# [SI-LIST] Re: Transmission line capacitance

• From: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
• To: "rahul kumar" <rahul05kumar@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
• Date: Mon, 26 Oct 2009 08:11:48 -0400

```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.
>>>
>>> Thanks
>>>
>>>
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