Microwave and RF engineers have long assumed that true high-frequency connectors, with the exception of blind-mate types, must have a threaded securing ring for a reliable ground connection. Because of this, the BNC connector has often been relegated to low-frequency applications in which S-parameter performance is not highly critical, and this type of connector has been limited to applications from
DC to 2 GHz. In fact, the BNC connector is capable of higher-frequency performance, essentially the same performance level as its closely related, threaded sibling the TNC connector, with added benefits of comparably lower cost, easier connect/disconnect, and performance that holds up well against the universe of connector types.
The BNC connector was developed in the United Kingdom in the early 1940s and named the Bayonet Neill Concelman (BNC) connector after the coupling mechanism and the names of the two inventors (it is at times wrongly called the British naval connector). The connector has a center pin connected to the cable conductor and a metal tube connected to the outer cable shield. A rotating ring outside the tube locks the cable to the female connector.
The way in which this simple design is mounted might suggest that, because there is some movement of the outer rotating ring after full engagement, the potential exists for degradation of the ground path between the male and female components. This is not true, however, since the rotating ring (or coupling sleeve) does not provide the primary ground connection, but is instead enabled by finger springs inside the connector jack. As a result, the rocking motion, which has been criticized in other "non-threaded" connectors as a possible source of ground interrupt, is of little or no concern with the BNC connector.
Typical printed-circuit-board (PCB)-mount BNC jacks have four legs for stabilization that carry the ground path from the surface of the circuit board to the body of the connector and a single pin for the center conductor. To simplify assembly, some more-sophisticated jacks feature an elongated center contact that facilitates fixing the jack into the PCB: the longer center pin is set in the hole, the part is rotated, and the other legs easily drop into place. In contrast, some simpler BNC jacks are designed with two separate parallel center conductor contacts. This approach permits undesirable crosstalk characteristics at higher frequencies and is not recommended for higher-frequency applications.
Return-loss performance, perhaps the single most important specification, varies significantly among BNC plugs and jacks. Return loss is influenced by the choice of dielectric material, the spacing of the ground shield or interior of the connector body from the center contact, and the number and extent of abrupt transitions in impedance or geometry. The characteristics of the transition contribute most to return-loss characteristics. The solution to this is no different in a connector design than it is in general microwave circuit design: the smoother the transition, the better the return-loss performance. Sweeping the transition in a radius significantly improves this situation. In general, at 3 GHz and below, −20 dB is acceptable and −15 dB is marginal—the best BNC designs achieve return loss of −30 dB.
There are four basic types of BNC connectors for PCB mounting: 90-deg., surface-mount, 45-deg., and edge-mount types (see figure). Some are better than others in reducing the severity of (or ideally eliminating) discontinuities that can increase return loss. This standard microwave problem is generally irrelevant at the frequencies to which BNC connectors are usually exposed (below about 1 GHz), but as frequencies increase the problem of abrupt right angles becomes a major consideration. Even at 4 or 5 GHz, the limit of the best BNC connectors, these discontinuities can significantly reduce return-loss performance.
Consequently, the best choice for higher-frequency applications is the edge-launch version. However, even 90-deg. types can deliver optimum performance if their center conductor is "swept" or curved rather than mitered or simply abruptly changed in direction by 90 deg. In fact, edge-launched BNC jacks almost entirely remove the discontinuity that occurs in getting the RF signal on and off the circuit board. Nevertheless, it is common to find BNC jacks with abrupt transitions from vertical to the horizontal plane of the board.
The right-angle connector uses a swept center contact to minimize return loss. However, the signal must still go through a 90-deg. transition when it hits the pad on the board. This is not remarkably better than the straight surface mount connector, and the 45-deg. connector is slightly better than the right-angle connector, but still doesn't address the 90-deg. transition at the board. The edge-launch version has the best return-loss performance because the transition into the jack is in the same plane as the board. Of course, edge-launched BNC jacks cannot be used in all applications, but their return loss can be as good as −26 dB at 4 GHz, which puts them in the same performance category as some of their more revered "precision" microwave counterparts. Nevertheless, through careful attention to providing a "swept" rather than abrupt center conductor, excellent return loss can be obtained even in right-angle types.
The properties that have made the BNC popular in networking applications have been largely overlooked by microwave engineers for applications to 5 GHz. Yet, the best BNC connectors are usable at these frequencies. The edge-launch BNC jack is extremely rugged, connecting as is does both above and below the board. Its "straight-on" design comes closest to making the jack all but transparent from the perspective of signal reflection. Trompeter Electronics, 5550 E. McDowell Rd., Mesa, AZ 85215; (800) 982-2629, e-mail: [email protected], Internet: www.trompeter.com.
