Transmission lines are often taken for granted in the design of high-frequency circuits and integrated circuits (ICs), since the focus is often on getting signals into and from an active device. But by understanding the capabilities of different microwave/RF transmission lines and how to optimize them, design iterations, time, and unnecessary cost can be saved from a project. Part 1 of this two-part article will show how a careful design process should include an analysis of requirements through final documentation, allowing a designer to balance such tradeoffs as electrical performance, size, cost, and reliability.

The major parameters that define an RF/microwave design are cost, volume, weight, electrical performance, power, and reliability. Cost, for example, can be controlled through the use of a low-cost technology process, a low-cost substrate material, a simple assembly, low-cost components, minimal interconnections, an effective electrical ground contact with good heat flow and heat sinking, and multilayer construction. Of course, this is not an exhaustive list of guidelines, and cost will also be influences by limitations imposed by such factors as required receiver sensitivity, size, receiver selectivity, and power consumption.

The size and weight of a high tance. For example, suppose that k₁ is the cost coefficient, k₂ is the volume coefficient, k₃ is the weight coefficient, k₄ is the electrical performance coefficient, k₅ is the supply power coefficient, and k₆ is the reliability coefficient. For calculation and optimization, each parameter should be normalized: C/k₁, S/k₂, W/k₃, E/k₄, P/ k₅, and 1/(Rk₆). Then the normalized parameters should be added by using the summation formula shown in Eq. A in the box at the bottom of p. 106. For an optimized transmission line, the combined parameter, Σ, must be minimized.

Figures 2-7 show common planar transmission lines. All printed transmission lines have strip conductor(s) implemented on a relatively thin substrate. Printed transmission lines can be classified as being uniform or nonuniform; homogeneous or inhomogeneous in their surrounding area; lossless or lossy; shielded or nonshielded; planar, multilayer, or threedimensional; and based on different substrate types, including dielectric, ferrite, or semi-insulating materials.

In a uniform line, the characteristic impedance does not vary with position along the line. A nonuniform printed transmission line exhibits characteristic impedance that varies as a function of the longitudinal coordinate. Usually, this change in impedance is achieved by changing the conductor strip width. Tapered transmission lines can be fabricated with smooth changes in conductor width and characteristic impedance as functions of distance along the line.

For transmission lines with inhomogeneous or mixed dielectric, such as a microstrip line (Fig. 2), the velocity of propagation depends on both the cross-sectional geometry of the line and the dielectric constants of the different dielectric media (air and substrate material). The effective dielectric constant of the line can be justly expected to be greater than the dielectric constant (e) of air (e = 1) and less than that of the dielectric substrate material. For this type of transmission line, the propagation of the electromagnetic (EM) waves does not take place in a purely transverse electromagnetic (TEM) manner. Usually, in a low-loss printed transmission line, the conductor thickness is greater than three to five times the skin depth. Lossy transmission lines with conductor thickness significantly less than the skin layer thickness can be used for distributed planar attenuators or terminations.²

For some regular printed transmission lines and combinations of different transmission lines, a multilayer design is necessary. The main objective of a multilayer RF construction is to significantly increase the density of an RF module. Also, a multilayer design provides the opportunity to combine both RF and digital functions in a single module. This provides benefits of size and weight reduction, enhanced performance, improved reliability, and decreased cost. In addition to normal planar interconnections, the interconnections in a multilayer module include vertical transmission lines or via-hole interconnections. In a threedimensional design, the transmission lines make it possible to send signals along orthogonally placed conductors. For an RF network, a three-dimensional or horizontal-vertical design is attractive.^1,2 A three-dimensional configuration can consist of various combinations of multilayer, horizontalvertical, and flexible structures.

Typical nonshielded transmission line structures (Figs. 2a, 2c, 2d, 5, 6a, and 6c) are open to the air, so it is usually desirable to protect them from environmental influences, as well as to prevent radiation and electromagnetic interference (EMI). Such protection can be realized by the use of shielded printed transmission lines (Figs. 2b, 3, 4, 6a, and 7).

Selection of a transmission line substrate depends on the technology process. Hybrid microwave integrated circuits (HMIC) use printed transmission lines with dielectric or ferrite substrates. In monolithic microwave integrated circuits (MMICs), the transmission lines are fabricated on a Optimum impedance matching between the transmission line and the interface results in better efficiency in the overall RF network. As Table 2 shows, the impedance range of printed transmission lines is somewhat limited. In terms of electrical performance, the most important parameter of a transmission line is insertion loss, and Table 3 compared the insertion-loss performance for different standard printed transmission lines.

Design tradeoffs usually deal with conflicting parameters, such as insertion loss versus transmission line physical dimensions. Decisions for optimization should be made according to the transmission-line integration index2:

where

a_Σ = the total insertion loss of a printed transmission line (in dB) and V = the volume of a transmission line in in.³, V_Σ= L_Σ x W_Σ x H_Σ where L_Σ, _ΣW, and _ΣH = the total (equivalent) length, width, and height of a transmission line, respectively.

The transmission line with minimum integration index, i_min, is optimal, having the smallest physical dimensions with the minimum insertion loss. Keep in mind that integration index varies linearly with frequency.

The total insertion loss for the printed transmission line is

where

a = the insertion loss (in dB) of the quarter-wavelength guide segment;

? = ?/(eeff)0.5 = the guide wavelength;

? = the wavelength in free space; and

eeff = the effective dielectric constant of the printed transmission line.

From Eqs. 1 and 2, the integration index is

Using the normalized coefficient, (4L_Σ)^4/3/?, the normalizedintegration index is

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