TI SDI transmission scheme based on BNC connector(2)

Common problems in BNC connector to circuit board connection

Most surface mount BNC connectors have large signal pins with a diameter of approximately 30-40 mils. In order to properly solder the signal pins on the PCB, a connection pad with a width of about 50 mils is required. To facilitate wiring, 8-15mil thin surface traces are usually used to transmit signals from BNC connectors to integrated circuits with multiple pins.

Figure 7 shows the top and cross-sectional views of the unoptimized edge-mount BNC footprint. In order to achieve a characteristic impedance of 75?, a 12mil microstrip line was specially designed and placed 15mil above the GND layer. The connection pad of the BNC connector is equivalent to a 50mil microstrip line. Because there is a GND layer 15 mils below the pad, the characteristic impedance of the pad is much lower than the characteristic impedance of the trace. The pad causes the impedance to drop significantly, which will affect the signal quality and increase the parasitic capacitance, thereby reducing the BNC bandwidth.

Top and cross-sectional views of unoptimized edge mount BNC connector footprints

Most video equipment will use plug-in BNC because it has better placement robustness. BNC connector is usually mounted on the top layer of the circuit board, its signal pins are soldered in larger metallized through holes, and the signal wiring is located on the bottom layer of the circuit board. The cylindrical metal pillars of the metallized vias will generate a small amount of inductance. Each internal power layer will provide parasitic capacitance for the metallized via, and the specific capacity depends on the gap between the internal power layer and the metal pillar. Large metallized vias with small gaps will generate excessive capacitance, resulting in a significant drop in impedance. If the signal wiring is on the same layer of the BNC connector, the metallized via will become a stub hanging on the signal trace, and generate a larger parasitic capacitance, and even cause a larger impedance drop.