Few components have the paradoxical attributes of the microwave connector. These precision-crafted devices play a vastly greater role in determining system performance than their diminutive size would suggest. For as long as there have been transmission lines to connect, the microwave industry has been searching for the "ideal connector." This connector "Holy Grail" would be a lossless, reflectionless interface. Because it would have no length, it would contribute nothing measurable to circuit performance. In the physical world, this goal can never be reached.
Designing a microwave connector involves a number of trade-offs:
- The larger the connector, the more power it can handle. Yet the frequency at which it will operate mode-free will be lowered with any increase in size.
- The smaller the pin gap (the sum of the pin depths of two interfacing connectors), the better and more predictable its performance will be. If either pin is too long, however, there will be a greater chance of female pin damage (assuming a sexed connector). In addition, the precise machining that is needed to tightly control the pin gap increases costs.
In an ideal connector, the inner conductor's outside diameter and the outer conductor's inside diameter would remain constant along the connector's length from the male connector to the female connector with which it mates. There would be no supports for the center conductor, as the dielectrics that support the center conductor would not be completely uniform. Even if the diameters were nearly identical within a mated pair of connectors and from there to the coaxial cable, the interface would still not be perfect.
A discontinuity will exist at the interface between connectors even if the diameters of the conductors are precisely the same. The metal-to-metal contact area causes this discontinuity. A small amount of RF energy will penetrate the interface, causing an inductive mismatch.
In real connectors, return loss can be significantly degraded by poor surface finish, hard metals that don't conform to each other, dirt, oils, and incorrect amounts of applied torque. For a clean, well-made connector, the problems caused by contact discontinuity are usually small (return loss of about 49 dB for a K connector pair, for example). Compared to other design challenges, they are a minor consideration.
Degradation in a dirty connector will be much worse, however. It also can dominate the return loss. For a good-quality connector, the plastic support beads are usually the dominant contributors to return loss. These beads keep the male and female pins centered in the connectors that employ air dielectric at the connector interface.
Slotless Versus Slotted
Some connector designsparticularly metrology-grade versions of 3.5- and 2.4-mm connectorsare called " slotless" (Fig. 1). If there are no slots in the female pin, the center conductor's diameter won't change significantly. The coaxial line's characteristic impedance will be nearly constant through the connector. This benefit is delivered at the expense of increased machining, however. Such machining is needed to keep the male and female pin diameters exactly right so insertion force doesn't become too great or too small. The insertion force (and therefore the wear) of a slotless connector is generally higher than that of a slotted connector.
In contrast, slotted connectors reduce the mating force and decrease wear on both the center pin and the outer conductor. The springy fingers of the female socket, which are generally made of heat-treated beryllium copper, move to accept the male conductor. They then apply force to the conductor to make sure that there is a good electrical connection. The fingers move during mating, which tends to increase the life of the connector by reducing friction. The change in the center-conductor diameter of a slotted design is greater than that of a slotless design, however. As a result, the characteristic impedance may be less constant along the connector's length.
The slotless design is not problem-free in this area either. The point at which the male and female center conductors touch is a critical juncture in a slotless connector. The actual point of contact is not usually at the very end of the female conductor. Rather, it is some distance inside that conductor. An inductive discontinuity results at the interface between the two connectors. This discontinuity can be seen as a bump in the Zo when a connector pair is measured away from the reference plane (assuming that the measurement is displayed in the time domain so variations in Zo along the transmission line can be seen).
Most current vector network analyzers provide this capability. The discontinuity can't be seen at the reference plane. At that point, it has been calibrated out (assuming a vector network analyzer with slotless connectors at the reference plane).
Outer Conductor Thickness
The durability of any coaxial connector is directly related to the thickness of its outer conductor. The original SubMiniature Version A (SMA) connector design has a very thin outer conductor. This conductor limits the number of times that the connector can be mated before wear degrades performance (Fig. 2). The maximum mode-free design frequency of an SMA connector is 26 GHz. Later connector designs mitigated this problem by greatly increasing outer-conductor thickness at the mating interface.
The 3.5-mm connector was one of the first to improve on the original SMA design (Fig. 3). Its original intention was to produce a metrology-grade connector. SMA connectors cannot be made in metrology grade because their Polytetrafluoroethylene (PTFE) dielectric causes major performance variations. The 3.5-mm design uses an air dielectric. It has a thicker outer-conductor mating surface. Although the mechanical interface can mate with an SMA connector, it provides better reliability and electrical performance.
