The Research Of Microstrip Filter Based On SIR And DGS Technology

In recent years, the mobile communication technology in order to meet the needs of the market has been rapid development, which not only need increase the quantities of the microwave/RF filters, but also raise higher requirements for filter performance, these requirements include:Efficient filtering characteristics, smaller size, lighter weight and lower production cost.In this paper, the mainly effort is the research of miniaturization of microstrip filters and high-performance technology of filter characteristics in today's wireless communication circuits, starting from the traditional microstrip filter's design, through the analysis shows that the traditional uniform impedance resonator (UIR) which form the filter have the shortcomings-large size of the design of microstrip line and spurious frequency response of transmission, and the introduction of the stepped impedance resonator (SIR) which is of a smaller size and have the ability to remove the spurious resonant frequency. Different SIR have been detailed analyzed and compared, while that in order to reduce the size of SIR microstrip filters further, realize the miniaturization, it's need to fold the half-wavelength SIR to form an characteristic of internal coupling effect. It will improve the reliability and reduce the production costs of resonator. A modified hairpin internal coupling SIR is proposed in this paper, the low-pass filter characteristics of the resonator and the corresponding equivalent circuit are given. By analyzing the simulating transmission characteristics figure, it shows that:the advantage of the hairpin internal coupling SIR is with steep cutoff frequency, but the disadvantage of stop-band is not broad enough. In order to overcome the shortage of hairpin internal coupling SIR, DGS (Defected Ground Structure, DGS) is introduced.Furthermore, another effective constructing method can realize miniaturization of the filter is introduced in this article:defected ground structure (DGS), which is proposed in recent years, it make non-periodic or periodic structure which is etched on grounded surface of a planar circuit, and change the effective dielectric constant of the circuit substrate, thereby changing the distribution capacitance and inductance of the microstrip line, and that change the electromagnetic transmission properties in the microstrip line. In this paper, DGS is researched deep and the practicality of the existing DGS'equivalent circuit is discussed, several representative DGS are analyzed and compared; some important DGS equivalent circuit among them are deduced and researched, the performance of the classic dumbbell- shaped and U-shaped DGS are stressing analyzed and researched.The main works are done in this paper as following:1. A new shape of hairpin internal coupling symmetric SIR is proposed, and its lumped equivalent circuit is derived. The structure of this resonator is simple, easy to facture, and with the excellence characteristic of compact dimension, steep cutoff frequency when made a filter, its transmission characteristics equivalent the three-branch elliptic function low-pass filter. The disadvantage is that the stop-band is narrow, it can be changed by altering the structure and size of the microstrip to expand the stop-band properly, but at the expense of the original steep cutoff frequency characteristics.2. A type of double equilateral U-shaped DGS is introduced, this structure has the characteristics of dual finite attenuation poles and can be properly regulated by adjusting the internal and external side length of U-shaped DGS, A controllable attenuation pole low-pass filter can be made by these periodic arrangement of structural units, thereby it can restrain parasitic resonant frequency as much as possible and expand the stop-band of the low-pass filter. A low-pass filter is designed and simulate by microwave simulation software HFSS. Design parameters are given as:the pass-band is 0～2.5GHz, the attenuation is lower than 1.5dB, the stop-band attenuation of 20dB can reach to 11.21 GHz. The size of this proposed filter is 80mm×22mm. (Length×width)3. Based on hairpin internal coupling symmetric SIR, and With the steep cutoff frequency and broad stop-band attenuation characteristic, a compact and with good performance low-pass filter is designed by inducting the SIR and the conventional dumbbell-shaped DGS. Finally, a low-pass filter is fabricated and simulated By ADS and HFSS, with the pass-band is 0～2GHz, pass-band attenuation is less than 0.5dB, it can achieved 20dB attenuation at the lowest frequency of 2.42GHz, the stop-band range from2.42 GHz to 8.04GHz above 20dB attenuation, the maximum attenuation is up to 42.19dB among stop-band. The whole dimension of the filter is 20mm×13.83mm (length×width).