Si-List has an interesting thread on recent advances in low-loss, low dispersion pcb materials. Some good to 100GHz.
The thread starts at
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[SI-LIST] RF vs. High Speed Digital PCB MaterialsThere is another thread earlier on placing ground vias next to diff pairs.
Previous threads have much useful information of interest to sed.
They've been around for a while, and have been mentioned here before.
-- Bill Sloman, Sydney
Vias are a few picoseconds long, and few signals are fast enough to care about a tiny impedance discontinuity. A diff signal should have low common-mode component, and parallel planes are already glued together by plane capacitances and bypass caps, so vias aren't necessary to glue the planes together. The reference planes may be different DC potentials anyhow.
IF the vias are so small that they raise the diff pair impedance, the vias can add effective ground references, and lower the impedances back to the nominal, for the short length of the via. That could cancel an inductive discontinuity.
It seldom matters. Dispersion in the traces is likely worse than vias; really high speed stuff often has adaptive equalization in the receivers to fix problems like this.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Microstrip (on the surface of a board) is inherently dispersive. Buried - symmetrical - strip-line shouldn't be, any more than good coax is - some dielectrics are better than others. John Larkin doesn't seem to have found out about any of this.
-- Bill Sloman, Sydney
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