Designing with fast processors, microcontrollers with enormous computing power, and network devices using the newest wireless communication standards is a significant undertaking. Still, sound system design requires the use of both active and passive components.
You may ask yourself why passive components are essential in modern designs. As most of us take wireless communication for granted, radio waves with frequencies in the gigahertz range are ubiquitous. Moreover, our demand for enormous computing power in ever-shrinking space leads to embedded high clocking frequencies in our everyday devices. In the end, we have radio frequency (RF) radiation in our pockets and everywhere around us. Considering the sensitivity of electronic circuits to RF noise, interference should be quite common, leading to malfunction.
So why do modern electronics work at all? The answer: Modern updated passives help shield sensitive electronics and mitigate the undesirable effects of Electromagnetic Interference (EMI), also called Radio-frequency Interference (RFI), to acceptable levels. Passive components have evolved with improvements in manufacturing technologies and a better understanding of the underlying physics to support modern electronics’ demands.. In conjunction with various electromagnetic devices, passives allow the high-powered (active) processors, memory, and transmitters to dwell in their glory.
Strangely, it is common practice for many engineers to select passive devices as an afterthought. They just pick them from a list of standard components. This practice does not suffice in the demanding world of high-frequency amplifiers, data converters, or other challenging circuits. It is crucial to select the requisite passive components to obtain a specified level of performance. Here, we will detail how to select the right passive components for your design.
Role of Passive Components
Passive devices, such as resistors, inductors, capacitors, ferrite beads, and transformers, neither generate energy nor require power to operate. By definition, they don’t amplify electrical signals and can’t control circuits. Those components can attenuate or control signals, cause a phase shift, or produce feedback.
At the heart of any modern electronic system is a printed circuit board (PCB) that carries connectors as well as passive and active components. Devices are connected via conductive paths embedded into the board material ( non-conductive stuff such as resin) or placed on the surfaces. Here are a few basic PCB design rules that minimize noise and their adverse effects:
- Signal traces should be as thin as possible. To reduce capacitive coupling, a common source for noise, paths are supposed to be less than 8mm thick.
- The separation between adjacent traces should be greater than the width of the traces; otherwise, significant cross-talk can occur between them.
- Sharp bends, such as 90-degree turns, cause interference and should be avoided.
- It should be avoided to run any traces underneath oscillators.
- As the high-frequency digital noise produced by digital circuitry can induce errors on all kinds of circuits, digital and analog components should be sufficiently separated.
Most common PCB design tools will point out violations of those rules and offer alternatives.
Selecting Passive Components
Arguably the most common passive component is the resistor. It is used for impedance matching and biasing, be it wire-wound, carbon-composite based, or a film resistor. At high frequencies, wire-wound resistors—basically coils of wire—become inductive. Even though film resistors consist of thin metallic film loops that also become inductive at high frequencies, they can still be used in some high-frequency circuits. As the end caps of a resistor are parallel to each other, they also generate capacitance. High-ohm resistors can have a capacitance that appears to exist in parallel with its resistance. At high frequencies, a high-value resistor can have lower impedance.
Capacitors themselves electrostatically store energy as a charge across two or more conducting plates separated by a dielectric. An electronics designer uses capacitors for filtering and to decouple at supply and signal lines. However, at high frequencies, they also tend to behave strangely. Parasitic inductance and resonance can occur with electrolytic and film capacitors, and thus harm RF performance. They can actually cause an equivalent series resistance (ESR) when the parasitic resistance combines with the capacitor plates’ resistance. To accomplish ripple and noise rejection, decoupling capacitors must have a low ESR. Ceramic capacitors with their smaller plate size have less self-inductance. Besides, they provide stability in the high-frequency ranges and offer a suitable solution for decoupling integrated circuits. A good example of these features is the High-Frequency MLCC series WCAP-CSRF by Würth Elektronik that features a 0.20pf to 33pF capacitance range and a 25VDC to 50VDC voltage rating. Because high-temperature grade aluminum/tantalum capacitors, such as the H-Chip Aluminum Polymer Capacitors by Würth Elektronik, have stable temperature and bias characteristics, the devices work well for decoupling supply lines.
A different type of energy storage device is the inductor, basically a wire coil. An ideal inductor stores indefinitely and doesn’t lose heat; the device is called lossless. In reality, inductors have non-ideal characteristics. Each wire has a specific resistance, and as the turns of wire touch, parasitic resonant capacitances form and limit the upper frequency. A thermally very stable kind of inductor with practically no variation in inductance over its temperature range from -40°C to +120°C is the series of multilayer inductors WE-MK by Würth Elektronik.
The right combination of active and passive components is the key to good system design. Using capacitors connected in parallel from power supply pins to ground minimizes noise. Different capacitor values set in parallel allow for a low AC impedance across a wide frequency range. Because at lower frequencies larger value capacitors present a low-impedance path to ground, low impedance over the frequency range can be obtained by using other values. With proper consideration given to selecting passive and active components, it is possible to eliminate any unnecessary parasitics in a high-frequency design.
About the Author
As Technical Content Specialist, Marcel is the internal contact person for technical questions in Mouser’s EMEA marketing team. Originally a physicist, he used to work as editor for special-interest magazines in electronics. In real life, he’s juggling two kids with too many chromosomes, a penchant for electronic gadgets and a fondness of books and beer. Until now, none has dropped.
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Source: Mouser Electronics