Transmission line-like resonances at ~30MHz

Joel Kolstad · 2005-02-09T22:23:59+00:00

I have a design on an FR-4 PCB that has a few long traces on it, about 12" each. When I examine those trace's responses on a network analyzer, they have significant (>15dB) dips at 30MHz, then 90MHz, 150MHz, etc... this led me to suspect that the traces were behaving like quarter wave transformers (the traces have various components connected to them, but they're all supposed to be pretty high impedances), but if I assume an effective dielectric constant of ~2.35, I compute lambda/4 as ~60 inches. Indeed, the trace lengths were originally set such that I would (hopefully) be able to assume the system was behaving as a lumped network up to at least 30MHz!Anyone have suggestions on what else might cause periodic resonances such as the ones described?Thanks,---Joel Kolstad

Feb 09, 2005 22 Replies

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John Larkin 21 years ago

The DLL clock buffers in the newer Xilinx FPGAs can be programmed for negative propagation delay! Now if I just build a long shift register, and run the stock ticker data through...

John

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Jeroen Belleman 21 years ago

That's precisely the thing that made Matthew Orman think he had FTL propagation...

Jeroen Belleman

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Keith Williams 21 years ago

Sure. The clock carries no information so it can go FTL. ;-)

....been done. There was an interesting insider attack on the horse racing pari-mutual industry too. They only got caught because they were a little too greedy and (un)lucky (several long-shots came in).

Keith

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