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)

BNC Testing
BNC is a coaxial connector designed to support up to 3Gb/s video transmission. Its performance is primarily determined by the coaxial structure inside the BNC. The transition from the BNC connector to the printed circuit board will heavily influence the performance of the BNC. A well-designed BNC footprint is necessary to preserve the BNC’s bandwidth and its characteristic impedance.

Time domain reflectometer (TDR) is a very good tool to quickly check out the intrinsic performance of the BNC’s coaxial structure without its signal pin or its footprint. A simple way to do this measurement is to launch a TDR step into the BNC, with its signal pin shorted to its shield pins using a flat metal blade. By measuring the reflected signal from the launched TDR step, the instrument is able to derive the impedance over the time that the step travels.

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Figure 4: Impedance profile of a good BNC

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Figure 4 illustrates the impedance profile of a good BNC. This right-angle BNC has a uniform coaxial structure with its 75Ω characteristic impedance practically constant inside the BNC. Its footprint should be designed to achieve the same characteristic impedance as the BNC.

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Figure 5: Impedance profile of a fair BNC with impedance drop

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Figure 5 shows the impedance profile of a fair BNC. This right-angle BNC shows sign of non-uniformity in its coaxial structure. At the right-angle bend, the characteristic impedance starts to decrease from the nominal 75Ω. In this case, its footprint can be designed to have slightly higher characteristic impedance in order to offset the imperfections from the BNC.

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Figure 6: Impedance profile of a poor BNC with impedance fluctuations

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Figure 6 shows the impedance profile of a poor BNC. This right-angle BNC shows multiple signs of non-uniformity in its coaxial structure. At the right-angle bend, it has difficulty to maintain its characteristic impedance. In this case, it will be challenging to design a footprint with good return loss performance for this BNC.

NEXT: Common Problems in BNC-to-Board Transition

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