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.

Page 6 of 8
Video/Imaging DesignWire
(9/10/2010 1:05:28 AM)

Transparent BNC Footprints – Through-hole BNCs
For a through-hole BNC, its footprint is made up of two structures – the plated-through-hole, and its exit trace. The plated-through hole is typically 30-50 mils in diameter. Large clearance (anti-pad) in the power planes is necessary to maintain the impedance of the plated-through hole to 75Ω. The size of the anti-pad is determined by the diameter of the plated-through hole as well as the number of the power planes in the board. With a large anti-pad, the exit trace within the anti-pad region loses its GND reference and its impedance increases. To overcome this problem, a short strip of metal is extended into the anti-pad for preserving the exit trace’s impedance. The metal strip extension is needed for the first power plane above the bottom exit trace, and its width is typically 3-5 times the trace’s width.
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Figure 13: Use of GND strip above exit trace for through-hole BNC footprint

Figure 13 illustrates the BNC footprint with this technique. Another commonly used technique is to widen the exit trace within the anti-pad region to lower the exit trace’s impedance.

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Figure 14: Use of wider exit trace for through-hole BNC footprint
Figure 14 illustrates the BNC footprint implemented with this technique.

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Figure 15: Use of GND guards to exit trace for through-hole BNC footprint

Figure 15 illustrates an improved footprint. In this example, two GND strips are placed at either side of the widened exit trace on the bottom metal layer. The ground strips are placed at a pre-determined distance from the exit trace, such that they introduce just enough ground coupling to achieve the desired impedance for the short exit trace. This structure has the advantage of allowing independent adjustments of the anti-pad in the power planes for controlling the impedance of the plated-through-hole, and the gap of the ground guards to control the impedance of the exit trace.

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