Re: [SI-LIST] : Excessive clock overshoot

D. C. Sessions (dc.sessions@vlsi.com)
Wed, 13 May 1998 14:23:14 -0700

fabrizio zanella wrote:
>
> I have a question regarding an FCT clock (TTL levels) driving a heavily
> loaded backplane. On the driver pin we see excessive overshoot on the L-H
> transition which increases as we increase the clock frequency. This
> overshoot goes from 5V at 33MHz to 6.5-7.0V at 45MHz. The stub impedance
> is 75 ohms, backplane impedance 25 ohms loaded. There is a clamping diode
> on the H-L side but not on the L-H. The H-L side does not have any
> undershoot.
> I have asked the manufacturer and they have never seen this phenomena, nor
> do they have an explanation for it.
> Any ideas on what could be causing this?

1) Is the line unterminated?
2) Is the loaded one-way delay anything near 2.7ns?

-- 
D. C. Sessions
dc.sessions@tempe.vlsi.com