Third try:  Using 8-layer board, 4 cores, 4 prepreg and a foil.

Laminate:   (4x2) + (8x6) = 56 mils
Copper: 2x(.7+.3) = 2 mils 1/2 oz copper plated to 1.0 for outer layers
Copper: (7x.7) = 4.9 mils inner layer 1/oz copper
Total:   56 + 2 + 4.9 = 62.9mils

Impedance:  about 52-54 ohms for FR4, all traces.

Since my source resistance is 53 ohms, I have less than 2% mismatch.

For the price of the extra layers, I get a good match and have the  luxury
of an extra routing layer and voltage plane to play with.

Thanks for the help.


Tim Hutcheson
Institute for Human and Machine Cognition
40 S. Alcaniz St.
Pensacola, FL.  32503

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