Hi, one of the problems with the PCB trick described in the previous email is that PCB substrates are not very accurate in terms of impedance matching: +-10% accuracy on the impedance is already asking a lot, especially when using Asian manufacturers. Problems get worse when going inter-layer through vias. These discontinuities and impedance mismatches cause signals to bounce around and cause ringing etc. It helps significantly to add a series resistor at the source since these ringing effects will then be terminated at both ends of the PCB trace, rather than only at the end of the line. Without the series terminator at the signal source, the reflected wave would reach the source again, be reflected again, and end up at the end termination resistor a second time... >It's a common trick on PCBs to series terminate point to point clock lines with the >impedance of the transmission line. You get a simple 2:1 divider from the source resistor >and the transmission line, so the signal goes out at half height. When the signal gets to >the far end where there isn't any termination resistor, it bounces off resulting in a full height >signal. That's what the receiving chip sees. When that reflection gets back to the source, >it sees the source termination resistor and there are no reflections.
_______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts