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

Arthur Collard (artc@lobsang.sps.mot.com)
Wed, 24 Sep 1997 16:38:43 -0500

But are we still missing some of the picture? We can damp the signal =
bounce=20
but, what about supply bounce? Output drivers switching at 500pSec or =
less can=20
cause power supply bounce effecting other components especially if the =
supply=20
bounce is internal to an IC. Now we need to talk about decoupling , =
pwr/gnd=20
pinout routing and packaging techniques.

regards,
Art Collard =20
Motorola - Transportation Saftey and Chassis Systems=20
Circuit Design =20

[Gratuitous deletions...]

>=20
> If it were not for source resistance, there would be very little
> damping, and the system would ring for a long time (more than 100 =
nSec).
> PCI drivers are great for this because they are strong (close to 5 =
Ohms
> output impedance). A series resistor at the source that is about 2/3 =
of
> transmission line Z0 cleans up the ringing very nicely. (A good target
> is to have the driver Ro + Rseries =3D 2/3 Z0. A slightly under =
damped
> system makes crisper edges.) A wavefront of slightly more than half
> the power supply voltage proceeds down the transmission line and =
doubles
> at the open circuit far end, giving just the voltage you wanted to see
> at the receiver. The reflected wavefront is almost completely =
absorbed
> in the series resistor and driver. Sophisticated drivers will attempt
> to match the transmission line impedance so that no external =
components
> are necessary.
>=20
> Daisy chain nets behave similarly. Clustered loads are somewhat the
> same. Star topologies introduce multiple Eigen values, so there
> will be multiple resonant frequencies on the net, associated with the
> several lengths of the star stubs. =20
>=20
> regards,
> Larry Smith
> Sun Microsystems.