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RF/Microwave Connectors on Printed Circuit Boards

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RF/Microwave Connectors on Printed Circuit Boards High Frequency Design CONNECTORS ON PCBs RF/Microwave Connectors on Printed Circuit Boards By Gary Breed Editorial Director hen mounting This month’s tutorial takes a an RF/micro- look at the practical matter Wwave or high of characterizing, then speed digital connector to installing, an RF connector a printed circuit board, on a printed circuit board the transition from the connector body and inner conductor pin to the printed traces is often the source of excessive mismatch. The discontinu- ity in size, shape and surrounding conductors Figure 1 · Connectors must provide a tran- results in an area with a characteristic sition from a round coaxial transmission line impedance that can be much different (usual- to the planar microstrip or stripline structure ly lower) that the system impedance of 50 of a p.c. board. ohms (RF/microwave) or 75 ohms (data or video). An additional challenge is the transition increases the characteristic impedance in the from the round coaxial structure of cables and area where the required solder pad is much their connectors, to the planar stripline or wider than a normal 75-ohm microstrip line. microstrip structure of signal paths on a p.c. Figure 2 illustrates the problem by showing board. The connector shown in Figure 1 the relative widths of the solder pad and demonstrates one approach to solving the microstrip lines on the top metal layer. Figure problem. The connector body is designed to 3 shows how the top metal and the metal of minimize the VSWR “bump” caused by the the next lower layer are modified to create a change from coaxial to planar transmission region with higher characteristic impedance. lines, but the pin that is soldered to the board An alternate solution is to mechanically has as added vertical thickness, and almost machine the p.c board to create an air space always will require a solder pad that is wider adjacent to the solder pad. This lowers the than a stripline with the desired characteris- effective dielectric constant of this part of the tic impedance. board, which will increase the characteristic Reference [1] offers a good description of impedance without changing the pad and the problem and typical solutions. Although trace widths. the author covers the issue from the perspec- Top-mounted connectors present a differ- tive of 75 ohm BNC connectors, the informa- ent set of structural variations that affect the tion applies to 50 ohm systems and other con- impedance match between the connector and nectors, as well. As shown in Figures 2 and 3 the p.c. board traces. This type of mounting on the next page, one solution is to modify the offers greater mechanical strength than edge- ground metal layer under, and adjacent to, the mounting, and is the only option for installing connector. Wider spacing between the signal connectors in locations other than at the edge conductor and the ground/shield conductors of a board. 60 High Frequency Electronics High Frequency Design CONNECTORS ON PCBs Figure 2 · Cross-section of a p.c. board “landing pad” Figure 3 · Cross-section of a p.c. board “landing pad” area for an edge-mounted BNC connector, with no area for an edge-mounted BNC connector, with the compensation for impedance mispatch (adapted from ground metallization modified to provide an improved Ref. [1]. match to a 75 ohm (adapted from Ref. [1]. Surface-mount connectors will quency increases, the effects will hole connectors. perform similarly to edge-mounted become increasingly apparent as Of course, the magnitude of the connectors, with round-to-planar increased VSWR and its attendant problems will vary with the connec- transitions and impedance variations loss, and time-domain reflections tor type and mounting method as due to solder pad size. However, that can affect the modulated wave- well. Well designed and precision through-hole mounted connectors form of RF signals, or the waveform manufactured surface-mount connec- must have vias to provide both signal shape (and eye closure) of high speed tors are available with performance and ground connections. As shown in digital signals. specified into the tens of GHz. Figure 4, the length of the via has an At 2.45 GHz, with typical FR-4 inductance, and the gap between the material dielectric constant of Summary via and intervening metal layers has approximately 4.4, the thickness of a At high frequencies and fast edge a capacitance. The average values p.c. board is in the range of 1/17 rates, the interface between an will determine the characteristic wavelength. At this frequency, prob- RF/microwave or high speed digital impedance of the via “tube,” but the lems would not be significant. connector and the printed circuit layered structure means that the However, a common rule of thumb is board can be the most critical loca- impedance will vary along the length that wavelength-related problems tion in the signal path. Designers of the via. will arise when dimensions are in the must be aware of the potential prob- If the space between the layers is range of λ/10. As the operating fre- lems that can arise, and be prepared small relative to the wavelength of quency rises above 2.45 GHz, design- to use appropriate techniques to pre- the highest frequency desired sig- ers should be prepared to implement vent their occurrence. nals, the variation will have little compensating techniques to avoid effect on performance. But as fre- performance issues with through- References 1. T-K Chin, National Semicon- Top View Cross-section View ductor Corp., “Optimizing BNC PCB C C Footprint Designs for Digital Video Equipment,” available at: www. C C samtec.com/technicallibrary/ L C C white_papers.aspx 2. “PCB Design Guide,” Trompeter C C Electronics, (available from several sources—do an Internet search by Capacitance Inductance title + Trompeter) between via due to length 3. S. McMorrow, J. Bell, J. Ferry,“A wall and each and diameter layer’s metal of tubular via Solution for the Design, Simulation and Validation of Board-to-Board Figure 4 · Thru-hole, top-mounted connectors have an impedance that is Interconnects,” High Frequency Elec- affected by vias, mid layers and metal layers of the p.c. board. tronics, Jan. 2005. 62 High Frequency Electronics.
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