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Design007-Aug2021

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AUGUST 2021 I DESIGN007 MAGAZINE 59 ence an eye height reduction of less than 1.1 % vs. B. e eye height faced by TL model A with through-hole via stub is diminished by 21 % vs. B, to 517 mVpp. Meanwhile, regarding Figure 6, at 32 Gbps, TL model B with a stacked microvia expe- riences an eye height of 502 mVpp. e eye height encountered by TL model C with a staggered microvia drops by 12% to 442 mVpp. Model D and E with shortened and widened short bars in a staggered microvia experience an eye height reduction of less than 9% ver- sus B. e eye faced by TL model A with a through-hole via stub is almost closed. Summary In this article, the performance of various via structures are compared relative to impedance discontinuity, insertion loss and eye diagrams. e analysis results indicate that stacked microvias encounter the least impedance dis- continuity and insertion loss versus the other via structures. However, if staggered microvias are selected for signal routing, the short bar shall be kept at the minimal length to minimize impedance mismatch to comply with DFM standards. DESIGN007 References 1. Image courtesy of Sierra Circuits. 2. T. Kushta et al, "Resonance stub effect in a tran- sition from a through-via hole to a stripline in multi- layer PCBs," 2003. 3. E. Bogatin, "The Quarter-Wave Stub Frequency: Rule of Thumb #17," EDN, 2014. 4. Keysight EMPro. 5. Keysight ADS. Chang Fei Yee is technical lead in hardware and signal/ power integrity for Keysight Technologies. Figure 6: Eye diagram for TL model A, B, C, D and E at 32 Gbps transmission.

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