Re: [SI-LIST] : PCB track length and ringing

Danwei Xue (danwei@pico.apple.com)
Tue, 23 Sep 1997 11:23:19 -0700

As Hans has pointed out, reflection will cause stair-case like
waveforms. These waveforms, however, might be different from the ringing
waveforms. Also, if there is no transmission line or the transmission
line effect is very small and can be ignored, then there will be no
reflection, or to be more acurate, no stair-case like waveforms.

Ringing, on the other hand, is due to the resonance of the L, C , and R
circuit, no matter whether transmissin line is involved or not. Because
the rise and fall edges of the pulses contains high frequency components
which may fall near the resonant frequency of the L and C. In this
condition, ringing occurs. You can do simple SPICE simulation to verify
this phenomenon.

According above discussion, I personally advocate that it is better to
distinguish ringing from reflection because they are caused by different
mechanism.

These are just my own opinion, please correct me if anything is wrong.

Cheers,

Danwei
Apple computer, Inc.

HANS_MELLBERG@NON-HP-Cupertino-om5.om.hp.com wrote:
>
> Simply due to the reflections caused by impedance mismatch somewhere
> in the trace between source and destination. Mismatch can occur for a
> multitude of reasons such as vias, changing planes, stubs, tees,
> capacitive inputs, inductive traces etc., etc. That is based on
> transmission line theory found in elementary electromagnetics
> textbooks.
> Hans
>
> ______________________________ Reply Separator _________________________________
> Subject: [SI-LIST] : PCB track length and ringing
> Author: Non-HP-gkrishna (gkrishna@rri.ernet.in) at HP-ColSprings,mimegw5
> Date: 9/22/97 8:32 PM
>
> Hi,
>
> Can someone explain how the PCB (printed circuit board) track (signal
> trace) length causes ringing in the High Speed Digital Signal systems.
>
> Regards,
> Gopalkrishna