This application claims the benefits of People's Republic of China application Serial No. 201410326631.2, filed Jul. 10, 2014, and Serial No. 201510175623.7, filed Apr. 14, 2015, the subject matters of which are incorporated herein by reference.
Field of the Invention
The invention relates in general to a microstrip line filter, and more particularly to a microstrip line filter having a defected ground structure (DGS).
Description of the Related Art
As technology advances, wireless communication technologies, such as Wi-Fi, ZigBee, and Bluetooth, have been widely used in people's everyday life. A filter is an important component in wireless communication products because the filter is capable of removing unwanted frequency components from signals, which improves signal quality of wireless communication products.
In response to a trend of small and lightweight communication products and the user's request for a higher communication quality, there is a need for reducing the area occupied by the filter, decreasing the manufacturing cost, and providing the filter with better frequency characteristics.
The invention is directed to a microstrip line filter having better frequency characteristics and requiring smaller circuit area.
According to one embodiment of the present invention, a microstrip line filter is disclosed. The microstrip line filter includes a dielectric substrate, a first hairpin resonator and a metal ground layer. The first hairpin resonator is disposed on a first layer of the dielectric substrate. The metal ground layer is disposed on a second layer of the dielectric substrate, and includes a first defected ground structure. The first defected ground structure includes a first defected area, a second defected area and a third defected area. The projection of the first defected area on the first layer is located inside the hairpin structure of the first hairpin resonator. The projection of the second defected area on the first layer is located in a direction opposite to an opening direction of the first hairpin resonator. The third defected area connects the first defected area and the second defected area.
The microstrip line filter provided in this disclosure achieves better frequency characteristics because the microstrip circuit on the printed circuit board is accompanied with a corresponding defected ground structure. Furthermore, because the defected ground structure may be aligned with the microstrip line circuit disposed on the printed circuit board, equivalent inductance can be increased and hence the same filtering function can be accomplished with a smaller circuit area.
The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiment(s). The following description is made with reference to the accompanying drawings.
A number of embodiments and accompanying drawings are disclosed below for elaborating the invention, not for limiting the scope of protection of the invention.
According to a conventional approach, a filter, such as a low-pass filter, a high-pass filter or a band-pass filter, is formed by lumped elements including resistors, capacitors and inductors. For a filter consisting of lumped elements, in order to achieve greater attenuation on the second harmonic and the third harmonic frequency components, more lumped elements are required. However, more lumped elements occupy a larger circuit area and increase manufacturing cost, which limits possible applications of the filter.
According to the embodiments of the invention, the filter may be formed by microstrip line circuits on a printed circuit board (PCB). The performance of the filter thus formed is better than that of the filter formed by lumped elements in particular regarding the high-frequency portion of the frequency response (the lumped elements generate a parasitic effect at high frequencies). Since the filter may be directly formed on a printed circuit board, there is no need to purchase additional elements and the manufacturing cost is effectively reduced.
In order to achieve better frequency characteristics, for example, better attenuation effect on signals at the stop-band, defected structures are created in the ground plane of a printed circuit board to form a specific pattern. For example, a defected ground structure is formed by removing a portion of the metal on the ground plane by an etching process. The defected ground structure can form equivalent inductors and capacitors, and thus change current distribution and achieve better filter frequency characteristics. Since the filter in this disclosure is formed by microstrip line circuits on PCB instead of lumped elements, the filter can be manufactured consistently and has little variation in frequency characteristics. Detailed descriptions of the embodiments of the invention are disclosed below.
The first hairpin resonator 11 is disposed on a first layer of the dielectric substrate 10, such as an upper surface of the dielectric substrate 10. The metal ground layer 12 is disposed on a second layer of the dielectric substrate 10. The metal ground layer 12 includes a first defected ground structure 13. The first defected ground structure 13 includes a first defected area 131, a second defected area 132 and a third defected area 133. The projection of the first defected area 131 on the first layer of the dielectric substrate 10 is located inside the hairpin structure of the first hairpin resonator 11. The projection of the second defected area 132 on the first layer of the dielectric substrate 10 is located in a direction opposite to an opening direction of the first hairpin resonator 11. The third defected area 133 connects the first defected area 131 and the second defected area 132. Detailed descriptions of each element are disclosed below.
Referring to
The U-shaped metal strip 113 used in the present embodiment includes a first metal segment 1131, a second metal segment 1132 and a third metal segment 1133. The first metal segment 1131 is connected to the first metal strip 111. The second metal segment 1132 is perpendicular to the first metal segment 1131 and the third metal segment 1133. The third metal segment 1133 is connected to the second metal strip 112.
