Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 44

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.