TI SDI transmission scheme based on BNC connector(1)

More and more video equipment now runs at gigabit rates, and they are interconnected by relatively large coaxial BNC connectors. Although these connectors are generally of good quality, their performance in the device depends on how well they are mounted on the PCB. Non-optimized connector footprint design can cause impedance mismatch, reflection, signal loss, and reduce the signal fidelity of the device. BNC occupancy PCB layout design tasks are generally undertaken by circuit layout designers and hardware engineers, but they usually do not have the time or suitable tools to complete the task smoothly. This article introduces several common problems in BNC footprint design, and illustrates examples of footprint design for edge mount and plug-in connectors with illustrations. These connectors can be used with National Semiconductor's LMH0384 3G/HD/SD adaptive cable equalizer, LMH0303 cable driver, and LMH0387 configurable I/O devices.

Types of BNC Connector

Video equipment has historically used BNC with 75? coaxial cable. Video images used to be transmitted at a standard-definition rate (270Mbps), later increased to a high-definition rate (1.485Gbps), and now converted to 3Gbps. The BNC connector must support 3Gbps signal transmission with minimal signal loss, while maintaining a characteristic impedance of 75? and minimizing reflections.

Many connector suppliers provide different types of BNC connector according to the mounting method on the PCB. Based on mechanical considerations, these connectors can be vertically mounted, right-angle mounted, or edge mounted. On the electrical side, the signal pins are either surface-mounted on the connection pads on the top layer of the circuit board or soldered in the metallized through holes, and the signal wiring is located on the other side of the circuit board. Figure 1 shows some examples of BNC Plug Figure 2 is an example of an edge-mount BNC with surface-mount signal pins.

BNC connector test

BNC connector is a coaxial connector designed to support video transmission up to 3Gbps. Its performance mainly depends on the coaxial structure in the BNC. The conversion from the BNC connector to the PCB will seriously affect the performance of the BNC. A well-designed BNC footprint is essential to maintain the BNC bandwidth and its characteristic impedance.

Time Domain Reflectometer (TDR) is a good tool for quickly verifying the internal performance of BNC coaxial structures without signal pins or occupying positions. The simple method for this test is to short-circuit the signal pin of the BNC Connector and its shield pin with a flat metal sheet, and then transmit a TDR step pulse into the BNC. By measuring the signal reflected from the transmitted TDR step pulse, the meter can measure the impedance during the transmission of the step pulse.