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)

Common Problems in BNC-to-Board Transition
Most surface-mount BNC connectors have large signal pins of about 30-40 mils diameter. Landing pads of about 50-mil width are necessary for soldering the signal pins properly onto the printed circuit board. For ease of routing, thinner surface traces of 8-15 mil widths are commonly used to route signals from the BNC connectors to high pin-count integrated circuits.

Figure 7: Top and cross-sectional diagrams of a non-optimized edge-mount BNC footprint

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Figure 7 shows the top and cross-sectional views of a non-optimized edge-mount BNC footprint. A 12-mil width microstrip, placed at 15-mil above its GND plane, is designed to achieve the 75Ω characteristic impedance. The BNC’s landing pad is effectively a 50-mil wide microstrip. With a GND plane 15mil below the pad, the characteristic impedance of the pad is significantly lower than that of the trace. The pad introduces a large impedance drop that will impact the signal quality and add parasitic capacitance that reduces the BNC’s bandwidth.

Many types of video equipment typically use through-hole BNCs because of the better mounting robustness. The BNCs are usually mounted on the top side of the board, with their signal pins soldered into fairly large plated-through holes, and signal routing is done on the bottom side of the board. Figure 8 shows the top and cross-sectional views of a non-optimized through-hole BNC footprint. The inner ground and power layers are isolated from the plated-through hole to avoid shorting the signal pin. The cylindrical barrel of the plated-through-hole introduces a small amount of inductance. Each inner power plane introduces parasitic capacitance to the plated-through hole, the amount of which depends on the clearance distance from the barrel. A large plated-through hole with a small clearance exhibits excessive capacitance that results in a large impedance drop. If the signal is routed on the same side of the BNC, the plated-through-hole becomes a stub hanging on the signal trace and presents a large parasitic capacitance and even larger impedance drop.

Figure 8: Top and cross-sectional diagrams of a through-hole BNC footprint

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NEXT: Effect of Non-Optimized Signal Launch

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