[SI-LIST] Re: Trace length vs Delay; Trace length vs Rise/Fallti me

  • From: Jim G Roberts <jgroberts@xxxxxxxxxx>
  • To: James_R_Jones@xxxxxxxx
  • Date: Thu, 20 Feb 2003 12:39:04 +0100

Hi James & Jagdeep,
            I would like to add if the absolute delay is <1ns then the trace is
has little attenuation
and is relatively short for rise times in the order of 1 ns (<ns is mentioned)
and therefore of
moderate consequence on the load of the receiver. i would assue that you would
match the
line impedance to the receiver impedance with a terminating resistor.
        It the source is an oscillator and presumably free-running, why would
you have a
requirement for an absolute delay of <1ns.
            Have I misunderstood or could you clarify.
--
Regards,    __________   James G Roberts
           /___  ____ |  jrobert@xxxxxxxxxxxxxxxxxxxxx
 Jim      __  / /___/ /  jgroberts@xxxxxxxxxx
         / /_/ /---| |   Room: BE436, Hilversum
         \____/    /_/   Tel: +31 35 687 4308 Fax: 5976

"Jones, James R" wrote:

> 1.  You could use a rule of thumb of 180ps/inch for stripline, say 160ps for
> microstrip.  Keep in mind that you would need to use a 2D field solver if
> you want to get a closer approximation for propagation velocity.
>
> 2.  Yes, trace length does have an effect on rise/fall times.  Let me
> explain...  for an infinitely long trace, the impedance looking into the
> trace is equal to the characteristic impedance.  For an infinitely short
> trace, the impedance looking into the trace is the load of the receiver,
> usually much larger than the characteristic impedance.  You will be using a
> trace of some finite length, so the load that the driver sees will be
> somewhere in between these two extremes.  As you can imagine, the amount of
> driver loading will affect the rise/fall time.  I recommend simulating this
> if you know that you have to meet a certain rise/fall time spec.
>
> Hope this helps,
> James R. Jones
> Dell
>
> -----Original Message-----
> From: jagdeep singh [mailto:jagsim@xxxxxxxxx]
>
> 2. Given the device parameters, how does trace length effect 10-90 Rise/Fall
> time of a particular signal. I thot rise/fall times are a factor of only
> knee freq., but does trace length has any effect on rise/fall times?
>
> >From an oscillator to a device, how do I make sure that the rise/fall time



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