
length to show the reasoning. If you were to have a 1meter long piece of
something that you modeled as a lump and try to put a 100ps edge into it
what would happen?
The delay = (Length*Square root of the dielectric)/Speed of light (3e8m/s)
= 6.23ns (assuming Er=3.5). You usually assume in a transmission line that
the time delay is no larger the 1/10 of the rise time. Pretty far off using
our 1meter long piece of something. Now if we were to look again, but this
time make our piece of something only 2mm in length...Delay = 12.47ps. Now
we are pretty close to 1/10 of the rise time.
Ok...so what does this mean? Well let's picture a ladder. In a lumped
model you would only have one rung in the ladder meaning that there would be
one L and C to describe your whole 1 meter long piece of something. If you
were to break your 1meter long piece of something up into small 2mm sections
(distributed) and cascade 500 of them together (500 rungs in the ladder) to
make your 1m long piece of soemthing you would now have 500 L's and 500 C's
that describe your piece of something. And, each rung of the ladder would
have a delay of 12.47ps which would allow you to capture all of the
discontinuities along the 1meter piece of something. If you were to use the
lumped model you wouldn't be able to see these discontinuities due to the
time delay through the 1meter.
The choice of lumped vs. distributed is based on the old cliche' that size
does matter...and so does speed. The relationship beteween the 2 is the
deciding factor. This is the reason that for high frequency you would use a
distributed model. High frequency meens BW is high and in the
"approximation" BW=.35/Trise the .35 is fixed. That means that if you
increase BW you decrease the Trise. This in turn means that you have to cut
up your 1 meter of something proportionatly so that you don't exceed the
1/10 "rule of thumb" due to the delay.
Regards,
Craig Clewell
-----Original Message-----
From: Jason D Leung [mailto:jleung@xxxxxxxxxxxxxxx]
Sent: Friday, November 30, 2001 3:19 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: lumped model vs distributed model
Hi everyone:
For transmission line we can always use a lumped R,L,C model to
represent a simple transmission line, or we can use a distributed model
.(I know that the distributed model is more accurate and for high freq
application we should use this model)
But my question is : what is the main difference between the lumped
model and distributed model?
If we are just using the lumped model for our SI simulation, what are we
going to miss ?
looking forward for your insight
thanks
Regards
Jason Leung
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