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Picket-fenced ground gias next to microstrip traces
I've noticed that various data sheet application circuits for items such as RF
switches, amplifiers, etc. (e.g., http://www.hittite.com/product_info/.../hmc349lp4c.pd f ) use what I would call "microstrip traces surrounded by a ground pour 'guard' to reduce coupling to adjacent traces." Someone else, however, has suggested that the application circuits were really designed as co-planar waveguides (with grounds). Anyone else want to venture an opinion? For a 62.5 mil board, a 50 ohm microstrip's width is around 115 mils using FR-4 (k=4.7). Using TxLine 2003, for a CPW w/ground I have to reduce the gap to ~50 mils before the signal trace width reduces ~10% to ~104 mils. I take this to imply that there's not a lot of coupling between the signal trace and the copper pours (instead of the ground plane) until the gap width is comparable to the board thickness. In general, my impression has been that the use of copper flooding is more to provide isolation between adjacent traces than to change the form of the transmission line, and the use of the picket fenced vias was to insure that a large chunk of copper pour didn't suddently turn into a resonator. Does that sound correct? ---Joel Kolstad |
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