Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 45

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.