Ronald W. Brewer
Vice President
EMC Technical Services
Instrument Specialties Co. Inc.
Delaware Water Gap, Pa.
Interconnections not only can be a source of troublesome radiated EMI or RFI, but also may provide a path for electrical noise to enter the system. Cabling is of particular concern because it creates the largest loop area for radiated emissions, as well as the largest area into which environmental interference can couple. This area varies depending upon whether the energy couples into the differential-mode or common-mode loop.
The area of the differential-mode loop equals the distance between the signal and return leads times the length of the cable. The common-mode loop area is the length of the cable times its distance from the common-mode ground reference. The latter is by far the larger and as such represents the biggest vulnerability for both radiated and coupled EMI/RFI. Minimizing cable length helps reduce both common and differential-mode loops.
For frequencies to 250 MHz, twistedpair cables can help guard against noise problems. Twisting the two conductors results in small regions in which the currents are 180° out of phase, thereby canceling each other. There should be at least two twists for every half wavelength of the highest offending frequency, preferably with an even number of twists.
Shielded cable provides more differential- mode attenuation at higher frequencies. But it is important to ground shields properly to prevent them from acting as antennae. This can happen if the cable length is equal to or greater than a quarter of the offending frequency’s wavelength. Grounding the shield at multiple points along its length prevents the antenna affect.
It should be noted that flat-ribbon cable can be configured in numerous ways. Designers should avoid cable containing multiple signal leads and just one ground wire at the edge because the configuration creates a large loop. The preferred layout is one where multiple ground wires alternate with signal leads to minimize loop areas and cross talk. Another acceptable configuration, which lets the designer maximize the number of signal leads in the cable, consists of multiple ground wires with signal leads on each side.
At low frequencies, a small piece of wire, or pigtail, can be used to tie the cable to ground. But its grounding effectiveness begins to deteriorate at the point where the combined distances of the pigtail length, the distance between the pigtail tie-point and exposed cable, and the length of any unshielded cable, is about one-tenth of a wavelength for noise frequencies of interest. The technique stops working entirely as this distance approaches a full wavelength. Critical circuits should use a configuration with an integral ground plane and reference, not a pigtail.
For high-frequency applications, the loop must vanish. The only way to accomplish this is through connectors with shielded backshells. Whether designs use shielded connectors or shielded terminals, the cable shield must make circumferential contact with the exterior of the enclosure at the point of entry.
In cases where cables connect to printed-circuit boards that are tied to ground, electrostatic discharges can travel along the cable into the board and destroy the electronics. ESD can also radiate into the board if the cable to the PCB is grounded on the inside of the enclosure. Cable shields tied to the outside of an enclosure can still create an RF pickup or radiating loop as well. The best solution is to attach the cable shield directly to the outside of an enclosure using a right-angle terminal or 360° shielded connector.
Common mode coupling is even more difficult to deal with because it involves a larger loop area. It is sometimes impractical to reduce this area, but grounding the cable at just one point helps. The location of the ground point is set by whether the problem is emission or susceptibility.
Even with one end of the cable floating, RF current will flow through the capacitive reactance of any parasitic capacitance. Thus the aim of shielding should be to minimize parasitic capacitance, perhaps through PCBmounted boxes or other measures. A particularly effective method of cutting common-mode radiation or coupling is to shield the entire loop area. Unfortunately, this is not practical for large systems.
It is important to note that shielded cable, which reduces differential-mode radiation and coupling, is ineffective against common-mode noise problems. In fact, it can actually make things worse if both ends of the cable shield are grounded. Shielded cable used to reduce differential-mode problems should be grounded at only one point. Particular care is in order when working with coaxial cable, which is typically grounded at both ends. Use of special ungrounded coaxial cable connectors can help prevent conditions that exacerbate common mode coupling.
Noise problems sometimes arise from neighboring circuits that induce circulating currents in ground reference networks. Currents result when there are even slight differences in voltage between two grounded points, as can happen in shielded cables with multiple ground points relatively far apart. The magnitude of currents induced in common-mode loops are a function of ground material impedance. From Ohm’s Law, Vab = IZ, whereVab= potential, V, between grounding points a and b; I = current, A; and Z = impedance magnitude or resistance at dc plus dominant reactances resulting from higher frequencies, Ω. If current flows through a common-mode loop in a grounding network, it is susceptible to modulation from external voltages, thus potentially generating RF noise currents.
Moving points a and b closer together, preferably to a single point, minimizes the phenomenon. Note that a grounding grid is better than a wire, and a plane is better still. Ground reference materials should also have as little impedance as possible — preferably copper or copper-plated metal to improve RF performance.
Other means of defeating differential or common-mode noise problems include low-pass filters, optical couplers, and fiber optics. Filters which attenuate RF noise components are usually most economical in low-frequency circuits. On the other hand, optical couplers provide good isolation over a wide frequency range. But their performance begins to degrade above about 100 MHz because of parasitic capacitance. Fiber optic interconnects don’t have this limitation.