k 1 Introduction to Design Using Microstrip and Planar Lines 1.1 Introduction The microstrip line is the most widely used interconnect at radio frequency (RF) and microwave frequencies. The microstrip line, shown in Figure 1.1, is the main member of a broad class of transmission lines that are built using printed circuit board technology. Here the microstrip line is typically created starting with a low loss dielectric slab or substrate that has a metal sheet bonded to both sides. The strip of the line is formed by patterning the top conductor and etching away the unwanted metal. Sometimes the metal pattern forming k the strips is patterned by silk screening or by growing metal in the appropriate place. At RF, k microwave and high-speed digital frequencies it is necessary to provide a return current path as well as the signal current path defined by the strip. At low frequencies, below a few tensof megahertz, it is also necessary to provide a signal return path but then it is less critical that the cross-sectional geometries be precisely established. The cross-sectional geometry, the width, , of the strip and the thickness, h, of the substrate define the ratio of the voltage and current signals traveling along the microstrip line. This ratio is called the characteristic impedance of the line and it is critical for reliable signal transmission, that is, good signal integrity, that the cross-sectional geometry be the same along the line as then the characteristic impedance of the line is constant. When microwave engineers refer to microstrip design they are referring to the design of RF and microwave circuits using the major types of planar transmission line technologies. While simple in concept, it is a technology that needed to be invented. As well as conceptualizing a transmission line that can be realized by etching a planar metallic conductor on the printed circuit board, it is essentialCOPYRIGHTED to provide the analytic tools MATERIAL that enable the propagation charac- teristics of the line to be calculated and enable structures such as couplers and filters to be synthesized using planar transmission lines. Foundations for Microstrip Circuit Design, Fourth Edition. Terry C. Edwards and Michael B. Steer. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd. k k 2 Foundations for Microstrip Circuit Design Strip t ε h w r Figure 1.1 Microstrip transmission line. 1.2 Origins of Microstrip The origins of microstrip trace back to the development, by Rumsey and Jamieson during the early 1940s, of a coaxial line with a flat center conductor forming a rectangular coaxial line [1]. At this time there were also concepts developed for a conductor between two metal slabs, most notably Hewlett Packard Company’s slabline tuner. At the same time printed circuit boards were employed for low-frequency circuits. These came together in an understanding in 1949 by Barrett that the thick center conductor of the rectangular coaxial transmission line could be very thin with little effect on the properties of the line. This then meant that low-frequency printed circuit board techniques could be employed in microwave circuits and the transmission line system became known as stripline [1, 2]. The conceptual evolution of stripline is shown in Figure 1.2. The stripline configuration is developed by sandwiching a metallic strip between two metal-clad dielectric sheets. As initially envisioned, the strip could be stamped out or k silk-screened using silver ink. Today it is most common to begin with a continuous metallic k sheet bonded to one or both sides of a dielectric sheet. A pattern of an etch resistant material is then photolithographically defined on the sheet and the strip pattern appears after etching. (a) (b) Ground plane Strip Ground plane (d) (c) Figure 1.2 Evolution of the stripline transmission line: (a) coaxial line with a round center conductor; (b) square coaxial line with a square center conductor; (c) rectangular coaxial line with a flat center conductor; and (d) stripline. k k Introduction to Design Using Microstrip and Planar Lines 3 While stripline has tremendous manufacturing advantages compared to the coaxial line, it is difficult to attach lumped components to it (it is after all buried) and to make circuit adjust- ments such as mechanically adjusting the patterned circuit. The next advance came with the development of microstrip by Grieg and Engelmann in 1952 as they removed one of stripline’s ground planes [3]. This microstrip line has become the most important microwave transmis- sion line and the basis for all printed microwave circuits, microwave monolithically integrated circuits (MMICs), and radio frequency integrated circuits (RFICs). Stripline, which can be extended to having multiple levels of strip, is mostly used where the interconnect density must be high, such as with integrated circuit packages and with high speed digital circuits. Fundamentally the most important aspect of microwave circuits is the provision of a signal current path and a signal return current path on the ground planes