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I'm pretty happy with the top side Samtec ARF6 launch simulations now.

S11:
* Red: No cutout, -12 dB at 10 GHz, -7 at 20 GHz, -4.8 at 30 GHz
* Blue: Rectangular ground cutout the size of the pad, -22.5 at 10 GHz, -16.4 at 20 GHz, -11.7 at 30 GHz
* Green: 30um larger than the pad on all sides (1.06 x 0.41 mm), -30.5 at 10 GHz, -22 at 20, -15 at 30.

With the oversized cutout it's more than good enough for QSGMII and 25Gbase-R.

TDR:
* Red: No cutout, 64Ω Zdiff at the launch
* Yellow: Pad-sized cutout, 86Ω
* Orange: Oversized cutout, 94Ω