In the first hairpin resonator 11, both the first metal strip 111 and the first metal segment 1131 connected to the first metal strip 111 are rectangular; the line width of the first metal segment 1131 is not equivalent to that of the first metal strip 111. The line width LW3 of the first metal segment 1131 is smaller than the line width LW1 of the first metal strip 111. The third metal segment 1133 and the first metal segment 1131 have the same shape and the same size. The second metal strip 112 and the first metal strip 111 have the same shape and the same size (the first metal strip 111 and the second metal strip 112 form parallel coupled microstrip lines). Therefore, the first hairpin resonator 11 is a symmetric structure. The line width LW4 of the third metal segment 1133 is smaller than the line width LW2 of the second metal strip 112.
Descriptions of the first defected ground structure 13 of the present embodiment are made with reference to
In the specification, the descriptions regarding the relative position between the first defected ground structure 13 and the first hairpin resonator 11 all refer to the projection of the first defected ground structure 13 on the upper surface of the dielectric substrate 10. To make the specification more concise, the term “projection” will not be repeatedly hereinafter. The first defected area 131 is located inside the first hairpin resonator 11. In the present embodiment, the first defected area 131 is surrounded by the U-shaped metal strip 113. The second defected area 132 is located in a direction opposite to the opening direction of the first hairpin resonator 11, that is, an opposite direction of direction D1, and located outside the hairpin structure of the first hairpin resonator 11. The third defected area 133, which connects the first defected area 131 and the second defected area 132, crosses the first hairpin resonator 11.
In the present embodiment, the first defected area 131 is rectangular. The second defected area 132 is rectangular, and has the same size as that of the first defected area 131. The third defected area 133 is rectangular. The third defected area 133 is across and perpendicular to the second metal segment 1132 of the U-shaped metal strip 113. Therefore, the first defected ground structure 13 has a dumbbell shape. One end of the dumbbell is located inside the hairpin structure of the first hairpin resonator 11, and the other end of the dumbbell is located outside the hairpin structure of the first hairpin resonator 11. The center portion of the dumbbell is across and perpendicular to the second metal segment 1132 and passes through the middle point of the second metal segment 1132.
The microstrip line filter 1 formed by the first hairpin resonator 11 and the first defected ground structure 13 is a low-pass filter capable of removing high-frequency components of the signal. The microstrip line filter 1 includes a signal feed point 14 and a signal output point 15. The signal feed point 14 is near the one end of the U-shaped metal strip 113 connected to the first metal strip 111. The signal output point 15 is near the other end of the U-shaped metal strip 113 connected to the second metal strip 112. The frequency response of the microstrip line filter 1 may be changed by adjusting the sizes of the first metal strip 111, the second metal strip 112, the U-shaped metal strip 113, the first defected area 131, the second defected area 132, and the third defected area 133. For example, parameters related to the frequency response may be changed, including cutoff frequency, return loss, insertion loss, and attenuation on the second and third harmonics.
The present embodiment discloses the shapes of the first hairpin resonator 11 and the first defected ground structure 13. However, a person skilled in the art would understand that the shapes of the first hairpin resonator 11 and the first defected ground structure 13 are not limited to the examples disclosed in the present embodiment, and may be designed according to the application and the desired frequency response. A number of other possible implementations are disclosed below. Any design in which the first defected area 131 is located inside the hairpin structure of the first hairpin resonator 11, the second defected area 132 is located in a direction opposite to the opening direction of the first hairpin resonator 11, and the third defected area 133 connects the first defected area 131 and the second defected area 132 is within the scope of the invention.
The first defected ground structure 13 may be S-shaped. For example, the first defected area 131 is rectangular, the second defected area 132 is C-shaped, one end of the second defected area 132 is connected to the third defected area 133, and the third defected area 133 is connected to a corner of the first defected area 131. The S-shaped defected ground structure is illustrated in
The microstrip line filter of the disclosed embodiments has a defected ground structure, which effectively reduces the size of the microstrip line circuit on the upper layer of the dielectric substrate and accordingly reduces the area required by the filter. Specifically, to achieve the same cutoff frequency, the U-shaped metal strip 113 used in a microstrip line filter having a defected ground structure can be shorter than that used in a microstrip line filter without a defected ground structure. In addition, the defected ground structure also improves the frequency characteristics of the microstrip line filter, such that the filter has a greater attenuation at the stop-band. Frequency response of a conventional first order filter has a slope of −6 dB per octave. If a larger attenuation rate is desired, the filter must be a higher order filter, which requires a larger circuit area. The microstrip line filter having a defected ground structure as disclosed herein requires a small circuit area and achieves good filter frequency characteristics.
The microstrip line filter 1 of the first embodiment includes a first hairpin resonator 11 and a first defected ground structure 13. To achieve better filter frequency response, one more stage of filter may be connected to the signal output point 15 of the first embodiment. Detailed descriptions of the second embodiment of the invention are disclosed below.
In the embodiment illustrated in
The filter parameters may be adjusted according to the required specifications. For example, the design of the filter can be adjusted according to the insertion loss at frequency of 2.4 GHz, the insertion loss at frequency of the second harmonic, and the insertion loss at frequency of the third harmonic. Table 1 shows actually measured frequency response of the low-pass filter designed according to different needs.
