[SI-LIST] Re: Transmission line capacitance
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
- Date: Mon, 26 Oct 2009 09:36:04 -0700
Hi Jim,
actually, you can, under two conditions:
- the transmission line's propagation delay is short compared to the
signal rise time
- the far (receiver) end of the transmission line is unterminated, i.e. no
ohmic load
In that case the line behaves just like a capacitance to very good
approximation, and can be lumped together with the receiver capacitance.
That said, if the receiver end of the line is shorted (instead of being
open ended), again the length being short against the signal rise time,
then the first-order lumped model would instead be the total line
inductance, and the capacitance plays a negligible role.
Of course with shorter rise times you have to use more sophisticated
models, getting more complex as the rise time decreases (and the necessary
model bandwidth decreases):
(1) pi-model (split capacitance it two and connect by inductance) - L, C
(2) loss-less transmission line - Tpd, Zo
(3) lossy transmission line with frequency dependent skin loss and
dielectric loss
Rahul does not explicitly state the signal rise time (250 MHz signal
frequency can still use very fast rising signals), but for a 3cm line (Tpd
~ 200ps) the lumped approximation (only C) is good for any signal with a
rise time of maybe 10 - 20 times Tpd, i.e. 2...4ns or slower.
Regards,
Wolfgang
"Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
10/26/2009 05:11 AM
To
"rahul kumar" <rahul05kumar@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
cc
Subject
[SI-LIST] Re: Transmission line capacitance
Regarding Rahul's point #1 below:
I'm stating the obvious here, so pardon me, but you can't go adding up
the total capacitance of a t-line, if placed in between each capacitor
in your equivalent t-line model is an inductor.
I suggest reading a good book on transmission lines. (t-lines are
amazing.)
Regards,
Jim
Honeywell
> Hello all,
> Thanks for the reply, i understood it. One more related point to
clearify .
>
> I am doing IO driver simulation running at frerquency less that 250
> Mhz . Capacitive load at farend is given equal to 4pF along with
> txline of 3cm. (Z0 = 50, Co = 1.5pF / Cm , Td = 60pS / cm )
>
> 1. Can i take equivalent capacitance to 4pF + (1.5pF *3) = 8.5pF for
> my first order IO calculations. ( Transitions ) .
>
> 2. I am taking ELDO ( simulator ) lossless txline model which takes
Z0
> and Td as parameter. By doing this can i assume safely that i have
> taken account of Transmission line capacitance of 4.5pF.
> Or do i need to take lossy transmission line model and ask for more
parameters.
>
> Thanks for the help.
> Rahul
>
>
>
>
>
> <rtatikola@xxxxxxxxx> wrote:
>
>> Rahul,
>>
>> Definitely not frequency dependent. The units show that it is
dependent on
>> the length of the transmission line and calculation is based on
>> cross-section (2D approximation).
>>
>> Ramani
>>
>> On Sat, Oct 24, 2009 at 5:34 AM, rahul kumar
<rahul05kumar@xxxxxxxxx> wrote:
>>
>>
>>> Hello all,
>>> I have seen in standards quoting transmission line capacitance
equal to
>>> 1.5pF per cm . How this number is calculated?
>>> Is this value frequency dependent? transition dependent ?
>>>
>>> Kindly help.
>>> Suggest link to know more information about the same.
>>>
>>> Thanks
>>>
>>>
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