All electronic systems generate both wanted and unwanted energy that may result in electromagnetic interference. This excess energy, whether from wanted signals, such as clocks and bus lines that have been improperly decoupled, filtered, or terminated, or from unwanted signals such as cell phone or power supply noise, can cause incorrect output or early system failure.
For today’s engineers, the main challenge is to recognize EMI suppression as a design parameter early in the process to optimize board layout with large power and ground planes. Recent developments in passive components, including a variety of technologies based on zinc oxide (ZnO) and barium titanate (BaTiO3), are adding to the options offered to designers.
ZnO Varistors
ZnO varistors, or voltage-dependent resistors, are protective devices for limiting surge voltages and absorbing energy pulses. They provide ESD and broadband EMI suppression for bandwidths greater than 300 MHz. When connected in parallel with the devices they are protecting, ZnO varistors divert transients and avoid overvoltage conditions.
The varistors are made of several million grain interface boundaries and form a series-parallel network of resistors and capacitors that appear like a multijunction semiconductor. The resistance of the varistors decreases drastically when voltage is increased. Surface-mount varistors offer the fastest turn-on time of any protection device, miniature size, and virtually no wearout characteristics.
ZnO varistors protect equipment subjected to controllable interference, such as switching of inductive or capacitive loads, and those which occur at random, such as high-voltage surges. In its rest state, a varistor has a very high impedance of several megohms, usually much higher than the impedance of the component it is protecting. As a result, it does not change the characteristics of the electric circuit. In fact, multilayer varistors can exhibit leakage currents of about 1 nA.
In the presence of a transient, the impedance of the varistor drops dramatically and shunts the transient around the component. When choosing a varistor for a particular electronic system, the operating voltage, leakage current, maximum clamping voltage, maximum current passing through the varistor, the energy of the pulse to be dissipated through the varistor, and the average power to be dissipated all must be considered.
Surface-mount varistors offer better EMI attenuation compared to back-to-back zener diodes due to the varistor’s available capacitance ranges and its low body inductance. Zener diodes must use a resistor capacitor network to achieve comparable attenuation. Surface-mount varistors also deliver the smallest size and highest reliability in EMI protection available today.
BaTiO3 Capacitors
BaTiO3 feedthrough capacitors provide excellent EMI suppression, broadband I/O filtering, and Vcc power-line conditioning. They achieve broadband EMI reduction in a small, surface-mount package.
Feedthrough capacitors deliver an optimized frequency response across a wide RF spectrum and have a minimized parallel inductance and an optimal series inductance that broadens the frequency response curve. Attenuations of greater than 100 dB may be achieved depending on the complexity and size of the filters involved.
A discrete series element (inductors or resistors) may be used for EMI reduction and noise limiting. It is inexpensive and easy to use, but can only reduce noise -3 dB to -10 dB.
Feedthrough capacitors can reduce costs by eliminating some types of LC filters, increasing the system’s reliability, and saving PCB area. For these reasons, feedthroughs are suitable for digital-to-RF interface filtering, control-line high-frequency decoupling, power-line high-frequency decoupling, and high gain and RF amplifier filtering in computers, power supplies, test equipment, cell phones, and medical instrumentation.
Dual-Resonance Capacitors
A dual-resonance capacitor provides two distinct, self-resonance frequencies for signal attenuation in RF bypassing applications (Figure 1). Using a proprietary electrode configuration and BaTiO3 technology, dual-resonance capacitors offer the functionality of two discrete capacitors in one monolithic block. The device has two distinct resonant points with greater than 20-dB insertion loss at both resonant frequencies.
The resonance points are picked by determining the amount of capacitance needed for resonance at specific frequencies. Each capacitor tolerance can be maintained to within 10% of the nominal value, which ultimately allows the resonance tolerances to be very accurate. With noise being generated in every facet of high-frequency design, the performance characteristics make a dual-resonance capacitor optimal for EMI filtering in dual-mode cellular applications.
Resistor/Capacitor Chips
Recent advances in resistor/capacitor circuit design are the result of the development of a proprietary technology that enables construction of resistor and capacitor combinations in one monolithic block (Figure 2). In signal bus termination, the series RC termination scheme ultimately conserves power and maintains signal integrity. The resistor is chosen to match the characteristic impedance of the transmission line. The capacitor blocks the DC average current and provides a time delay.
Essentially, the resistor eliminates signal reflections or noise on the transmission line while the capacitor minimizes DC power consumption. The results are reduced EMI emissions and power consumption for all laptop and hand-held digital systems.
About the Authors
Ron Demcko is the application engineering manager at AVX Raleigh Products. In more than 17 years in the industry, he also has served as a product engineering manager at Corning Electronics and as a product engineer at Corning Military Capacitors. AVX Raleigh, 3900 Electronics Dr., Raleigh, NC 27604, (919) 878-6224.
Ben Smith is an applications/research engineer at AVX. He has been at AVX for more than 2 year and involved in the RF/microwave field for 4 years. Mr. Smith has a B.S.E.E. from the Georgia Institute of Technology. AVX, 801 17th Ave. S., Myrtle Beach, SC 29578, (843) 448-9411.
Copyright 1999 Nelson Publishing Inc.
June 1999