High-frequency signals must survive many transitions in an RF/microwave system, with one of the more challenging being the point at which signals are “launched” from a coaxial connector to a printed-circuit board (PCB). Managing that transition without interruptions to the signals requires not only proper mechanical alignment but careful electrical optimization. Signals can propagate through many different kinds of coaxial connectors and many types of PCB materials, and the combinations require proper preparation to form the most seamless transitions. Following some general guidelines can help improve the effectiveness of an RF/microwave signal launch in double-copper-layer and multilayer PCBs, even when they contain different types of transmission-line formats, such as microstrip, stripline, and coplanar-waveguide (CPW) transmission lines.
Successful high-frequency signal launches require a tight match between a coaxial connector and a PCB. Because of the wide number of choices for each, there are no automatic combinations that ensure this tight match. But following some basic design guidelines and, hopefully, access to computer simulation software, such as a commercial three-dimensional (3D) electromagnetic (EM) simulator, can help in optimizing the signal launch from a coaxial connector to a PCB. In general, signal launches require good impedance matches between a coaxial connector and a PCB’s transmission lines. Good signal launches are easier to achieve at lower frequencies, and generally easier to achieve for narrowband designs than for broadband designs.
Many different coaxial connector types are used at high frequencies, including BNC, Type N, and SMA connectors, each with its own frequency range and mechanical and electrical characteristics. Connectors are differentiated by gender (male or female) as well as by different styles, such as standard straight connectors, edge-launch connectors for PCBs, and right-angle connectors for PCBs. These connectors and, often, their connected coaxial cables, will have a characteristic impedance, such as 50 or 75 ?. A key to achieving a good signal launch to a PCB is to minimize disruptions in the impedance from the connector’s center conductor to the metal transmission line on the PCB, so that high-frequency signals can launch or flow without insertion loss or reflections (return loss) from the connector to the PCB’s transmission lines.
Signals and their electromagnetic (EM) fields propagating through a cable and connector have a cylindrical orientation, compared to the signals and EM fields in a PCB which have a planar or rectangular orientation. In changing from a connector to a PCB, the signals change orientation to adapt to the new propagation medium, and anomalies can occur in the form of signal loss or reflections. Different types of transmission lines on the PCB can present different challenges in making the connector-to-PCB transition, with grounded-coplanar-waveguide (GCPW) transmission lines providing the easiest transition, followed by microstrip transmission lines, and then stripline transmission lines being the most difficult to make the transition because of their “buried-within-the-substrate” nature.
Circuit designers have learned a few tricks in launching signals from different connectors to different types of transmission lines and PCBs. For one thing, the ground path on a PCB is a very important part of any successful signal launch from a connector to a PCB, since a continuous ground return path is essential to the uninterrupted, low-loss propagation of high-frequency signals from a connector through the PCB. The length of the ground path can also affect the quality of a signal launch from a coaxial connector to a PCB. Even such things as minimizing differences in conductivity between the solder used to join a coaxial connector’s metal parts to a PCB’s conductor metal can make an impact in improving the transition and the performance, especially at higher frequencies. These small losses and impedance mismatches are increasingly noticeable at higher frequencies.
One of the most basic practices in achieving a good signal launch is to minimize dimensional differences between a connector’s conductor and the circuit’s conductor on the PCB to which it is connected. At higher frequencies, connector dimensions shrink, and there is more of a tendency for the PCB’s conductor to be much wider, resulting in a capacitive spike at the transition from the connector conductor to the PCB conductor. Circuit designers have learned that by tapering the circuit conductor to create a more narrow transition where it meets the coaxial connector’s conductive pin, the transition becomes more inductive and less capacitive in nature, and the capacitive spike at the transition can be reduced or minimized. Impedance mismatches in the signal launch interface between a connector and a PCB are due to changes in the electrical characteristics of the circuit. An increase in impedance is due to a rise in inductance at the transition while a decrease in impedance is the result of an increase in capacitance at the transition.
Modifying the inductive or capacitive nature of the transition from the coaxial connector to the PCB will result in frequency-dependent changes to the nature of the signal launch. The PCB’s ground-plane spacing can also play a role in these frequency-dependent changes depending on how it changes the inductive/capacitive characteristics of the PCB and the transition. The length of the taper used to narrow the PCB’s transmission lines closer to the dimensions of the coaxial connector’s conductor can also impact the frequency response of the circuit
How does the choice of PCB material impact the quest for a high-performance signal launch? One of the more important characteristics of any circuit material for achieving a good signal launch is consistent dielectric constant (Dk) value throughout the material. Not only will this ensure consistent impedance for the transmission lines on the PCB but it will aid in achieving the desired impedance match at the transition from the coaxial connector to the PCB. The choice of circuit material Dk value will impact circuit dimensions at a given frequency, with higher Dk values resulting in narrow conductor widths and smaller circuits for a given frequency compared to circuit materials with lower Dk values. The thickness of the PCB material will also affect the transition from connector to PCB, since thicker PCB materials will yield wider transmission-line conductor widths for the same impedance, which may require additional tapering on the PCB in order to achieve a conductor width that is more closely matched to the coaxial connector’s conductor width.
Multilayer circuits (with layers usually numbered according to the number of conductor layers) can produce their own set of headaches for achieving good signal launches with coaxial connectors, since they offer added complexity and may even include different transmission-line technologies together. Coaxial connectors are typically mounted on the top of a multilayer assembly, and plated through holes (PTHs) are used to create electrical paths to the different PCB conductor layers. A connector’s signal launch typical employs a PTH for electrical connections to the PCB’s inner conductor layers. Access to inner layer conductors may be more difficult, but the essential guidelines referenced for achieving good signal launch, such as trying to match connector and PCB conductor dimensions, still hold.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.