Look Deeper at Shielding

After writing last month's column, "An Update on Shielding," I dug further and found the Audio Engineering Society AES48-2005 standard that covers grounding and electromagnetic compatibility (EMC). The standard stresses the need to properly shield cables to reduce coupling with noise sources. It says in part, "the shield contact and the shell of the equipment connector shall have a dc connection to the shielding enclosure via the lowest impedance path possible." The AES strongly recommends designers make this connection to the outside of the chassis or shielding enclosure. If a design lacks a shielded enclosure, run all grounds to a single ground point via low-impedance connections. The 11-page AES standard provides more useful shielding information and examples of what not to do.

I also contacted Henry W. Ott, an EMC consultant and author of a text on EMC, to ask for his thoughts about cable shielding. He explained a grounded shield protects against electrical fields, and grounding at one end prevents 60- or 50-Hz currents from flowing in the shield and possibly coupling to the shielded signals. According to Ott, this type of single-point shield grounding should occur at the signal source end, because that ground is the reference for the signal. But if the source "floats" and has no ground connection, ground the shield at the instrument end.

Ott notes that a shield, even one grounded at both ends, provides no magnetic-field shielding at low frequencies. To shield signal wires from high-frequency magnetic fields, Ott recommends grounding the shield at both ends. This type of shield grounding at both ends lets a magnetically induced current from the ambient noise source flow in the nonmagnetic (copper, aluminum, etc.) shield and return through a ground path. That current flow cancels the noise magnetically induced on the shielded signal conductors. Thus you want as low a resistance as possible in the ground loop. In fact, the amount of magnetically induced noise on a shielded signal is directly proportional to the shield resistance. Thus, you want a low-resistance shield.

A shielded cable has a shield cutoff frequency (f_c) (usually in the audio frequency band) below which the magnetic-field protection drops off quickly. At 5f_c, the drop is about 2 percent, which means that below about 5f_c the shield protects against electric fields only.

According to Ott, shielded cables longer than 1/20th wavelength and with the shield grounded at only one end can act as efficient antennas. So, most shielded cables used above audio frequencies, as well as many used at or below audio frequencies, should have their shield grounded at both ends. The primary reason for grounding shields at both ends is to provide magnetic field protection. This protection only occurs at frequencies above the audio range.

Look Deeper at Shielding_A