Magic trace

02.05.2010 12:48

A while ago I wrote about an old SMC barricade wireless router that's been acting up. It's still having weird issues with both wired and wireless interfaces, which magically disappear when I open the box. It's probably time to invest in some new hardware, but I gave it one last shot at finding the problem. It's most likely something trivial, like a bad soldered point somewhere.

Anyway, when closely inspecting the circuit board I found this interesting feature:

Detail of the SMC Barricade G circuit board

What is the purpose of the long U-shaped trace at the bottom of this photograph (click on the image for a larger version)? The via at one end doesn't appear to be connected anywhere (although it is possible I missed a connection on some inner layer).

It looks like an antenna, but is surrounded by the ground fill and doesn't have any obvious connections to the RF front-end above (plus this router uses two external antennas).

My guesses are some test feature for measuring the parameters of the circuit board (unlikely in a production version) or a small capacitance needed by some part of the circuit (provided it's actually connected. But really, why use this instead of one more SMD capacitor? The circuit already has a hundred of them).

Maybe it's just a modern integrated version of the magic/more magic switch.

Posted by Tomaž | Categories: Analog


To what is this trace connected (if anything)?

Posted by Gašper Korinšek (via Facebook)

I don't know. As far as I can see the via on the left isn't connected anywhere, but it is possible there's some connection on an inner PCB layer that I can't see.

Posted by Tomaž (via Facebook)

Interesing. If this is a 1/4 whip antenna and if carrier frequency is known then length can be calculated and compared against actual length of magic trace.

Posted by Gašper Korinšek (via Facebook)

U trace makes nice optional open L/2 notch filter at TX antenna output. Coax cable versions often used in L/4 or L/2 config for HF harmonic filtering on amateur radio.

Posted by Mario

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