As shown in Table 1, the filter proposed in this disclosure has good attenuation on the second harmonic and the third harmonic. The return loss can also fulfill actual needs. No matter the filter is used in a transmitting end or in a receiving end of wireless signals, signal distortion and interference will be greatly reduced if the harmonic components can be attenuated by a large amount. For example, the transmitter can avoid transmitting the harmonics generated by internal circuits, and the receiver can filter out the harmonic components of the signal when receiving a signal.
The microstrip line filter 2 of the second embodiment is a low-pass filter. To realize a band-pass filter, a high-pass filter may be cascade connected to the output end or the input end of the microstrip line filter 2. Therefore, the microstrip line filter of the third embodiment of the invention is realized by cascade connecting a high-pass filter 4 to the second hairpin resonator 24 of the second embodiment, wherein the high-pass filter 4 may be formed by lumped elements.
The microstrip line filter 5 includes a first hairpin resonator 51, a second hairpin resonator 54 and a connecting metal strip 55. The metal ground layer 52 includes a first defected ground structure 53 and a second defected ground structure 56. Since the second defected ground structure 56 and the first defected ground structure 53 have substantially the same structure, only the first defected ground structure 53 is described below.
Referring to
The microstrip line filters disclosed in the above embodiments are formed by microstrip line circuits on a printed circuit board. A specific pattern is formed on the metal ground layer of the printed circuit board to form the defected ground structure. The filter formed by the defected ground structure along with the microstrip line circuit on the upper surface of the printed circuit board achieves good frequency characteristics, particularly in attenuating harmonic components. Moreover, the filter may be formed directly on the printed circuit board, and hence additional lumped element or additional special manufacturing process on the printed circuit board is not required. The filter thus formed not only occupies a smaller circuit area but also reduces the manufacturing cost. Although the filters in the above embodiments have a pass band around 2.45 GHz, the filter proposed in this disclosure may be adapted to any frequency band, for example, a pass band around 5 GHz may also be applicable.
As shown in
As shown in
The microstrip line filter 6 may be used as a radio frequency filter (RF filter) capable of transmitting alternating current signals. When a metal conductor transmits alternating current signals, the current density is largest near the surface of the conductor and decreases with greater depths in the conductor due to the skin effect. In
For the alternating current signals, at a particular time instant, assume the current I1 on the first edge 6101 flows upwards, meanwhile, the current I2 on the second edge 6102 flows downwards, the current I3 of the metal ground layer 62 near the third edge 6313 flows upwards, and the current I4 of the metal ground layer 62 near the fourth edge 6314 flows downwards. Current generates an induced magnetic field. Therefore, when the current on the upper surface and the current on the lower surface flow in the same direction at the place where metal edges are aligned, for example, the current I1 and the current I3 flow in the same direction, the direction of the induced magnetic field generated by the current I1 will be the same as that generated by the current I3. The magnetic lines of force generated by the current I1 superimposed on the magnetic lines of force with substantially the same direction generated by the current I3 increases the mutual inductance.
Therefore, when the first edge 6101 is aligned with the third edge 6313, equivalent inductance will be increased. Likewise, when the second edge 6102 is aligned with the fourth edge 6314, equivalent inductance will be increased as well. Since the equivalent inductance is increased when the structure on the upper surface is aligned with the structure on the lower surface, the same inductance value can be achieved with a smaller circuit area. That is, the circuit area can be effectively reduced, and the same frequency response can still be achieved.
As shown in
Like the previous embodiment, at a particular time instant, assume the current I5 on the first edge 7101 flows upwards, meanwhile, the current I6 on the second edge 7102 flows downwards, the current I7 on the metal ground layer 72 near multiple third edges 7313 flows upwards, and the current I8 of the metal ground layer 72 near multiple fourth edges 7314 flows downwards. Since the current on the upper surface and the current on the lower surface have the same direction at the place where metal edges are aligned, the equivalent inductance is increased. The present embodiment discloses a defected ground structure whose shape is different from that disclosed in previous embodiments. The filter of the present embodiment increases the equivalent inductance and thus requires smaller circuit area.
In the microstrip line filter 5 of
According to the microstrip line filters of the above embodiments, a defected ground structure is combined with a microstrip line circuit disposed on the upper surface of a printed circuit board, such that the filter achieves good frequency characteristics. Furthermore, because the defected ground structure is aligned with the microstrip line circuit disposed on the upper surface of the printed circuit board, the equivalent inductance can be increased, and hence the same filter performance can be accomplished with smaller circuit area.
While the invention has been described by way of example and in terms of the preferred embodiment(s), it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.
Number | Date | Country | Kind |
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2014 1 0326631 | Jul 2014 | CN | national |
2015 1 0175623 | Apr 2015 | CN | national |
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Entry |
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Xiao et al.; “Novel Compact Hairpin SIR Bandpass Filters with Defected Ground Structure”; Jul. 2007. |
Number | Date | Country | |
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20160013533 A1 | Jan 2016 | US |