Per Dr. Howard Johnson, a well known expert in high speed signal integrity and RFI:
Cable Shield Grounding
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 2 Issue 2
In high-speed digital applications, a low impedance connection between the shield and the equipment chassis *at both ends* is required in order for the shield to do its job. The shield connection impedance must be low in the frequency range over which you propose for the shield to operate. The measure of shield connection efficacy for a high-speed connector is called the ground transfer impedance, or shield transfer impedance, of the connector, and it is a crucial parameter. In the example you cite, the ground transfer impedance at one end of the cable would be 100 ohms, rendering the shield useless.
In low-speed applications involving high-impedance circuitry, where most of the near-field energy surrounding the conductors is in the electric field mode (as opposed to the magnetic field mode), shields need only be grounded at one end. In this case the shield acts as a Faraday cage surrounding the conductors, prevent the egress (or ingress) of electric fields.
In high-speed applications involving low-impedance circuitry, most of the near-field energy surrounding the conductors is in the magnetic field mode, and for that problem, only a magnetic shield will work. That?s what the double-grounded shield provides. Grounding both ends of the shield permits high-frequency currents to circulate in the shield, which will counteract the currents flowing in the signal conductors. These counteracting currents create magnetic fields that cancel the magnetic fields emanating from the signal conductors, providing a magnetic shielding effect.
For the magnetic shield to operate properly, we must provide means for current to enter (or exit) at both ends of the cable. As a result, a low-impedance connection to the chassis, operative over the frequency range of our digital signals, is required that *both* ends of our shielded cable. (See Henry Ott, ?Noise Reduction Techniques in Electronic Systems?, 2nd ed., John Wiley & Sons, 1988.)
There are shielding approaches that provide a low ground transfer impedance at high frequencies, while at the same time providing a much higher impedance at 60 Hz. These approaches involve the use of shields that are capacitively- coupled to the chassis. They are used where high-frequency shielding is needed, but where there is a desire to limit the circulation of 60-Hz currents.
For a capacitively-coupled shield to work, the impedance of the capacitor, at the frequency of operation, must be very low. For example, if the signal wires couple to the shield through an impedance of 75 ohms (that?s another way of saying that the common-mode impedance of the cable is 75 ohms), and the shield is tied to ground through an impedance of 0.1 ohm, then we would expect to measure on the shield a voltage equal to (0.1/75) = 0.0013 times the common-mode signal voltage. The shield in this case would be giving us a 57dB shielding effectiveness. These are the specifications that our IEEE 802.3z 1000BASE-CX copper cabling groups feels are necessary to meet FCC/VDE regulations.
For any shield to work in the Gigabit Ethernet application, we will therefore need a ground transfer impedance (that is the impedance between chassis and the shielded of the cable) less than about 0.1 ohms at 625 MHz. If you check the specifications for the BERG MetaGig shielded connector, it beats this specification. It provides a direct metallic connection between chassis and shield that goes all the way around the connector pins, completely enclosing the signal conductors.
To achieve equivalent performance with a capacitively-coupled shield, the effective series inductance of the capacitor would have to be limited to less than about 16 PICO-henries. That small an inductance cannot be implemented in a leaded component, it would have to be a very low-inductance distributed capacitance, possibly implemented as a thin gasket distributed all the way around the connector shell, insulating the connector shell from the chassis. We have seen proposals for this type of connector, but have not seen one work in actual practice.
I do not advocate the use of capacitively-coupled shields for our application because: (1) It would add complexity, (2) It hasn?t been demonstrated to work, and (3) It would not expand the range of our applications. Keep in mind that the short copper link we are discussing (P802.3z clause 39) is intended for use inside a wiring closet. It only goes 25 meters. It will be used between pieces of equipment intentionally tied to the same ground (we call out in the specification that this must be the case). Between such pieces of equipment there will be no large circulating ground currents. For longer connections, we provide other links types which do not require grounding at either end (multimode fiber, singlemode fiber, and category-5 unshielded twisted pairs). Direct grounding of the shield at both ends is the correct choice for our application.
Best Regards,
Dr. Howard Johnson
