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). 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. In the 75-ohm case, I used the same 3cm/50-ohm trace and DVI connector but changed the source and load resistors to 75-ohm and modified the cable to approximately 2m of 75ohm cable. Result : In this case, the signal at the load sees a slight "rounding" of the single pixel event. Given the bandwidth limitations in a real system, both at the video card and monitor ends, I'm thinking that this rounding will for the most part get swamped and nobody will be able to detect the effect of the mismatch. I guess I will try simulating the 50 and 75 ohm "traces in parallel" solution. My concern here is that the additional resistors required to implement this represent a larger impedance discontinuity than the 50 ohm trace in a 75-ohm system, leading to worse results, but we shall see. Howard. On 3/21/06, Timothy Miller <[EMAIL PROTECTED]> wrote: > On 3/21/06, Michele Carla` <[EMAIL PROTECTED]> wrote: > > If I understand correctly > > > > there are the following possibility for the hig-res head connector: > > > > DVI: > > PROs: > > -no space problem to place it on the board > > -cheap ? > > CONs: > > -low bandwidth > > > > 5-BNC > > PROs: > > -cheap > > CONs: > > -no space on the board to place them in > > > > 13w3 > > PROs: > > -no space problem to place it on the board ? > > CONs: > > ??? > > The plan was for three connectors on the face-plate: two DVI-I and > one s-video. We'd really like to to have the DVI connectors do > double-duty for the analog, even the high-speed. Howard said > someone's successfully done it before. > > > What about impedence: is really nedeed 50 Ohm ? > > There are definitely monitors that have 50 Ohm impedence, and we have > worked with them before. Jack proposed a solution to this to allow 75 > and 50 Ohm traces on the same board, and Howard is looking into using > that or a similar solution. > _______________________________________________ > Open-graphics mailing list > [email protected] > http://lists.duskglow.com/mailman/listinfo/open-graphics > List service provided by Duskglow Consulting, LLC (www.duskglow.com) > _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
