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S-parameter Extrapolation for Improving Near-fmax Accuracy in 2x-thru Calibration
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  • Chiu-Chih Chou ,
  • Po-Chen Kuo ,
  • Stone Lin ,
  • Carr Wang
Chiu-Chih Chou
National Central University

Corresponding Author:[email protected]

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Po-Chen Kuo
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Stone Lin
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Carr Wang
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The 2x-thru calibration often faces the problem that at the highest measurement frequency fmax, the calibrated results exhibit spurious ripples or spikes that are clearly unphysical such as violating passivity. Though being a common phenomenon, it has attracted little attention in the literature. In this paper, the cause of this problem is identified, and a solution is proposed which is to pre-extrapolate the S parameters before carrying out 2x-thru algorithm. Meanwhile, a novel extrapolation method, called the Sun-Earth-Moon (SEM) fitting, is proposed, which is shown to be able to provide smooth extension of S parameters for several GHz beyond fmax. The effectiveness of SEM extrapolation on reducing the spurious response at fmax is demonstrated by measurement examples. The results also indicate that with extrapolation, the calibration accuracy can be slightly improved even at lower frequencies. Comparison with three commercial 2x-thru tools reveals that this near-fmax problem shows up also in the industry-standard tools, indicating that the proposed method has the potential of advancing the state-of-the-art.Â