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Coaxial connectors continue to move higher in frequency, with ever-smaller dimensions, driven in part by the development of test equipment such as vector network analyzers (VNAs) for higher-frequency use. But designing reliable higher-frequency coaxial connectors is more than simply a matter of shrinking connector dimensions and developing novel connector interfaces. Connectors reaching towards higher millimeter-wave frequencies have been helped by VNA advances, but also by solid mechanical design of shrinking connector components—to the point where coaxial VNA testing at 100 GHz and beyond is becoming almost trivial.

As coaxial connectors have risen in frequency, they have historically relied on two approaches to mating center conductors: hermaphroditic contact and male-pin/slotted-female contact. The non-sexed approach offered many advantages, the main one being that only one type of connector was required. As frequencies increased, these connectors were made smaller to remain in single-mode operation.

The success of the SMA connector essentially foretold the end of non-sexed connectors. As connectors became smaller for higher-frequency use, it became difficult to make butt-type connectors—pin depth tolerances were critical and the smaller sizes made it difficult to achieve a resilient contact. Such factors would raise the cost of the connectors well above the simple male pin/slotted female contact.

The male/female contact approach became the standard for connectors at higher frequencies. For metrology applications, a slot-less female design was introduced, but it became impractical following the 50-GHz 2.4-mm connector. Slotted connectors are not without problems, however. Designs such as the SMA connector and its two-slot female contact are inexpensive to produce, but easy to damage. A half-round feature is not very flexible, and the SMA connector was initially rated for a lifetime of only 500 connections. SMA designs with a long male pin, which allowed the center conductors to engage before the outer conductors aligned the connectors, meant that careless mating would damage the female contact.

A four-slot contact is much more resilient, such as those used in 3.5-mm connectors designed for compatibility with SMA connectors. With their air-interface design, these precision connectors were needed for calibrated VNA use, but they had their own problems. For SMA compatibility, the size of the male pin was set at 0.914 mm (0.036 in.). The 3.5-mm connector center conductor diameter is 1.52 mm (0.060 in.), which creates two problems.

First, the wall thickness of the female fingers is 0.3 mm (0.012 in.), which is quite thick for such a small-diameter contact. After slotting, the fingers are closed and the part is heat treated. If closed by too little, the contact will be unreliable. If closed by even slightly too much, the insertion force required to mate the connectors will become quite high.

Such high force introduces excess wear and may ever distort the support beads holding the center conductors in place. The large wall thickness of the 3.5-mm connector introduced more pin gap reflection. The impedance of the gap section is 80 Ω. The higher-impedance line section created by the exposed male pin also yields pin gap reflections.

The 40-GHz 2.92-mm K connector, introduced around 1985, minimized many of these problems. A short male pin ensured that, before center conductors could engage, the outer conductor parts aligned the two connectors so the male pin could not damage the female section by being inserted at an angle. The center-conductor diameter of the K connector was designed as 1.27 mm (0.050 in.), leading to a finger wall thickness of 0.18 mm (0.007 in.). This meant that the fingers were more flexible and the insertion pressure was greatly reduced. As a result, K connectors were rated for 4000 connections.

The 50-GHz 2.4-mm connector and the 65-GHz 1.85-mm connector interfaces were introduced by Agilent Technologies. Launch of the firm’s coaxial 50-GHz VNA required the development of the 2.4-GHz connector interface in support of millimeter-wave frequencies.

The 1.85-mm V connector was introduced in support of a coaxial 60-GHz VNA by Anritsu/Wiltron Co. Improvements in V connector bead design made it possible for coaxial VNA frequency coverage to rise first to 65 GHz, then to 67 GHz, and then to 70 GHz. A 110-GHz 1-mm connector was introduced by Agilent, followed by a 110-GHz W connector from Anritsu with the introduction of their 110-GHz coaxial VNA. In development of a 70-kHz-to-145-GHz coaxial VNA system, Anritsu has presented a 0.8-mm coaxial connector.

As connector dimensions shrink, however, they also become more fragile. The thinner wall designs of the K-band and V-band connectors yielded connectors at 1 mm and smaller with female contacts that were too fragile; thus, thicker walls were required. But thicker walls raise the pin gap impedance. Still, it was preferable to a fragile female contact with very thin walls. These connectors are quite expensive and a distorted contact is very undesirable.

Finding machine tools to produce such connectors becomes a challenge, although small drills are available—as small as 0.05-mm diameter. Drills are required to make the holes in the support beads. In addition, a fine saw is needed to cut slots in the female center conductor, with the capacity to cut deep enough to make long enough slots. The finest-dimensioned saw is 0.05 mm (even a 0.025-mm saw was available); the resulting slotted female contact would be very fragile. With a thin-walled design, the insertion force would be very slight, as would be the contact pressure. With a thick-walled connector design, the contact pressure would be greater but the finger flexibility would be slight.

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