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Comment by cnvogel | Hacker News Reader

Here's a paper that shows reflections (Fig. 3, "S11") and insertion loss (Fig. 4, "S12") on coplanar waveguides, when using a sharp 90° bend, a 90° bend with a champfer and the case where the bend is replaced by two successive 45° turns ("final design").

The frequency scale spans 10..90 GHz.

http://tentzeris.ece.gatech.edu/ECTC09_Rida.pdf

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I'm a chip designer and this article kinda cracked me up. For microwave signals on submicron lines, losses get very significant. It seems that PCB design inherits our practices in places these issues aren't so critical.

But then I wonder about applications. If you ignore this rule of thumb, and gang a few hundred boards together sharing a clock... will your signal survive?

Sometimes a 'superstition' is just common sense regarding edge cases - walking under a ladder won't have cosmic effects, but a dropped bucket of paint can leave a mark

> If you ignore this rule of thumb, and gang a few hundred boards together sharing a clock... will your signal survive?

No. But then if you need a few hundred boards off a single clock I assume your phased array radar budget can handle the extra engineering.

Yah anyone who has done RF engineering knows that "just a little bit of capacitance" can be a lot of capacitance.

If you are in RF, you may well be already using "cornerless" smooth traces. Google "topological router"

That or using discontinuities in impedance intentionally. I've designed and had built a handful of successful UHF planar PCB filters using Sonnet. In a lot of them I use very small changes in trace width as the place to put resonating elements. ie, http://superkuh.com/stepped-impedance-bandstop-filter.html

The difference between square corners and compensated is real. And it gets more real if you're working in generic FR4 with 1.6mm thickness and 2-3mm wide traces for the sweet spot between 50 and 75 ohms.

You can play around with this type of simulations quite easily. Have been using free version of Sonnet planar EM solver [1] for this over the years. Helpful in developing the industry standard magic RF intuition.

[1] http://www.sonnetsoftware.com/products/lite/

The "final design" is the chamfered bend with additional vias, a design with two 45° bends was not studied in this paper. Page 2, second paragraph of section A :

> The final design that includes both the chamfered bend and the vias

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