Chapter 3 3-D Embedded CPW
3.6 Conclusion
In this section, a 3-D embedded CPW structure was developed to solve the loss problems of the low-impedance CPW lines and to extend the usable impedance ranges. Micromachined technology was employed to bended both the center conductor and ground plane partially overlap with each other. A systematic and comparative study has been performed to characterize the 3-D embedded CPW lines. Compared with the conventional CPW, the fabricated 3-D embedded CPW lines with 20- μm trench width showed lower losses (less than 3 dB on silicon substrate at 20 GHz) over a wider range (20–90Ω). To demonstrate practical usefulness of the 3-D embedded CPW lines, an X-band stepped-impedance LPF was fabricated using 3-D embedded CPW lines. The 3-D embedded CPW LPF showed distinct advantages over the conventional CPW filter such as lower loss and reduced size, together with improved spurious responses, including sharp skirt and wide stop-band characteristics. Thanks to their wide impedance and low-loss characteristics, the proposed 3-D embedded CPW is expected to be very useful for integration with various uniplanar microwave/millimeter-wave integrated circuits.
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Chapter 4 Applications in Monopole Antenna
4.1 Introduction
Nowadays, wireless communication systems are becoming increasingly popular. However, the technologies for wireless communication still need to be improved further to satisfy the higher resolution and data rate requirements. That is why ultra wideband (UWB) communication systems covering from 3.1 GHz to 10.6 GHz released by the FCC in 2002 [31] are currently under development. Various antennas for wideband operation have been studied for communications and radar systems for many years [32], [33].
The design of wideband antenna is a very difficult task especially for hand-held terminal since the compromise between size, cost, and simplicity needs to be achieved. In UWB communication systems, one of key issues is the design of a compact antenna while providing wideband characteristic over the whole operating band. Due to their appealing features of wide bandwidth, simple structure, omidirectional radiation pattern, and ease of construction several wideband monopole configurations, such as circular, square, elliptical, pentagonal, and hexagonal have been proposed for UWB applications [34]–[36].
In this thesis, a new microstrip-fed patch antenna with a tuning 3-D trench is presented. In order to improve the impedance bandwidth, a 3-D trench on the patch and stepped ground are used in the design of the antenna.
4.2 Antenna Design
Fig. 4-1 (a) shows the evolution of the proposed antenna fabricated on the quartz substrate, which consists of a rectangular patch with two notches at the two lower corners of the rectangular patch and a truncated ground plane with the notch structure. These notches alter the electromagnetic coupling between the rectangular patch and the ground plane. Matching improvement can also be obtained by inserting a slot in the truncated ground plane. It is found that much enhanced impedance bandwidth can be achieved for the proposed antenna.
Moreover, the 3-D embedded technology mentioned in chapter 2 will be applied to this monopole antenna in order to improve the band-width and matching. The proposed antenna has compact dimension of 36 mm X 16 mm with thickness of 0.52 mm and relative dielectric constant of 3.7. From the simulation, the optimized dimensions are: W=8.4 mm, L=10.6 mm, Wg=16mm, Lg=23.5 mm, Ws=6 mm, Ls= 1mm, Wn=1.6 mm, Ln=1.4 mm, We=2.4mm, Lf1=1mm, Lf2=23mm and the width of fed-in line is 1mm. The detail trench dimensions is shown in Fig. 4-1(b)
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Lf1 Lf2
(a)
(b) D
4.3 Simulation Results
Fig. 4-2 (a) and (b) show the comparison of the simulation results of the antennas with and without 3-D embedded structure. The bandwith for -10 dB return loss covers the range of 3.28~11.2 GHz. Comparing with the antenna without 3-D embedded structure, the proposed antenna shows better impedance matching.
The geometrical parameters related to the trench include the dimensions of the trench (0.05mm X 0.01mm) and the metal depth (D). Fig. 4-3 exhibits the effects of varying the parameters on the impedance matching. From Fig. 4-3, we can see that the length of the metal (D) would affect the impedance matching especially at the lower operating frequencies around 5 GHz and increasing the length extend the higher edge frequency of the bandwidth. The depth of the metal (D) and location of the trench slightly affect the higher edge frequency. In general, all the trench-related parameters influence the impedance matching to a certain extent.
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(a)
Fig. 4-2 (a) proposed antenna with 3-D embedded structure;
(b) the antenna without 3-D embedded structure (b)
4.4 Conclusions
In this section, a monopole patch antenna has been proposed for promising ultrawideband applications. The antenna has been designed to supply a new matching structure by placing the trench on the radiator. The study has shown that the trench is able to vary the matching of bandwidth of the antenna and offer a better matching characteristic.
Fig. 4-3 The effect of varying the parameters (D) on the impedance matching.
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Chapter 5 Conclusions and Future Works
5.1 Conclusions
The 3-D embedded structure mentioned in this thesis shows several advantages as below:
(i) compatibility with MMIC/VLSI circuit layouts and processing, (ii) ability to realize low characteristic impedances,
(iii) reducing current density near conductor strip edges, thus reducing conductor loss,
(iv) allowing the use of thick substrates during processing, reducing damage during handling and enhancing yield,
(v) eliminating the need for via holes, thus reducing associated parasitics,
(vi) exempting from crosstalk due to presence of ground plane between any (20–90Ω). Moreover, the 3-D embedded CPW LPF showed distinct advantages over the conventional CPW filter.
The monopole antenna has been designed to supply a new matching method by etching the trench on the radiator. The designed antenna satisfies the 10 dB return loss requirement from 3.2 to 11 GHz and provides good monopole-like radiation patterns.
5.2 Future Works
The results presented in this thesis represent a work in progress, and hence there are still other efforts can be done. The 3-D embedded CPW mentioned in chapter 3 is considerable to integrate to MMIC/VLSI process.
Besides, other applications of 3-D embedded structures are described as follows:
(1) DC Block
DC Block is widely used in microwave circuit in order to isolate the DC voltage level from different blocks. In general, DC block must have very low insertion loss and tolerable to high voltage. In normal high frequency circuit, DC block is composed of electromagnetic coupling structure. Fig. 5-1 and Fig. 5-2 show the layouts of traditional DC blocks.
However the structures in Fig. 5-1 and 5-2 are a 2-D circuit, they consume big area of the substrate to get the desired capacitance value.
Moreover, it can only operate in high frequency because the capacitance value is not big enough. By using the 3-D embedded structure, high coupling effect can be achieved due to the increase of the effective capacitance and the area consumption will be less. Both top and cross section view is shown in Fig. 5-3.
(2) Coupler
As mentioned in last paragraph, the 3-D embedded structure can also facilitate to fabricate coupler. The size of the coupler would be reduced due to the high coupling effect in the 3-D embedded structures. Fig. 5-4 shows the construction of the coupler with 3-D embedded structures in it.
Comparing with the traditional planer (vertical type) RF-MEMS device, the 3-D embedded structure shows more advantages. For instance: size
48
reduction, compatible with CMOS process, low cost, easy to fabricate (1~2 masks can be done) and etc. Besides, with proper design, movable parts would be added in the 3-D embedded structure and thus the ability of tuning could be achieved.
Fig. 5-1 DC block made by series CPW
Fig. 5-2 coupling CPW DC block
Fig. 5-3 Newly embedded DC block
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Fig. 5-4 newly embedded coupler
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Kai Chang “Encyclopedia.of.RF.and.Microwave.Engineering”
Hong Xiao “ Introduction to Semiconductor manufacturing technology”