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Lowpass filters (LPFs) are invaluable in communications and other electronic systems for their capabilities to screen out higher-order interference and noise. To achieve a 1.8-GHz microstrip LPF in a relatively small size, a unit was designed with quasi-Yagi defected ground structure (DGS) and compensated capacitors. It achieved a sharp cutoff frequency response with low passband insertion loss and wide rejection band, with about 20-dB attenuation from 2.8 to 11.0 GHz. With the aid of Microwave Office circuit simulation software from AWR Corp., an equivalent-circuit model was derived and circuit parameters were extracted using a simple circuit-analysis method. The rejection band can be adjusted by controlling the dimensions of the Yagi structure. A prototype of the filter fabricated on commercial printed-circuit-board (PCB) material measured just 0.45λg x 0.35λg, with 0.45λg = 44 mm.
Circuit designers have been discovering the usefulness of defected microstrip structures (DMSs) and defected ground structures (DGSs) in planar topologies, along with the added benefits of their relatively inexpensive fabricating using photolithographic techniques or PCB technologies.1,2 Periodic or nonperiodic DGSs can achieve good rejection at high frequencies, making it possible to reject undesired spurious and harmonic responses in microwave circuits.2,3
A DGS with a simple geometry (such as a rectangular shape) or a more complicated shape can improve the rejection band of a LPF.1,2 Typically, DGS units are used to design and improve filters,1 patch antennas,3,4 branch line couplers, power dividers, and power amplifiers.5 One technique for realizing DGSs is to etch a few defect patterns in the backside metallic ground plane of a PCB under a microstrip transmission line. The defect patterns disturb the shield current distribution in the ground plane, modifying the transmission-line characteristics of the microstrip (an increase in the effective inductance and capacitance) and achieving a slow-wave effect with bandstop properties.
Based on previous work using rectangular DGSs,6-17 a rectangular DGS with arms along the slot channel—essentially a quasi-Yagi DGS—was used to control the coupling between the cascaded resonators to improve the rejection-band response, reduce passband loss, increase the sharpness of the transition domain, and thus minimize the size of the filter structure. Four tuning stubs were added to the lowpass filter to create a wide rejection band and low passband insertion loss. Measurements performed on a fabricated filter show close agreement with simulations.
Figure 1 shows a three-dimensional (3D) view of the quasi-Yagi DGS unit, which consists of two rectangular heads, connected through horizontal and vertical thin slot-channels (arms) in the ground plane. Figure 2 shows a lossless equivalent-circuit model of the quasi-DGS unit, based on a Chebyshev π-network, where Cs represents the sum of the capacitances in the ground plane and Ls is the parallel inductance to Cs. The parallel capacitance, Cp, between the microstrip feed and the metallic plane is formed by the influence of the fringing field around the DGS unit. Resistance Rs accounts for radiation loss as well as loss from the dielectric substrate in the PCB material.