The precision sexless connectors are now found only in metrology labs. Their chief benefit was a repeatable connector that has identical characteristics for each connector. As such, it was easy to create a system calibration, and any part with such connectors could be inserted between two cables in either direction. This was important because in the past it was difficult to deal with “non‐insertable” devices from a calibration sense (a non‐insertable device is one with the same sexed connector on each port, e.g. female‐female). The 7 mm connector is often found on precision attenuators and airlines used as transfer standards. The 7 mm connector is also known as the GPC‐7 for general precision connector, and often as the APC‐7™ for amphenol precision connector. Because these connectors are sexless, there is no need for adapters to provide interconnections between devices or between devices and cables.
1.8.2.1 7 mm Connector (APC‐7, GPC‐7)
The 7 mm connector has a couple of interesting attributes: the center pin has no slots but contains spring‐loaded center collets that protrude slightly from the mating surface, as shown in Figure 1.18.
Figure 1.18 A 7 mm connector.
When mated, the collet from each connector floats against each other, providing a good center contact. There is a slight gap in the slotless outer sleeve of the center pin. As with almost all RF connectors, the outer conductor forms the physical mating plane. On most connectors, there is a slip‐ring‐threaded sleeve surrounded by a coupling nut. To mate, the threaded sleeve is extended on one connector and retracted on the other. On the retracted connector, the coupling nut is extended to engage the other's sleeve and is tightened. Only one coupling nut should be tightened, although it is common but incorrect practice to tighten the other coupling nut. In fact, tightening both coupling nuts can result in the center pins pulling apart and a poorly matched contact. Occasionally, one sees parts that contain only a solid threaded outer conductor (serving the purpose of the threaded sleeve) and no coupling nut. These are more common on older test fixtures intended to mount directly the 7 mm connectors of network analyzer test sets.
1.8.2.2 Type‐N 50 Ω Connector
The Type‐N connector is common in lower‐frequency and higher‐power radio frequency (RF) and microwave work. It has the same outer diameter (7 mm) as the 7 mm connector but is sexed. In fact, this connector has the unusual attribute of having the mating surface for the outer conductor (which is almost always the electrical reference plane) recessed for the female connector. Thus, the female pin protrudes (in an electrical sense) from the reference plane, and the male pin is recessed. Thus, the calibration standards associated with Type‐N connectors have electrical models that are highly asymmetric for male and female standards.
The Type‐N connector has precision forms, including ones with slotless connectors (metrology grade), ones with precision six‐slotted collets and solid outer conductor sleeves (found on most commercial test equipment), and commercial forms with slotted outer conductor sleeves and four or even two slotted female collets. Slotless connectors have a solid hollow cylinder for the female connector with an internal four‐ or six‐finger spring contact that takes up tolerances of the male center pin. As such, the diameter of the female center pin does not depend at all on the radius of the male pin. Typical female contacts with collets expand or contract to accept the male pin, and thus their outer dimension (and thereby their impedance) varies with the diameter tolerance of the male pin.
The commercial forms are found on a variety of devices and interconnect cables. The male version of these commercial‐grade parts present two common and distinct problems: there is often a rubber “weather‐seal” o‐ring in the base of the connector, and the outer nut of the male connector is knurled but has no flats to allow using a torque wrench. The first problem exacerbates the second, as the mating surface of the outer conductor of the male connector is often prevented from contacting the base of the female connector because the outer (supposedly non‐mating) surface of the female connector touches the rubber o‐ring and prevents the male outer conductor from making full contact. If one can fully torque a Type‐N connector, the rubber o‐ring would compress, and the contact of the male outer conductor would occur, but as there are no flats for a torque wrench, it is difficult to sufficiently torque the Type‐N connector to get good repeatable connections. This one issue is the cause of hundreds of hours of retest when components don't pass their return‐loss specs. The solution is quite simple: remove the rubber o‐ring from the base of the male connector, always, before any measurement. A pair of tweezers and a needle‐nose pliers are indispensable for the process of removing this annoying o‐ring. One will note that none of the precision versions of Type‐N connectors contains such an o‐ring. Figure 1.19 shows some examples of Type‐N connectors; the upper two are commercial grade, and the lower two are precision grade. Figure 1.20 shows the insertion loss measurement of a male‐to‐male Type‐N adapter mated to a female‐to‐female Type‐N adapter for a precision pair and a commercial‐grade pair, where the loss is normalized to the length of the adapter. The commercial‐grade pair is operational only to about 12 GHz, due to moding in the connector. The precision N is mode free beyond 18 GHz.
Figure 1.19 Examples of Type‐N connectors: commercial (upper) and precision (lower).
Figure 1.20 Performance of a precision and a standard Type‐N connector.
1.8.2.3 Type‐N 75 Ω Connector
Type‐N connectors also have a 75 Ω version, which has the same outer dimensions but a smaller center conductor. This is in some ways unfortunate as the smaller female collet of the 75 Ω version can be damaged when inserted with a 50 Ω male pin. There are a couple of versions of the 75 Ω female collet, one with short slots and six fingers, and one with long slots and four fingers. A precision slotless version is also available. The short slot version has the potential for better measurements, as the slots expand less so there is less uncertainty of the open capacitance. However, on many products with 75 Ω N‐connectors, the long slot connector is used; the long slots were designed to accept a 50 Ω male pin, at least for a few insertions, without damage. Often the 75 Ω components have an extra machined ring or line on the outer nut to help identify it. Versions of 75 Ω Type‐N connectors are shown in Figure 1.21. An example of the insertion loss measurement of a mated pair of a male‐to‐male adapter with a female‐to‐female adapter is shown in Figure 1.22, where the loss is normalized for length of the adapter. The frequency limit of Type‐N 75 is often stated as 2 or 3 GHz, but that is because the commercially available calibration kits were rated only to those frequencies. In practice, these connectors could be used up to 7 or 8 GHz without difficulty. The response of the commercial‐grade connector is likely limited not due to moding (since the loss signature is quite low Q) but rather due to poor impedance control in the center pin support bead, causing impedance mismatch.