[SI-LIST] : transmission line theory

Bodley, Andrew (andrew.bodley@intel.com)
Tue, 3 Mar 1998 08:50:08 -0800

Hi All,

I am interested in transmission line theory as suggested by Mark
below. Does anyone have suggested references both basic and

Andrew Bodley

> I find it necessary to go back to transmission
>line theory (reflection coefficients, etc.) and make sure what
>simulating makes sense!

> -----Original Message-----
> From: Greg Edlund [SMTP:Greg.Edlund@digital.com]
> Sent: Tuesday, March 03, 1998 5:06 AM
> To: 'Mark Nass'
> Cc: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : Driver Strength
> Mark,
> I have found that driver current ratings don't mean a whole lot today.
> They are a hold-over from TTL days, when each receiver drew a certain
> dc
> current and a driver had to be able to source the dc current of all
> its
> loads. In a CMOS environment, they are useful in a relative sense,
> i.e.
> you can guess that a 4 mA driver will probably have a lower output
> impedance than a 2 mA driver - within the same part family and the
> same
> vendor. However, don't expect vendor A's 4 mA driver to have the same
> output impedance as vendor B's 4 mA driver.
> Output impedance tends to be a much more useful parameter in a
> transmission line environment. Unfortunately, most vendors don't spec
> output impedance. If you have a non-linear output IV curve, it does
> vary over loading, making it a little difficult to spec. The vendor
> would have to guess what kind of environment driver will see and spec,
> say, a 60 Ohm resistive load to ground and Vdd. Even this information
> is more useful than nothing.
> The best thing you can do is to ask for an IBIS model of your driver
> that is verified against lab data.
> How many loads you can expect to drive varies widely with load
> capacitance, net topology, and timing. A behavioral simulator is an
> excellent tool for prototyping nets and answering these kinds of
> questions. It's important not to rely on the simulator to do all your
> work for you, though. I find it necessary to go back to transmission
> line theory (reflection coefficients, etc.) and make sure what I'm
> simulating makes sense!
> ----------
> Greg Edlund, Principal Engineer
> Server Product Development
> Digital Equipment Corp.
> 129 Parker St. PKO3-1/20C
> Maynard, MA 01754
> (978) 493-4157 voice
> (978) 493-0941 FAX
> greg.edlund@digital.com
> ----------
> From: Mark Nass[SMTP:markn@rccorp.com]
> Sent: Monday, March 02, 1998 10:35 PM
> To: si-list@silab.Eng.Sun.COM
> Subject: [SI-LIST] : Driver Strength
> Can someone explain to me what driver strength means? When
> a driver is spec'd by an ASIC vendor as a 12ma, 6ma, etc what
> does
> that mean as far as its expected VI curve and how many loads
> I can expect it to drive?
> Thanks,
> Mark Nass