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A HYBRID U TURN WITH SKEW COMPENSATION FUNCTION FOR PRINTED CIRCUIT BOARD

IP.com Disclosure Number: IPCOM000247322D
Publication Date: 2016-Aug-23
Document File: 5 page(s) / 176K

Publishing Venue

The IP.com Prior Art Database

Related People

Jianquan Lou: AUTHOR [+6]

Abstract

A hybrid U turn for a printed circuit board (PCB) layout consists of both 45 degree corners and round corners. This helps the differential pair to keep the same length with other differential pairs and to compensate for skew between its positive and negative traces.

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A HYBRID U TURN WITH SKEW COMPENSATION FUNCTION FOR PRINTED CIRCUIT BOARD

AUTHORS:

Jianquan Lou Xiaoxia Zhou Shun Li Alpesh Bhobe

Cindy Shu Jinghan Yu

CISCO SYSTEMS, INC.

ABSTRACT

    A hybrid U turn for a printed circuit board (PCB) layout consists of both 45 degree corners and round corners. This helps the differential pair to keep the same length with other differential pairs and to compensate for skew between its positive and negative traces.

DETAILED DESCRIPTION

     In a high speed printed circuit board (PCB) layout, one differential pair is needed to keep the same length with other differential pairs for functional considerations. It is also necessary to compensate for skew between its positive and negative traces to obtain better signal integrity (SI) and electromagnetic compatibility (EMC) performance.

    A new hybrid U turn is proposed herein. The new hybrid U turn consists of both 45 degree corners and round corners, which can help the differential pair to keep the same length with other differential pairs (first function) and to compensate for skew between its positive and negative traces (second function). The first function is similar as the current U turn and the second function can be achieved by the combination of the round corner and 45 degree corner. The skew compensation calculation depicted in FIG. 1 below.

Copyright 2016 Cisco Systems, Inc.

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FIG. 1

    The hybrid U turn has round corners in top and 45 degree corners in bottom or vice versa, as illustrated in FIG. 1 above.

    For the round corner section, there are short trace l1, and long trace l2. The difference of the length between these two traces is l2-l1=1.57P, where P=S+W, and S is the edge to edge distance between two traces, W is the width of the trace.

    For the 45 degree corner section, the difference of the length between the short trace and the long trace is 4*P*tan(22.50) = 1.66P.

    The long trace of the round corner connects to the short trace of the 45 degree corner while the short trace of the round corner connects to the long trace of 45 degree corner, and each hybrid U turn structure has two round corners and two 45 degree corners. As a result, every hybrid U turn can provide 2*(1.66P-1.57P)=0.18P skew compensation.

    So in application, when one differential pair needs to use a hybrid U turn to achieve the same length with other pairs and also to compensate skew between its positive and negative traces, it will have the following attributes:

    1. Each hybrid U turn can provide 0.18P skew compensation. When the skew compensation length provided by all of the hybrid U turn structures is shorter than the skew between positive and negative traces of this differential pair, then additional bend structures are needed to compensate the remaining skew. Th...