4.2 Basic Structures for Filter Design
4.2.2 Basic Types of Microstrip Bandpass Filters
Conventional microstrip bandpass filters include stepped-impedance filters, open-stub filters, semi-lumped element filters, end- and parallel-coupled half-wavelength or quarter-wavelength resonator filters, hairpin-line filters, interdigital and combline filters, pseudocombline filters, and stub-line filters. They are widely used in many RF/microwave applications. In this section, the designs of some of these filters will be briefly introduced.
4.2.2.1 End-coupled, Half-wavelength Resonator Filters
An example of end-coupled microstrip bandpass filter is shown in Fig. 19. The length of each open-ended microstrip resonator is approximately half-wavelength,λgo / 2, at the centre frequency foof the bandpass filter. The capacitive coupling between the two resonators is through the gap between the two adjacent open ends. The gap can be represented by inverters. These J-inverters tend to reflect high impedance level to the end of each resonator, which causes the resonators to exhibit a shunt-type resonance [138].
, 1
Zo θ Zo,θ2 Zo,θn
l1 l2 ln
Fig. 19 General configuration of end-coupled microstrip bandpass filter
4.2.2.2 Parallel-coupled, Half-wavelength Resonator Filters
Fig. 20 shows the general structure of parallel-coupled or edge-coupled microstrip bandpass filters using half-wavelength resonators. The adjacent resonators are parallel to each other along half of their length, which gives larger coupling for a given spacing between resonators compared to end-coupled resonators. This structure can be used for designing bandpass filters with wider bandwidth as compared to the end- coupled microstrip filters.
l1 l2 l3
l4
1
ln− ln Z1
Z2
Z3 Z4
1
Zn− Zn
Fig. 20 General structure of parallel (edge)-coupled microstrip bandpass filter
4.2.2.3 Hairpin-line Bandpass Filters
Hairpin-line bandpass filters can be obtained by folding the resonators of parallel-
is the so-called hairpin resonator. When folding the resonators, it is necessary to take into account the reduction of the coupled-line lengths, which reduces the coupling between resonators. If the two arms of each hairpin resonator are close to each other, they function as a pair of coupled line, which will affect the coupling as well.
Fig. 21 Layout of a hairpin-line microstrip bandpass filter
4.2.2.4 Interdigital Bandpass Filters
A type of interdigital bandpass filter is shown in Fig. 22, which consists of an array of n TEM-mode or quasi-TEM-mode transmission line resonators, each of which has an electrical length of 90° at the centre frequency and is short-circuited at one end and open-circuited at the other end with alternative orientation. Generally the physical dimensions of the resonators can be different in lengths and widths. Coupling is achieved through the fields fringing between adjacent resonators. This kind of filter requires the grounding microstrip resonators, which can be accomplised using via holes. Since the resonators are quarter-wavelength long, the second passband of the filter is centered at about three times the centre frequencyfo.
Fig. 22 General configuration of interdigital bandpass filter
4.2.2.5 Stepped Impedance Resonator (SIR) Filters
Filters with uniform impedance resonators (UIRs) [91] suffer from poor harmonic suppression. To alleviate the problem, the stepped impedance resonator (SIR) was developed to solve the problem. Stepped impedance resonators (SIR) are composed of transmission lines with different characteristic impedances. They provide an effective way to minimize circuit space and push spurious resonant frequencies away from the passband [62]. SIRs also provide a wide degree of freedom in structure and design and a wide range of applicable frequency through the use of various types of transmission lines (coaxial, stripline, microstrip, coplanar). The resonant frequencies of SIRs can be easily altered by tuning its geometric parameters.
4.2.2.6 Dual Mode Resonator Filters
Dual-mode resonators [90]-[92] have been widely used to realize many RF/microwave fitlers. A main feature and advantage of this type of resonator lies in the fact that each
number of resoantors required for a n-degree filter is reduced by half, resulting in a compact filter configuration.
For example, the field distributions of the two modes of a microstrip square patch resonator are orthogonal to each other. In order to couple them, some perturbantion to the symmetry of the caviry is needed, and the two coupled degenerate modes function as two coupled resonators. A microstrip dual-mode resonator is not necessarily square in shape, but usually has a two-dimentional symmetry. A small perturbation can be applied to each dual mode resonator at a location that is assumed at a offset from its two orthogonal modes. For example, a small notch or a small cut is used to disturb the disk and square patch resonators, while a small patch is added to the ring, square loop, and meander loop resonantors, respectively.
45o
4.2.2.7 Other Types of Bandpass Filters
There are some other types of bandpass filters, such as combline filters and pseudocombline filters, which comprises of an array of coupled resonators. Stub bandpass filters include λg0/ 4 short-circuited stubs or λg0/ 4 open-circuited stubs.
A lot of research has been conducted on designing compact, low-loss and good performance bandpass filters. In the next two sections, we will introduce several novel compact bandpass filters for microstrip structures and coplanar waveguide structures respectively.