150- W STP-A: Radiation and Safety

W STP A Radiation and Safety

Radiation from high-speed 150- W STP-A data links is controlled by proper grounding of the shield at both ends of the link. The 150- W STP-A shield must be connected to ground at both ends to be effective. This connection must have a very low impedance at all frequencies within the data spectrum.

Unfortunately, this dual-grounding requirement runs directly counter to AC power-safety grounding considerations (see Section 6.12.2, "Immunity to Large Ground Shifts").

Any form of shielded twisted-pair cable should, in this author's opinion, be used only for short connections within a wiring closet or computer room between pieces of equipment intentionally tied to the same ground. Your specification should explicitly state that this must be the case. Between such pieces of equipment there will be no large circulating ground currents. For longer connections, you should provide other links types that do not require grounding at either end, such as multimode fiber, single-mode fiber, or UTP.

The susceptibility and radiation budget for Gigabit Ethernet requires a ground transfer impedance (that is the impedance between the chassis and the shield of the cable) of 0.1 W or less at 625 MHz. Connectors that meet this ground-transfer impedance specification provide a direct metallic connection between the chassis and the shield that goes all the way around the connector pins, completely enclosing the signal conductors. This specification is difficult (if not impossible ) to achieve with any sort of pigtail connection of the shield to the chassis or with an AC-coupling capacitor in series with the shield.

A discrete capacitor can't be used to make an AC- coupled shield because the ground-transfer impedance requirement of 0.1- W at 625 MHz implies an effective series inductance of less than 25 pH. That small an inductance cannot be implemented in a leaded component. If you are dead-set on building an AC-coupled shield, try making a thin capacitive gasket of dielectric material distributed all the way around the connector shell, insulating the connector shell from the chassis, and then use plastic screws to hold the connector in place. I have seen proposals for this type of connector, but have not seen one working in actual practice.

POINT TO REMEMBER

• Pigtail and AC-coupled shields work at audio frequencies, but not at a gigahertz.