I have seen this many times before with FCT logic. In my experience it is
the worst choice of logic families from a signal integrity point of view.
The edge speed is very fast, and current drive is very high, and so the
speed is fast---but that is bad for signal integrity. I suggest you try
ABT, or F. There is also other types of FCT which are intended to be more
Charles Hill, consultant
Reply To: firstname.lastname@example.org
Sent: Wednesday, May 13, 1998 2:16 PM
Subject: [SI-LIST] : Excessive clock overshoot
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
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?
thanks and regards,