Coupling high-frequency electromagnetic (EM) energy between transmission lines is a critical step in the design of many RF/microwave components, notably couplers and filters. Coupling of energy occurs when transmission lines are close enough together to pass energy from one line to the other. While coupling can be accomplished with the two most popular microwave transmission-line formats, microstrip and stripline, many circuit designers prefer stripline for its excellent bandwidth and isolation characteristics compared to microstrip. In stripline, broadside and edgewise coupling are two widely used methods. Both techniques have strengths and weaknesses, and both rely on consistent quality from printed-circuit-board (PCB) materials.
Stripline is not a dispersive or radiating transmission-line medium like microstrip. In microstrip, transmission-line circuits are etched on one side of a PCB, with a conductive ground plane on the other side. Stripline is more like a flattened coaxial cable, with transmission-line circuits surrounded top and bottom by dielectric layers which in turn are sandwiched by top and bottom ground planes. Microstrip circuits are easier to fabricate, but stripline circuits can provide extended frequency response with excellent isolation between circuit traces. In stripline, coupling between transmission lines can be critical to the design of both couplers and filters, and it is often achieved by means of broadside or edgewise coupling.
What is the difference between the two coupling approaches? In a stripline broadside coupler, the conductors are one on top of the other, separated by dielectric material. In a stripline edgewise coupler, transmission lines to be coupled are side by side. The cross-sectional drawings compare the two types of couplers.
In both cases, the transmission lines to be coupled are considered to have equal lengths. Coupling is usually strongest along a section of the transmission lines that is one-quarter wavelength at the frequency of interest, and broadband coupling can be achieved by cascading a number of quarter-wavelength sections together. In general, the broadside approach can provide tighter coupling with higher coupling coefficients than the edgewise approach. Couplers are often described by their amount of coupling or coupling coefficient, such as a 3-dB coupler to transfer one-half of a given amount of power. But a 3-dB coupler is difficult to realize in PCB form, especially as an edgewise coupler, although it can be achieved with some broadside coupler designs.
In both stripline broadside and edgewise couplers, the distance between the transmission lines is a critical factor in determining the coupling, whether the transmission lines are positioned on top and bottom or side-by-side. For broadside couplers, alignment of the inner-layer signal conductors can be an issue, requiring a specific manner of building the circuit. An effective approach is the use of copper-clad laminate to define the inner circuit features and prepreg to bond the outer copper ground planes. By defining both of the inner signal layers with a copper-clad laminate, the alignment between circuit planes can be precisely controlled. The use of copper-clad laminates enables tight control of thickness tolerance, allowing the option to use prepreg to make these layers.
A variation in the dielectric constant will have a similar effect on coupling as a variation in the thickness of the material: coupling will vary if the electrical or physical spacing between the coupled transmission lines and the ground planes varies. All circuit materials exhibit some amount of dispersion, which means that the dielectric constant changes with frequency. Since stripline configurations are often used in wideband applications, a circuit material with a low amount of dispersion is beneficial for such designs. In addition, the dielectric constant of a PCB material can vary with temperature: any choice in dielectric material for stripline broadside couplers and edgewise couplers should exhibit good stability of dielectric constant with temperature.
The choice of a dielectric material and its dielectric constant can certainly have an impact when creating couplers. The signal layer in a stripline broadside coupler can be fabricated from a copper-clad laminate using a high-dielectric-constant material. RO3010™ or RT/duroid™ 6010.LM high-dielectric-constant laminates from Rogers Corp., both with dielectric constant of 10.2 in the z-axis at 10 GHz, provide consistent performance for the signal layer. When combined with a prepreg with a different value of dielectric constant, it can add flexibility in a coupler design. However, the coupler’s even- and odd-mode phase velocities will be different for different dielectric constants, which can cause issues with spurious and harmonic signal modes; this may not be an issue for narrowband couplers, but can be troublesome for wideband couplers.
The signal layers in a stripline broadside coupler are surrounded by top and bottom dielectric material, with top and bottom ground planes often added with the aid of a bondply material, such as 2929 prepreg from Rogers Corp., a hydrocarbon-based thin-film adhesive system with a somewhat lower dielectric constant of 2.9 at 10 GHz. Although prepregs and bonding materials will typically have a dielectric constant value considerably less than that of RO3010 or RT/duroid 6010.LM laminates, understanding the circuit models when using these materials can help account for any differences in dielectric constants. The use of well-characterized laminate materials for the signal layer helps to maintain the tight spacing requirements in a stripline broadside coupler, since the thickness of a material such as RO3010 or RT/duroid 6010.LM laminates can be tightly controlled to establish tight tolerances in the spacing between the signal lines themselves and the spacing between the signal lines and the coupler’s two ground planes.
Stripline edgewise couplers are less sensitive than stripline broadside couplers to the thickness tolerances of the circuit materials, but can be affected by differences in dielectric constant values for the prepreg or bonding layers and the laminate layer used for the transmission lines. Even as designers have formed stripline edgewise couplers on circuit materials with lower dielectric constants, such as Rogers RO3006™ laminates, which have a relative dielectric constant of 6.15 in the z-axis at 10 GHz, this is still a higher dielectric-constant value than available prepreg and bonding materials. Careful design approaches and proper use of computer-aided-engineering (CAE) software design tools are required to properly account for differences in the dielectric constants for the materials used in a stripline edgewise coupler.
In some cases, when circuit designers have sought to achieve a consistent value of dielectric constant throughout a stripline edgewise coupler to ensure consistent coupling, they have used laminates as bonding materials. For example, Rogers RO3006 laminate, with a dielectric constant of 6.15 in the z-axis at 10 GHz, might be used for the transmission lines and another layer of the same RO3006 laminate used as the bonding material, by laminating the material above its melt temperature as part of a fusion-bonding process. While this fabrication approach can yield both stripline broadside and edgewise couplers and filters with low dispersion and excellent spurious and harmonic performance, the fusion-bonding process is not commonly used among high-frequency circuit fabricators.
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