Subject: Clarification of my ramp measurement question

From: [email-masked] (Bob Ward)
Date: Mon, 28 Feb 94 15:35:07 CST

The other day I posted a message asking about the more correct interpretation
of the measurement spec for the ramp rate for an ibis model.  I got back some
traffic reaffirming the use of the 20% and 80% points, but the answers missed
the point of the question.  Let me try again.  The spec says that we are to 
measure the ramp rate into a 50 ohm load.  Because of the load there will be 
less than full voltage swing.  I fact I would expect VERY little, if any 
overshoot under such a load as this.  I would expect that the rising edge will
settle to a voltage rather under the rail voltage, and the falling edge will
settle to a voltage rather higher than the ground voltage.  So the question is
how are the 20% and 80% points determined?  Are they 20 and 80 % of the reduced
swing from ground to less than rail for rising edge and from rail to above
ground to falling edge, or are they 20 and 80 % of the rail voltage even though
the actual output doesn't swing that far?  I suspect that there would be
relatively little difference in the value of dV/dt no matter which was used,
but why not get it right?

Thanks,
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