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3.3: Planar Transmission Line Structures

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    Planar transmission lines are the most common transmission lines for high-speed digital, RF, and microwave circuits. Two planar transmission line structures are shown in Figure 3.2.1. The reason these are so popular is that they can be mass produced. For the microstrip line in Figure 3.2.1(a) the fabrication process begins with a dielectric sheet with solid metal layers on the top and bottom. One of these is covered with a photosensitive material, called a photoresist, exposed to a prepared pattern that defines the interconnect line network, then the photoresist is developed and the unexposed (or exposed, depending on whether the photoresist is positive or negative) metal on one side is etched away. The stripline in Figure 3.2.1(b) is fabricated similarly to microstrip but followed by one more step in which a dielectric sheet with a ground plane only is bonded on top.

    There are two major categories of planar transmission lines that can be sorted according to the uniformity of the medium surrounding the transmission line conductors. When the embedding medium is uniform the transmission line structures are referred to as homogeneous. If there are two or more regions with different permittivity the transmission line is called inhomogeneous. The most important planar transmission line structures are shown in Figure \(\PageIndex{1}\) longitudinal direction). Transmission lines where the longitudinal fields are


    Figure \(\PageIndex{1}\): Cross sections of several homogeneous and inhomogeneous planar transmission line structures.

    almost insignificant are referred to as supporting a TEM mode, and they are called TEM lines.

    The most important inhomogeneous lines are shown in Figure \(\PageIndex{1}\)(a–c). The main difference between the two sets of configurations (homogeneous and inhomogeneous) is the frequency-dependent variation of the EM field distributions with inhomogeneous lines. With inhomogeneous lines, the EM fields are not confined entirely to the transverse plane even if the conductors are perfect. However, they are largely confined to the transverse plane and so these lines are called quasi-TEM lines. The inhomogeneous lines are simpler to make. Each transmission line structure comprises a combination of metal (shown as dense black) and dielectric (indicated by the shaded region and having permittivity \(\varepsilon\)). It is common not to separately designate the permeability, \(\mu\)µ, of the materials because, except for magnetic materials, \(\mu=\mu_{0}\). The region with permittivity \(\varepsilon_{0}\) is air. In most cases the dielectric principally supports the metal pattern, acting as a substrate, and clearly influences the wave propagation. The actual choice of structure depends on several factors, including operating frequency and the type of substrate and metallization system available.

    3.3.1 Microstrip


    Microwave circuits on compound semiconductor substrates (e.g. GaAs) are called monolithic microwave integrated circuits (MMICs). Microwave circuits on silicon (Si) semiconductor substrates are called radio frequency integrated circuits (RFICs) Microwave integrated circuits (MICs) are also called hybrid MICs.

    Microstrip (Figure \(\PageIndex{1}\)(a)) is the simplest structure to fabricate beginning with a thin dielectric substrate with metal on both sides. One metal sheet is kept as the electrical ground plane while the other is patterned using photolithography. The metal is chemically etched to form a microstrip transmission network. Although microstrip has a very simple geometric structure, the EM fields involved are complex and cannot be determined analytically. However, simple approaches to the field calculations combined with frequency-dependent expressions yield quite accurate designs. Microwave integrated circuits (MICs) using microstrip can be designed for frequencies up to several tens of gigahertz. At higher frequencies, particularly into the millimeter wavelength ranges (above \(30\text{ GHz}\)), losses (including radiation) increase significantly, the transmission line characteristics vary greatly with frequency (called frequency dispersion), the field directions cannot be confined to the transverse plane, and fabrication tolerances become exceedingly difficult to meet as the required substrate thickness becomes very thin. With monolithic ICs, fabrication tolerances are much finer than with hybrid MICs and the options available for both microstrip and other transmission structures are extended.

    3.3.2 Coplanar Waveguide

    Coplanar waveguide (CPW) (Figure \(\PageIndex{1}\)(b)) [1] supports a quasi-TEM mode of propagation with the active metallization and the ground planes on the same side of the substrate. Each “side-plane” conductor is grounded and the center strip carries the signal, thus much less field enters the substrate when compared with microstrip. In conventional CPW the ground planes extend indefinitely, but in finite ground CPW (FGCPW), the extent of the grounds is limited. It is important to connect the ground strips every tenth of a wavelength or so. This is done using wire bonds, via structures, or in integrated circuit form using air bridges. CPW does a good job of suppressing radiation, it has low frequency dispersion, and is preferred to microstrip for large spatially distributed circuits at frequencies above \(20\text{ GHz}\) or so. It does have drawbacks, including the increased area required (compared with microstrip) and the need to use ground straps.

    3.3.3 Coplanar Strip and Differential Line

    This simple transmission structure (Figure \(\PageIndex{1}\)(c)) is formed by two conductors in the same plane. As with the embedded differential line, the possible existence of ground planes is incidental and ideally these should not influence the field pattern. In one realization, one of the conductors is grounded, and this form is called coplanar stripline or coplanar strip (CPS) [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. In this configuration, CPS is used as an area-efficient variation of CPW. When neither of the conductors is grounded and the line is driven differentially, the interconnect is called a differential line. A differential line is used extensively with RFICs and in critical nets in high-speed digital ICs. The two forms have essentially identical electrical characteristics, with differences resulting from interaction with other metallic structures such as ground planes.

    Silicon-based RFICs generally use differential signaling for analog signals to overcome the problem of field coupling in high-density circuits and problems due to the finite conductivity of the silicon substrate that results in high levels of circuit noise in the substrate. The currents on each of the differential signal paths balance each other and thus each provides the signal return path for the other. This design practice effectively eliminates RF currents that would occur on ground conductors.

    3.3.4 Stripline

    Stripline (Figure \(\PageIndex{1}\)(d)) is a symmetrical structure somewhat like a coaxial line completely flattened out so that the center conductor is a rectangular metal strip and the outer grounded metal is an extended rectangular box. The entire structure is \(100\%\) filled with dielectric, and therefore transmission is TEM and completely dependent upon the relative permittivity, \(\varepsilon_{r}\). Therefore the wavelength is simply the free-space value divided by the square root of \(\varepsilon_{r}\). Stripline is fabricated similarly to microstrip, but now a substrate with a ground plane backing is placed on top.


    Figure \(\PageIndex{2}\): A microstrip line modeled as a zero-thickness microstrip line. Also shown is a more accurate alternative simplified structure.

    3.3.5 Embedded Differential Line

    This simple transmission structure (Figure \(\PageIndex{1}\)(e)) is formed by having just two conductors embedded in a substrate with no specific ground plane. In this structure the possible existence of ground planes is incidental, and ideally these should not influence the field pattern. Essentially the substrate acts as a mechanical supporting element and a quasi-TEM mode forms the main propagating field distribution.

    This page titled 3.3: Planar Transmission Line Structures is shared under a CC BY-NC license and was authored, remixed, and/or curated by Michael Steer.

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