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.
Figure 1.21 75 Ω Type‐N connectors: commercial (upper) and precision (lower).
Figure 1.22 Insertion loss of 75 Ω connectors.
1.8.2.4 3.5 mm and SMA Connectors
The 3.5 mm connector is in essence half the scale of the N connector and provides higher‐frequency coverage. The center pin of the 3.5 mm connector is supported by a plastic bead, rather than solid dielectric, meaning it has mode‐free operation to a much higher frequency than Type‐N. Traditionally, 3.5 mm connectors are specified up to 26.5 GHz, but their first mode is nearly 30 GHz, and they are functional up to about 38 GHz. An interesting aspect of modes is that the first mode of a 3.5 mm connector is due to the bead (and its increased effective dielectric), but this mode is non‐propagating, so it is reasonable to use these connectors to even higher frequencies. The 3.5 mm female connector comes with several versions of center pin, the main varieties being a four‐slot collect and a slotless precision connection, found now on most calibration kits. Interestingly, even though the slotless connectors may have the center spring contact damaged due to oversized or misaligned male pins (under the microscope one or more fingers may be crushed back into the hollow of the female pin), the RF performance is almost unaffected due the robust solid outer conductor. In fact, one typically can tell if a slotless connector is damaged only by visual inspection, as the RF performance is substantially unchanged, as long as even one finger is left to make contact.
The SMA connector is mechanically compatible
