Coaxial Cable Connectors

There are too many different families of coaxial connectors. This is the natural result of a broad, diverse industry operating for many years without coherent standardization. Table 10.5 summarizes a selected group of the more popular connectors used in digital applications.

Within each family, the basic choices you will need to make relate to the frequency response, the plating materials used, the style of attachment, the connection to the cable, and the quality of the springs.

Above 100 MHz, you should always match the characteristic impedance of the connector to the cable. A matched coaxial connector balances the parasitic series inductance L and the parasitic shunt capacitance C of the connector in such a way that the natural impedance of the connector matches some particular value (perhaps 50 or 75 ohms). Even when this is done, the connector may still show imperfections at extremely high frequencies. Depending on the distribution of parasitics within the connector, there may be internal regions over which the matching is quite good and regions over which it is not so good. As the risetime of your signals shrinks to a value comparable with the propagation delay through the connector, the internal details begin to matter. In very high-speed applications, look for vendors that provide standing-wave ratio (SWR) data or reflection coefficient data to back up their claims of superior performance. Screw-on type connectors will always outperform quick-disconnect types because the threads accurately align the mated halves of the connector and provide a solid 360-degree ground connection concentric with the signal conductor. The electrical performance of the connector has more to do with the care taken during manufacturing to produce a uniform, properly dimensioned cavity than it does the physical dimension or other factors.

Table 10.5. Selected Coaxial Connector Families

Here's a handy little table that relates the SWR, reflections, and return loss specifications for connectors (Table 10.6). This table has been computed only for the case of sine-wave excitation . In general, if you want your digital signal to pass through the connector 99% intact, select a connector with a return loss greater than 17 dB at all frequencies from DC up to the knee frequency of your logic (1/2 over the risetime). Also keep in mind that connector distortions aggregate across all the connectors in a particular data link.

The contact plating serves to stave off corrosion and eventual failure of the contacts. For applications that require multiple connector insertions, always look for connectors with gold or stainless steel mating surfaces.

The attachment style may be either quick-disconnect or threaded. This is a tradeoff of ease-of-use versus reliability. My rule of thumb here is simple: If it goes on a boat, a car, a plane, or anything that moves, it's got to be threaded. A quick-disconnect part will not survive the tough U.S. military-standard "500- hour salt-spray test" or the "2-minute Saturn-5 vibration , heat, and shock test," or for equipment mounted near a gasoline-tank, the even more excruciatingly difficult "Ford Pinto heat and flame trial." [88]

[88] OK, I made up the last one.

Regarding the choice of crimped versus soldered cable attachment, this choice is based on the facilities available at the point of assembly and the electrical performance of the connector. Crimping has two basic advantages: It doesn't require access to AC power (soldering does), and it's fast. On top of a telephone pole, down in a cable tunnel, or anywhere power may not be available, crimping is the way to go. In a factory environment, where speed matters, crimping wins again. Crimping is not appropriate in situations where your field technicians won't have access to the special crimping tools required to press the connectors onto the cable or access to any spare connectors. In those applications, like on a ship or a spacecraft, the solder-type connectors may be best. In all cases avoid the popular "twist-on" style connectors. These seem to twist off just as easily as they twist on.

Table 10.6. SWR and Return Loss Conversions for Connectors

Crimp-type connectors tend to have the best dimensional control (because they don't have to accommodate blobs of solder) and so deliver the best control over impedance. This makes crimp-style connectors generally superior to the other types for high-frequency work, but always check the specifications for insertion loss and SWR at the frequency of operation.

Lastly, about the springs, always specify heat-treated beryllium-copper for critical contact springs. These maintain contact pressure for years, whereas ordinary copper or brass will soon deform and fail to connect.

Good practical information about coaxial connectors is found in [82] and [84] . General reference material concerning plating, crimping, and spring-loaded connector technology may be found in [83] .

POINTS TO REMEMBER

• Above 100 MHz, you should always match the characteristic impedance of the connector to the cable.
• Contact plating serves to stave off corrosion and eventual failure of the contacts.
• If it goes on a boat, a car, a plane, or anything that moves, use threaded connectors.
• Crimp-style connectors generally superior to the other types for high-frequency work.
• Always specify heat-treated beryllium-copper for critical contact springs.

For further study see: www.sigcon.com