Adam Seychell suggested this pattern on the Homebrew_PCBs Yahoo! group as a true test for whether or not you can do a specific line/space size reliably. To test a given rule size, use the files below to etch a board. The board passes if each of the two independent spirals are contiguous (outer pad to corresponding inner pad < 4K ohms) and not touching each other (between outer pads> 1M ohms).
The PCB files are for the gEDA suite PCB editor. The gerbers are just for the one copper layer; use the corresponding drill file if you have a board professionally made. The PDFs are 1:1 but note that most postscript printers make the lines 1 pixel wider than spec.
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