Optimizing BNC PCB Footprint Designs for Digital Video Equipment

How to choose a BNC connector and properly design the BNC footprint on a high-speed printed circuit board — with the goal of meeting the tight requirements for SMPTE return loss. This article provides an overview of the types of BNCs in the broadcast video market, the test to determine the BNC’s electrical quality, common mistakes in BNC footprint designs, techniques for designing good BNC footprints and the use of 3D simulation tools to determine layout decisions.

By Tsun-kit Chin
Applications Engineer, Member of Technical Staff National Semiconductor Corp.

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Video/Imaging DesignWire
(9/10/2010 1:05:28 AM)

An increasing amount of video equipment is running at Gigabit rates today. This equipment is usually interconnected through relatively large size coaxial BNC connectors. While these connectors are in general of good quality, their performance in the equipment depends on how they are mounted onto the printed circuit board. Use of non-optimized connector footprints introduce impedance mismatches, reflections, signal loss, and impair the signal fidelity of the equipment. The task of printed circuit board layout designs for BNC footprints falls into the hands of layout designers and hardware engineers who often do not have the time or tools to get it right. This article outlines a few common problems in BNC footprint designs, and illustrates examples of carefully designed footprints for edge-mount and through-hole connectors for use with National’s LMH0384 3G/HD/SD adaptive cable equalizer, LMH0303 cable driver, and LMH0387 Configurable I/O devices.

BNC Types
Video equipment has historically used BNCs with 75Ω coaxial cables. Video pictures are used to transport at the standard definition rate (270Mb/s), upgraded to the high definition rate (1.485Gb/s), and are now migrating to 3Gbit/s. The BNC connectors must be capable of supporting 3Gbit/s signal transmission with minimum signal loss, while maintaining 75Ω characteristic impedances in order to minimize reflections.

Many connector vendors offer different types of BNCs, depending on how they are mounted onto the printed circuit board. With regard to mechanical considerations, they can be vertical mounted, right-angle mounted, or board-edge mounted. For electrical, the signal pins are either surface mounted to landing pads on the top side of the board, or soldered into plated-through holes with signal routings on the opposite side of the board. Figure 1 shows some examples of the through-hole BNCs.

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Figure 1: Examples of Through-hole BNCs

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Figure 2 shows examples of edge-mounted BNCs with surface-mount signal pins.

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Figure 2: Examples of Edge-mount BNCs

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Figure 3 shows an example of a right-angle BNC with a surface-mount signal pin.

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Figure 1: Example of a Surface-mount BNC

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