Let’s hear it for the lowly wire. That mysterious device that can be a conductor, a resistor, a capacitor plate, an inductor, and an antenna all packaged in one exciting component. Soft and flexible, thick or thin, narrow or wide, rigid and stiff, a great conductor—or maybe not. All these terms describe a component with really unique properties that is largely ignored by electronics designers.
Most of the time, wire routing creates loops that provide paths for signal currents to move from a source to a load and return back to the source. Loops appear in ICs where they interconnect millions of transistors, on PCBs and backplanes where the trace loops are larger but fewer, and in cables where loop areas can be extremely large.
Sometimes the loop is so ill-defined that we have trouble comprehending that there really is a loop and that it is returning current to a source. This is especially true with logic circuits because they look so single-ended. But, nonetheless, they are loops. And if the circuit isn’t provided with a stable, well-defined return path, the current will return to its source the best way it can.
These illusive return paths may be unacceptable because they can result in increased radiated emissions from the circuit, capacitive and inductive crosstalk coupling between circuits, and the susceptibility of circuits closely coupled to the undefined path. If the loop leaves the confines of the system, the unwanted RF current in the loop represents a conducted emissions problem. If the loop is connected directly to an external source of unwanted RF current, then there is a potential conducted- susceptibility problem. In addition, loops make good antennas (especially in the near field) that work well for both radiated emissions and radiated susceptibility coupling. Handling the radiated coupling to and from the conductor generally is more difficult than handling the conducted coupling problem. This is mostly because conductors behave as low-pass filters. So in this article, the radiated characteristics of the loop will be addressed.
Although there are three principal coupling modes associated with any loop—differential, common, and antenna modes—the models and associated equations presented here apply only for the differential mode. Even so, understanding the radiated emissions and susceptibility characteristics of a differential-mode loop, along with some pedagogical extensions, will provide significant insight into the design practices used to meet radiated EMC requirements.
Understanding loop radiation coupling is important if the system must meet European or military EMC specifications that control both the radiated emissions and susceptibility (immunity) of a system. Both of these effects are created by time-variant electromagnetic fields propagated through free space.
The circuit radiated emissions and susceptibility characteristics are interrelated, requiring a coordinated systems-design program to cost-effectively meet both requirements. By studying the emissions and susceptibility characteristics of a loop, we can formulate a design strategy for meeting the radiated EMC requirements. Since the loop behavior is different for emissions and susceptibility, they will be discussed separately.
Radiated Emissions Coupling Loops
120 p 2 I A sin (q)
e (q) = ———————–
l 2 r
By orienting the loop for maximum emissions (something done during the EMC measurements by rotating the unit, raising/lowering the antenna, and changing the antenna polarization angle), sin (q) = 1; converting to frequency instead of wavelength, F (MHz) · l(m) = 300 m/µs; and introducing the attenuation provided by shielding, S, the equation can be rewritten as:
1.32 A I F2
e(max) = —————
where: e(max) = µV/meter
A = radiating loop area (sq cm), 0.1<(l/w)<10
I = drive current (amps)
F = emissions frequency (MHz)
r = measurement distance (meters)
S = shielding effectiveness ratio
Figure 1 is a graphical illustration of these parameters. A discussion of the circuit characteristics related to this equation reveals that:
The measurement distance (r) is dictated by the EMC specification, with commercial measurement distances ranging from 3 to 30 meters.
The radiated emissions levels (e) created by active loops are proportional to the square of the frequencies and linear for all other parameters. Since the levels are increasing as the square of the frequency, it is very important to operate the circuit only as fast as necessary to process the data. As clocks have increased in frequency from 1 to 1,000 MHz, the emissions levels have increased by 120 dB = 20 log (1,000/1)2.
The drive current (I) and switching frequency (F) are uniquely determined by logic family selection. Generally, nine to 11 harmonics are needed to adequately define the rise time for the signal.
Above the 9th or 11th harmonic frequency, low-pass filters can be used to reduce circuit bandwidth, providing the waveform is not affected and compensation is made for the added propagation delay. Since the propagation time is Tp = Ö LoCo, the blind application of filtering devices that change the transmission line L or C results in the deterioration of signal integrity.
The loop area (A) and shielding (S) are the only parameters totally under the designer’s control. They also are the only parameters that do not affect the high-speed operation of the PCB. Consequently, the major reductions of emissions come from reducing active loop areas and adding shielding.
Every loop can be both a transmitting and a receiving antenna. So long as the loop is nonresonant, the electric-field radiation from most rectangular conducting loops with an l/w ratio of less than 10, such as PCBs and backplanes, can be accurately calculated using this model.
