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Differential pairs for DVI/HDMI signals

EEBuckeye4Life

I am currently routing a board with several differential buses meant for HDMI signals.  I do not have the Performance package, I am using PCB Designer Pro 16.5.  What is more important, the coupling between the differential pairs or the phase?  With the connectors in the design, it is impossible to keep the pairs coupled their whole length and when a direct connection is made for the traces to the pins there is as much as 100 mils difference in the length of the pairs.  Should I accordian one of the traces to match the lengths as much as possible and forget about the coupling over that distance?

Thanks

Todd

  • From my considerable experience with high-speed layout of DVI/HDMI signaling, which runs at speed up to 2.5Gbps, I would say the phasing has the most significant impact on "closing the eye" of the signals.  Although coupling (and impedance control) also affect the signal propagation velocities, trace length is more significant (IMHO).

    I usually try to use a 50-mil maximum for pair matching.  This can be challenging with some connector pin layouts, especially if you are manually implementing it (without performance option capabilities).

    Good luck!

    Roger Green

  • There's no easy answer here.  Just saying 50 mils is better than 100 mils is not conclusive.  How far is the run?  What kind of material is the signal propagating through? (not just FR4, but what weave, thickness, etc...?)  Is it point-to-point? (IC to IC) or through connectors?

     100 mils over 30 meters does not really matter.  I used to build HDMI interfaces over these distances and 100 mils is much less than the phase error due to cable bending so in that case I would not worry about it.  I suspect you know that HDMI runs at 3.4Gbps (2.5????).  The bit duration is about 588ps.  The typical velocity of most PCB materials is around 170ps/in or 3.46in.  So 100 mils is about 2.8% of the bit period.  This can typically be disregarding if the distance of travel is greater than 1 bit period. Most receivers can absorb more than 25% phase error -- check with your vendor.

     If your distance exceeds a bit duration you might want to look at impedance matching as ISI can cause more issues than phase error of this minimal amount.  Remember that impedance discontinuity can occur at the connector/via so be careful to analyze this portion of your design.  

     Other issues can contribute as well to degradation such as noise (power) and clock jitter.

    Another good forum to get lots of advice on is the signal integrity forum (search google).

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