Literature survey

A microstrip line, as shown in Fig. 1-1, supports a quasi-TEM mode of propagation and is widely applied to design microwave circuits, such as filters, couplers, and etc. The mi- crostrip parallel-coupled filter proposed by Cohn [1] in 1958 has been extensively used in the microwave area because of its planar structure, insensitivity to fabrication tolerances, and well-known synthesis method. However, there are two drawbacks limiting the application of this type of filter. One is that the whole length of the filter is too long as the order of the filter becomes high. The other is that due to the unequal even- and odd-mode phase velocities, it suffers from the existence of the spurious response at 2f₀ (i.e., twice the center frequency), which may cause a poor attenuation level in the stopband [2].

SIR filters have been proposed to solve the drawbacks mentioned above [3]-[17]. They can be categorized into three major types, namely: 1) quarter-wavelength; 2) half-wavelength;

and 3) one-wavelength SIR filters. The resonant frequency of the SIR is primarily controlled by the impedance ratio of the line sections. For the same substrate, the characteristic impe- dance of the conventional microstrip line is only controlled by the width of the conductor.

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(a) (b)

Fig. 1-2. Cross-sectional views of the symmetric coupled microstrip lines. (a) Conventional.

(b) Modified with the ground-plane aperture.

Due to the restriction of the fabrication process, manufacturing tolerances may influence the performance of the filter and cause a shift of the center frequency. These effects are much more obvious on the SIR than on the uniform-impedance resonator (UIR). This is because for a constant amount of etching error on the conventional microstrip line, the percentage width variation (i.e., etching error divided by the normal width in percent) in the high-impedance section is much larger than that in the low-impedance section. Thus, the variation of the characteristic impedance of the high-impedance line is different from that of the low- impedance line, and this may cause the impedance ratio of the SIR to change largely. Thereby, the conventional microstrip SIR is very sensitive to fabrication tolerances. However, until now, there is still no works related to the sensitivity of microstrip SIR filters.

Coupled microstrip lines are extensively used to design directional couplers and edge- coupled filters. Tightly coupled directional couplers (especially a 3-dB coupler) and wideband filters are essential components in modern wireless communication systems. For the tight coupler and wideband filter design, strongly coupled microstrip lines [see Fig. 1-2(a)] are required. Nonetheless, the minimum line width and gap spacing in the conventional PCB process are only approximately 0.15 mm. On the other hand, most of the popularly used PCBs have low dielectric constants. It is inherently difficult to implement tightly coupled lines with low-dielectric-constant substrates.

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Various approaches have been proposed to design a single-section 3-dB coupler. The most famous one is the Lange coupler [18]-[20], which is used extensively in the monolithic microwave integrated circuit (MMIC). However, the line width and gap spacing for either a four- or six-line 3-dB Lange coupler will be far below the fabrication limitation of the PCB process if a common substrate (for example, a RO4003 substrate with a dielectric constant of 3.58 and a thickness of 0.508 mm) is used. The vertically installed planar (VIP) structure [21], [22] could fit the PCB process, but it needs some special tools to solder the vertical substrate.

The multilayer structure [23]-[27] requires multilayer substrates. A floating conductor along with a small dielectric layer [28]-[30] can be placed above the signal strips or between the signal strips and the microstrip ground plane. All the structures in [23]-[30] lead to higher fabrication costs compared to a single-layer substrate structure. A 3-dB coupler can also be fabricated on a single-layer substrate with broadside-coupled structures employing coplanar waveguides (CPWs) [31], [32]. Nevertheless, these structures may have the input and output ports on different sides of the substrate and might be difficult to apply to wideband filter design. Since there must be ground plane metals on two sides of the top layer, filter topologies such as interdigital, combline, or hairpin filters are not suitable.

Since broadband communication systems (e.g., ultra-wideband (UWB) system) are highly developed, multisection 3-dB directional couplers are necessary to increase the band- width. It is much more difficult to implement a multisection 3-dB directional coupler on the PCB since a very high even-mode impedance and a very low odd-mode impedance are required for the extremely tight-coupling inner sections. Although the Lange coupler [33] and the tandem coupler [34] have been used to design the tight-coupling section of a three-section 3-dB directional coupler, they require wire crossovers and may be unable to achieve very tight coupling on the low-dielectric-constant substrate. Thus, they are not appropriate to realize the tight-coupling section of a multisection directional coupler. The broadside-coupled and slot-coupled approaches can be applied to construct very tight-coupling structures [35]-[40],

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but they require multilayer substrates. The VIP structure has been used in the tight-coupling sections to design a five-section directional coupler with a wide bandwidth of 160% [41]. As mentioned above, it is complicated from the manufacturing point of view.

To design a microstrip, wideband, coupled-resonator BPF on the PCB, there are many published works. The ground-plane aperture technique and the defected ground structure (DGS), as shown in Fig. 1-2(b), are commonly used to enhance the coupling [42]-[44]. In Fig.

1-2(b), the aperture width WR varies according to the required coupling strength. Dual-plane and broadside-coupled structures [45]-[48] enable the stronger coupling, and filters with these structures inherently exhibit wideband characteristics. Other techniques, such as multilayer structures [30], [49], three-line microstrips [50], multimode resonators [51], [52], the cascade of lowpass and highpass filters [53], and the new coupling scheme in [54] are used to design wideband BPFs. However, the above-mentioned filters may be large, require multilayer technology, or have a narrow upper stopband. To summarize, it is more appropriate to design new coupling structures suitable for tight couplers, wideband filters, and other circuits that require strong coupling.