# [SI-LIST] Re: When a signal is considered high speed signal

• From: Praveen Gowda <praveenkumardr@xxxxxxxxx>
• To: Rama Mohan Reddy Boreddy <mohan.svuce@xxxxxxxxxxxxxx>
• Date: Mon, 21 Sep 2009 12:41:48 +0530

```The definition is based on two parameters
rise time and length f transmission line...
when the length of the transmission lines becomes comparable with rise time,
transmission line management has to be done.
Critical length = ( rise time / 7 ) *  ( C / sqrt (dielectric constant) )
i.e,                = ( rise time / 7 ) * (velocity of signal propogation)
e.g, if rise time = 1 ns, C L = 1 ns / 7 * 6 inch per ns = 0.85 inches...

also, there is another definition...Based on crosstalk, CL is also defined
as length at which backward crosstalk reaches maximum. i.e at CL =
Transition electrical length  / 2.
i.e, CL = 3 inches for a rise time of 1 ns ( TEL = 6 inches for rise time =1
ns, in 1 ns signal travels approx. 6 inches on a pcb)

On Mon, Sep 21, 2009 at 12:22 PM, Rama Mohan Reddy Boreddy <
mohan.svuce@xxxxxxxxxxxxxx> wrote:

> Hi,
>    Can anyone tell when a signal is considered as a high speed signal? Is
> there any definition for high speed signal in time domain and frequency
> domain?
>
>    I know that a rise time and PCB trace/Interconnect legth is one of the
> parameters to define a signal as high speed signal. Can any body provide me
> the formula
>
>    Can any body share a FAQ on high speed signals and Cross talk
>
> Regards,
> Rama Mohan Reddy
>
>
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