Impedance matching is an aspect of RF/microwave design that has challenged even the best circuit designers from time to time. High-frequency circuit designers generally aim for a characteristic impedance of 50 Ω, unless they are working on cable-television (CATV) circuits, which typically operate at 75 Ω. The 50-Ω impedance is not by chance, since it represents a state that supports the most efficient transfer of RF/microwave signal power from a source to a load, with the least number of signal reflections. Of course, high-frequency power can be transferred from a source to a load when they are at different impedances, and few sources and loads are precisely at 50 Ω. But mismatched conditions can lead to reflections and standing waves in high-frequency transmission lines, which can blend with desired signals and result in amplitude and phase distortion. For the lowest phase distortion and flat amplitude response, most RF/microwave circuit designers start with ensuring that all of the possible impedance mismatch points, such as transmission-line junctions, connections to components, and terminations with connectors, are as close to 50 Ω as possible.
Circuits in which the characteristic impedance is well matched throughout will suffer minimal reflections and exhibit few standing waves. Standing waves cause the impedance to fluctuate as a function of distance from the load. Circuits with well matched impedances throughout will exhibit a measurably low voltage standing wave ratio (VSWR). A perfectly matched transmission line would have a VSWR of 1.0:1. Under such ideal conditions, full power would be transferred from a source to a load, with no reflected power. In contrast, a severely mismatched line would have a VSWR that tends towards infinity, or a VSWR of ∞:1. For example, a short circuit or an open circuit, in which no power is transferred from a source to a load, both have a VSWR of ∞:1. Realistically, a circuit with VSWR value of about 1.50:1 is considered to have well matched impedance.
Maintaining a 50-Ω impedance throughout an RF/microwave circuit is no simple task. High-frequency circuit designers use numerous transmission-line technologies, including microstrip and stripline, and each has its own set of guidelines for determining characteristic impedance. For microstrip, for example, the impedance of a transmission line is dependent on the width of the signal trace, the thickness of the metal conductor used for the trace, the dielectric constant of the printed-circuit-board (PCB) material, and the height between the signal trace and the reference or ground plane, which is essentially the thickness of the PCB material.
Stripline circuits, which are formed as a sandwich of a signal trace between dielectric layers, also exhibit transmission-line impedance as inversely proportional to the line width and directly proportional to the thickness of the dielectric material layers. One difference between stripline and microstrip is that the height of the signal trace above the ground plane for stripline has less of an impact on impedance than the height of a microstrip signal trace above the ground plane. The air above a microstrip circuit will actually contribute to the characteristic impedance of the circuit, whereas a stripline circuit is contained within a sandwich of dielectric material. Because air has an effective dielectric constant of 1, it will always serve to lower the effective dielectric constant of any PCB material used in a microstrip circuit.
Because of this, the effective dielectric constant of PCB materials used in a stripline circuit will be higher than those same dielectric materials used in a microstrip circuit. Because of the impact of the air above a microstrip circuit—lowering the effective dielectric constant of its PCB material—the dielectric material in a stripline circuit must be planned for differently. To achieve the same controlled impedance in a stripline circuit as in a microstrip circuit, the distance from the circuit trace to the ground plane must be greater in the stripline circuit—essentially requiring thicker dielectric material.
Matching impedances within an RF/microwave circuit can also require careful planning and tight control of circuit parameters, such as trace width. Over time, circuit designers have developed a broad range of creative solutions for maintaining a matched 50-Ω impedance within their circuits even when employing complex circuit junctions and making interconnections to chip components, active and passive packaged components, and various connectors. Impedance is affected by such factors as the thickness of the metal conductor layer used for the transmission lines, the width of the signal trances formed on that layer, the thickness of the dielectric substrate, and the effective dielectric constant of the substrate. For example, a straight microstrip transmission line at a particular trace width may exhibit a 50-Ω impedance, but with an sudden change in direction, such as a 90° bend, the impedance of the transmission line can change. Circuit designers rely on a variety of circuit structures, such as single-stub and double-stub impedance tuners and quarter-wave transformers, to form impedance matches, for example between a 50-Ω transmission line and a high-impedance (300-Ω) source. These impedance matches become more difficult when they must be achieved over broad operating frequency ranges.
Trusted tools for achieving matched impedance include the Smith chart, for visualizing shifts in impedance, and the RF/microwave vector network analyzer, for measuring a circuit’s scattering (S) parameters, which are defined for impedance-matched conditions. And, of course, modern computer-aided-engineering (CAE) software tools can provide excellent assistance in developing impedance-matched RF/microwave circuits. Such software tools allow users to define the essential parameters of a circuit’s PCB material, including dielectric constant, which can have a tremendous impact on achieving a matched 50-Ω impedance.
Although the dielectric constant of a circuit-board material plays such a key role in determining the characteristic impedance of an RF/microwave circuit, it is a numerical value that represents a very complex property of a PCB material. The dielectric constant may vary across the length and width of a circuit board, and it can also vary as a function of temperature or frequency. The dielectric constant of a circuit material can be determined by a number of different methods; the value for a material may depend on the method used.
RF/microwave circuit impedance matching is not a simple topic, and books have been devoted to it. But a good starting point in understanding how to achieve reliable matched impedance in high-frequency circuits is by better understanding the role of the PCB material in determining a circuit’s effective impedance. To help with that understanding, the next Blog in this series will feature Part 2 to this impedance-matching discussion, focusing on two particular PCB materials--RO3010™ and RO3035™ laminates from Rogers Corp. (www.rogerscorp.com). Part 2 will provide a “teardown” of those two materials and how their various properties contribute to achieving successful impedance matching in narrowband and broadband RF/microwave circuits.
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