For example, this model predicts the emissions levels from a 10″ × 10″ loop to within 1%. It even is possible to apply this model to the parasitically coupled radiated emissions from short transmission lines and cables. But unless they are very short, it is a stretch to apply it to the differential mode. Even with this shortcoming, it still provides a superb way to visualize their emissions characteristics.
Radiated Susceptibility Coupling Loops
The same circuit can both radiate and be susceptible to RF energy. For a small nonresonant loop oriented at angle (q), the induced voltage created by an incident plane wave is:2
2 p e A sin (q)
Vi (q) = ——————
By orienting the loop for maximum pickup, sin (q) = 1; converting to frequency instead of wavelength, F(MHz) · l(m) = 300 m/µs; and introducing the contribution provided by both the circuit bandwidth-frequency response, Bpb and shielding, S, the equation can be rewritten as:
0.021 e F A Bpb
Vi (max) = ———————-
where: Vi (max) = induced loop voltage (V)
e = susceptibility field strength (V/m)
F = susceptibility frequency (MHz)
A = circuit capture area (sq meters), 0.1<(l/w)<10
Bpb = passband bandwidth-frequency response, in band Bpb = 1
S = shielding effectiveness ratio
Figure 2 is a graphical illustration of these parameters. A discussion of the circuit characteristics related to this equation reveals that:
The induced RF voltage is directly proportional to all parameters. When the induced RF voltage exceeds the noise threshold of the circuit, susceptibility occurs. This may be from RF overload or audio rectification.
Analog devices have continuous input/output transfer functions and are approximately 100 times (40 dB) more susceptible than digital devices.
Although the narrowband electromagnetic field strength at each frequency could be the result of the environment, generally it is dictated by regulatory agencies. The most severe commercial radiated broadband electromagnetic field strength results from electrostatic discharge.
Outside of the circuit passband, circuit RF attenuation determines narrowband signal response. Within the passband, the narrowband signal response essentially is constant. On the other hand, the broadband signal response is determined by the circuit attenuation and the bandwidth. Changing the bandwidth (BW) changes the coupled signal level by:
dB = k log (BW1/BW0)
where: k = 10 for noncoherent signals
k = 20 for coherent signals
Just as it was true for radiated emissions, it also is true for susceptibility loop coupling; that is, the loop area and shielding are the only parameters for which the designer has complete control. Consequently, major reductions in circuit susceptibility come from reducing the loop areas of the highest bandwidth, most susceptible circuits and adding shielding.
For accurate results, this model also requires that the loop be nonresonant and have an l/w of less than 10. It is best used for PCBs and backplanes.
When used within its limitations, this model is quite accurate. It has the same shortcomings as the emissions model when applied to transmission lines and cables. Still, it provides us with a way to visualize the RF coupling characteristics of a loop.
Three-Step Design Strategy
Examining the emissions and susceptibility characteristics of a loop permits us to formulate a three-step design strategy for meeting the radiated EMC requirements. All three steps are interlinked and will be expanded in future articles.
Layout circuits to minimize high-frequency loop areas. High-speed systems require high frequencies with broader bandwidths. Reduction of the area of the highest frequency circuit loops is the most important consideration. An easy way to accomplish this is to use multilayer PCBs. They reduce circuit-loop areas by as much as 40 to 60 dB.
Minimize circuit bandwidth. Sometimes this is a major point of contention because reducing system speed is not a popular option. However, many circuits are operating at high speeds or using high-speed logic devices when it is not necessary. Since radiated susceptibility increases with frequency and radiated emissions increase as a function of frequency squared, using slower speeds will desensitize the circuit to external susceptibility fields and dramatically reduce emissions.
Shield problem circuits. This noninvasive suppression technique reduces both the radiated emissions and susceptibility of a circuit. Since shielding is not inserted into the circuit, it should not affect high-speed operation. In fact, it is the only suppression technique that does not affect signal integrity.
1. Kraus, J.D., Electromagnetics, Third Edition, p. 665, John Wiley, 1984.
2. Terman, F.E., Electronics and Radio Engineering, Fourth Edition, McGraw Hill, 1955.
About the Author
Ron Brewer is vice president of EMC technical services at Instrument Specialties. He is a NARTE-certified EMC/ESD engineer with more than 25 years in EMC/ESD/Tempest engineering. Mr. Brewer serves on three technical committees and, as an internationally recognized EMC authority, has made more than 185 technical presentations in North America, Europe, Asia, and the Pacific. He also has been named a Distinguished Lecturer by the IEEE EMC Society. Instrument Specialties, P.O. Box 650, Delaware Water Gap, PA 18327, (570) 424-8510.
Copyright 1999 Nelson Publishing Inc.