As the frequency of microwave applications crept higher, connectors with smaller internal geometry were needed to increase the frequency of mode free operation. The 2.4-mm and K connector emerged to meet these needs. These connectors are mode-free to beyond 40 GHz. They are limited by the polyphenylene-oxide (PPO) support bead in the connector. Generally, moding first occurs in the transmission-line areas that have dielectric support materials.
As shown in Fig. 4 and Fig. 5, the male and female K connectors' outer walls are much thicker than those of the 2.4-mm design. The K connector is therefore much stronger. By retaining the 1/4-36 thread in the K connector, it remains physically compatible with its SMA and 3.5-mm counterparts. The 2.4-mm connector is physically compatible with the V connector, which is mode-free past 65 GHz (Fig. 6). The 2.4-mm connector is mode-free to 50 GHz.
The male center pins of the K and V connectors are shorter than those of the 2.4-mm, 3.5-mm, and SMA connectors. When connectors are being mated, the male center pin of the 2.4-mm, 3.5-mm, and SMA designs can touch the female center pin before the connectors are perfectly aligned. The female pin can be damaged if the fingers of the center conductor are bent. With the K and V connectors, such bending cannot occur. The male-connector mating surface is well engaged in the female connector before the center pins touch, assuring proper center-conductor alignment.
For optimum RF performance, the center conductor's diameter should remain constant in the mating region when the center pins of a sexed connector are mated. To achieve this goal, the mating surfaces must precisely touch but not apply force to each other. If they do apply force, bowing of one or both conductors will occur. Such bowing degrades the match. When the center conductors are forced together, damage to the conductors can result. In addition, bowing lowers the mode-free frequency by degrading the concentricity of the inner and outer conductors. Damage can occur with even a single mating of a connector pair with less than zero pin gap.
To achieve zero pin gap, machining tolerances must be extremely tight. Otherwise, adjustability will be required in the pin depth (perhaps accomplished through spring-loading). The pin depth of the connectors also must be measured regularly to ensure that they are still exactly the correct length. In the APC-7 and GR900 4-mm connectors, zero pin gap is achieved by using spring loading.
Extremely tight machining tolerances add significantly to a connector's cost. The same is true for the extra material and labor that are required if spring-loading is added to a sexed connector. The ongoing need to measure precision connectors also adds significantly to their cost of ownership.
If a small amount of pin gap can be tolerated, the cost of a connector can be lower. In the area where the ratio of center-pin-to-outer-pin diameter increases, the characteristic impedance increases. This increase degrades the connector's return loss. The thickness of the female contact area is directly related to the connector's sensitivity to the performance degradation caused by pin gap. The thicker the female contacts, the more the characteristic impedance changes in the pin-gap area.
Although the diameter of the male pin is relatively small, the 3.5- and 2.4-mm connectors have thick female contacts. Their performance is therefore more prone to pin-gap degradation than SMA, K, and V connectors, which have thin female contacts. The 3.5- and 2.4-mm designs do achieve slightly better repeatability. But that repeatability comes at the cost of increased wear, which is caused by higher contact pressure.
There are even more trade-offs to consider when choosing a connector type. Generally, larger connectors are more rugged than small connectors because of the amount of material involved. Yet smaller connectors generally operate to higher frequencies than larger connectors. It is not the outer dimensions of the connector, but its inner geometry that influences its ability to launch and propagate higher-order modes. Finally, a connector's power-handling capability will be greater if its internal geometry is larger.
The primary factor affecting connector cost is manufacturing precision. Assuming that the connector design is fundamentally sound, tighter manufacturing tolerances will produce better return-loss performance. Even the humble SMA connector, for example, can range widely in price. A simple Web search revealed prices ranging from $1.24 to $19.10 with special versions of SMA connectors costing even more.
The best system performance will almost always be achieved when the least possible number of connectors and adapters are used. After all, signals are degraded by each connector and adapter through which they must pass. The best architecture is one in which critical signals remain within a module—emerging only when they must face the outside world. Keeping this in mind, it is very important to choose the most durable connector. That connector should cover a frequency range that is wide enough to pass the signal without excessive attenuation. It also must deliver the best possible return-loss performance to minimize the signal reflections that can degrade overall system performance.