howard parkin wrote:
I've just done a spice simulation involving a representation of the
high speed analog output.
Anybody can do this same simulation using the free LTSpice program
downloaded Linear Technology
(http://www.linear.com/designtools/softwareRegistration.jsp) and the
attached text file. I simulated a single pixel event transitioning
from black to white to black in 250ps (400MHz dot clock).
In the 50-ohm case, I used an approximate 3cm/50ohm trace at the
source end( velocity=163E+6 m/s, 200ps delay), assumed the DVI-I
connector was 75ohm (velocity=2E+8, 60ps delay) and about 2 metres of
50 ohm cable (velocity=2E+8, 10ns delay).
The 3cm trace is in that wonderful gray area between a short line
<0.5*rise_time and a long line >2*rise_time so you shouldn't expect much
of a problem from the reflection. IIRC, the reflection should only
effect the rise time as long as the line has a delay of <rise_time.
What I wonder about is whether your representation for the DVI connector
is close enough. That is, I don't think that it is a question of delay
but rather one of capacitance.
Result : The effect of the 75 ohm-ish DVI connector can be seen to be
negligeable. There's a reflection back at the driver end but this
gets absorbed and the load sees > a clean signal.
I'm not surprised that it doesn't have much effect since the trace isn't
long enough to get a real reflection.
However, I am concerned by the specs which I didn't completely
understand that the DVI connector could degrade the rise time by up to
140 ps and that it was rated for a rise time of 700 ps.
--
JRT
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