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)

Effect of Non-Optimized Signal Launch
The Society of Motion Pictures and Television Engineers (SMPTE) publishes standards1 that govern the transport of digital video over coaxial cables. The SMPTE standards include input and output return loss requirements, which basically specify how well the input or output port resembles a 75Ω network. Figure 9 shows the SMPTE requirements on return loss specifications. A poor BNC or a non-optimized BNC footprint introduces impedance mismatches and makes it challenging to pass the SMPTE return loss limits.

Severe impedance mismatches cause reflections that will adversely affect the signal quality and reduce the voltage or timing margin of the data eye. Excessive parasitic capacitance at the signal launch reduces the bandwidth of the signal path, and introduces inter-symbol interference jitter.

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Figure 9: SMPTE return loss requirements for video ports

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Figure 10 illustrates an example of a signal waveform degraded by a non-optimized signal launch.

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Figure 10: Signal waveform degraded by non-optimized signal launch

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