in the case of stripline and microstrip. However at tens of megahertz and above the proper electrical design requires a precise ratio of the voltage and current waves on the lines. To design for this characteristic it is necessary to have analytic design formulas. Thus conceptualization of the planar trans- mission line structures is not sufficient, it is necessary to develop analytic formulas and design curves for the electrical properties of stripline and microstrip. It is not sufficient to rely on elec- tromagnetic (EM) simulation as there is little physical insight provided. Many of the effects that are important and derive from such parameters as strip thickness and roughness cannot be modeled. Indeed many of the effects that impact the performance of microwave planar cir- cuits, for example multimoding, cannot be predicted by EM simulation alone as the undesired effects usually will not show up when perfectly symmetrical geometries and uniform materials are considered. The essential electrical properties of a transmission line are its characteristic impedance, the ratio of the traveling voltage and current waves on the line, and its propagation k k coefficient which relates to the speed of propagation of the voltage and current waves onthe lines. Knowing these is essential in using the lines in RF and microwave circuits. Among the first useful formulas for the electrical characteristics of stripline were those develop byCohn in 1954 [4], and by Pease and Mingins in 1955 [5]. Grieg and Engelmann were the first to develop the formulas for the characteristic impedance of microstrip [3]. This situation is typi- cal of all microwave and RF design developments: it is not enough to conceptualize a structure, it is necessary to develop the design formulas that enable the structure to be used in design. Planar transmission line technology has developed considerably over the decades with con- siderable functionality, such as filtering, derived using patterned planar lines. Overwhelmingly the preference is to realize these functions and interconnections using planar technology and, if space and extremes of frequency operation are not of concern, preferably with microstrip that can be easily adjusted by the user. Once a microstrip design has been optimized, it can be faithfully and cheaply reproduced using photographically defined geometries. The value of printed microwave circuits was immediately recognized when stripline and microstrip were introduced. This resulted in significant investments in the development of ever more accurate design formulas as well as the development of circuit structures that enabled such functions as filtering and coupling to be realized in printed circuit technology. Thisbook collects the most important developments and the ones that have emerged as providing the best design insight. Many other types of planar lines have been developed that have proper- ties superior to those of microstrip and stripline in particular applications. All of the planar technologies that are currently used are considered in this book. k k 4 Foundations for Microstrip Circuit Design 1.3 RF and Microwave Modules The great majority of RF and microwave design engineers are employed in realizing microwave systems using RF and microwave modules. This includes the design of modules using smaller modules such as monolithically integrated circuits (ICs). Economics necessitate that RF integrated circuits be developed for multiple applications, and in RF and microwave module design ICs as well as discrete semiconductor devices are interconnected using planar transmission lines in printed microwave circuit technology. In this section a microwave module implementing a 15 GHz microwave receiver is considered with the aim of illustrating RF and microwave module design. This book considers the technology required to realize such printed microwave circuits. Most RF and microwave systems convert information at one frequency to information at another frequency that can either be more conveniently processed, in the case of a receiver, or more conveniently radiated, in the case of a transmitter [6, 7]. Figure 1.3 is a 15 GHz receiver module which itself consists of interconnected modules. This unit is used in a point-to-point microwave link (a microwave fixed service) employed mostly in cellular systems tocom- municate between base stations. The subsystem modules such as the amplifiers, frequency multipliers, mixers, circulator, and waveguide adaptor are available as off-the-shelf compo- nents from companies that specialize in developing such modules and selling them to a large user base. Using modules enables high-performance systems to be realized cost effectively. The receiver shown in Figure 1.3 has a local oscillator (LO) section with components A–K, an RF input section comprising a waveguide-to-microstrip adaptor L, a frequency conversion section, and an intermediate frequency (IF) section. The other parts of the printed circuit sur- k rounding the microwave sections provide the supply voltage and control logic.