[SI-LIST] Re: Trace length vs Delay; Trace length vs Rise/Fallti me
- From: "Jones, James R" <James_R_Jones@xxxxxxxx>
- To: "'jagsim@xxxxxxxxx'" <jagsim@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 18 Feb 2003 14:24:14 -0600
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]
Sent: Tuesday, February 18, 2003 2:13 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Trace length vs Delay; Trace length vs Rise/Fall time
Hello:
I am new to high speed design. Your Help is highly appreciated.
1. Is there a general rule of thumb to determine the end to end signal delay
for a specified trace length.
Between two devices, how to make sure that the end to end signal delay is
<1ns. How to determine maximum trace length for that delay value.
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
of the clock reaching the device is <1ns?
Best Regards,
Jagdeep
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