Cables are a major source of radiated emissions and receptors in electromagnetic (EM) coupling. Commercial equipment that is designed for electromagnetic compatibility (EMC) and that includes a well-thought-out grounding scheme and wiring and PCB layout can meet commercial EMC requirements with a number of unshielded cables connected to the equipment. When the equipment enclosure is also unshielded, the sources of emission are likely to be both the enclosure and the cables
Which is typical of analog, video, or low-level radio frequency (RF) circuits with linear power supplies, the level of noise current on unshielded cables may be low enough to ensure meeting the radiated emission requirements. However, the same low-signal-level equipment is likely to be susceptible to noise voltage induced in unshielded cables during exposure to the E field generated in the radiated susceptibility RS103 test. In many cases, even equipment with shielded cables fails these tests.
This chapter describes some of the reasons why this is possible. Apart from the EMC requirements, the severity of the EM environment may necessitate the use of shielded cables. Also, some standard interfaces, such as GPIB, MIL-STD 1553, and Ethernet, require the use of shielded cable. The use of fiber-optic cables may appear to eliminate the need for concern about cable radiation and coupling. However, with the present high cost of fiber-optic links and the need for power interconnections, the use of shielded cables is not quite obsolescent.
A vector network analyzer with an S-parameter test set, that is, a full two-port calibration will be established including the connecting cables used to connect the test setup to the test equipment. The reference planes for the calibration are the connector interface of the connecting cables. For networks that respond in a liner manner, they can be completely characterized by parameters at the network terminals without regard to the contents of the network.
S-parameters are useful in RF and microwave circuit design in determining current based on input voltage sources and power gain, voltage gain, and loss. The S-parameters are typically measured using a network analyzer. which reduces the long-line effects. Placing the cable insulation on the ground plane means that the velocity of propagation in the cable-to-ground plane transmission line is approximately equal to that of the shielded cable. This would not be true with a predominantly air space between the cable and the ground.
A spacer may be used between the cable and the ground plane that has a similar permittivity to the cable core insulation material to minimize this effect. In this test setup, the addition of the measuring device will present a complex impedance between the shield of the CUT and ground, in parallel with the terminating resistor. To control this impedance, many ferrite baluns should be placed on the shielded cable connecting the measuring device to the termination point. Even with the baluns in place.
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