Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 46

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 with the 3.5 mm connector but has a solid Teflon dielectric and thus a lower operating frequency due to moding. SMA is traditionally considered to be an 18 GHz connector, but the first propagating mode is well above 20 GHz, depending upon the type of cable that is connected to the SMA connector. The chief advantage of SMA connectors is low cost, especially when mounted to semi‐rigid coaxial cables. The dimensions are such that the center wire of the coax can be used a connector pin for SMA, and only an outer conductor sleeve needs to be added to the coax outer conductor to form a male connector, shown in the lower‐right picture of Figure 1.23. But these cables are notoriously bad at maintaining the proper dimensions for the center pin, and often the center pins are poorly trimmed and improperly chamfered so that they cause mating problems with their female counterparts. This is particularly true when mating them to 3.5 mm female connectors, slotless ones in particular. Figure 1.23 shows examples of 3.5 mm and SMA connectors, with 3.5 mm on the left and SMA on the right.

Figure 1.23 3.5 mm (f) and (m) (upper left); SMA (f) and (m) connectors (upper right); 3.5 mm (lower left) and SMA adapters (lower right).

Figure 1.24 shows measurement plots of a mated pair of 3.5 mm male‐to‐male with a 3.5 mm female‐to‐female, as well as two SMA examples. The moding of the SMA connector is clearly seen above 25 GHz (Marker 2 on the SMA1 and SMA2 trace). The moding of the 3.5 mm connector is seen just above 30 GHz (Marker 2) and again at 34 and 38 GHz. There two typical construction types for SMA, one with a press‐fit of the Teflon and center conductor (SMA1 in the measured response) and one where the Teflon is held in with a small dot of epoxy through a hole in the outer conductor (SMA2 in the measured response). The second method usually gives a poorer match, and we can see that with the small dip in the S21 response of SMA2 near 12 GHz and the larger dip just above 20 GHz.

Figure 1.24 Performance of SMA and 3.5 mm mated‐pair connectors.