An UWB 3-dB Directional Coupler
2.4 Design Procedure and Realization
2.4.2 Section 2 and Section 4 (Coupling= 7.54dB)
(a) VIP structure (b) Top view (c) Side view
1 2
3 4
Magnetic wall Electric wall
(a) (b)
Ca Ca
2Cb
width
length gap
Figure 2.13: A conventional parallel coupled line coupler
The parallel coupled line coupler is shown in Fig. 2.13.
The specifications of section 1 and section 5 are Z0e = 58.77Ω, Z0o = 42.54Ω,and Coupling = 15.92dB. Roughly calculating the gap, width and the length of coupled line, we obtained the gap = 15mil, width = 40mil, length = 233mil. By using the EM simulator such as Ansoft HFSS to simulate circuit, the simulated results of scattering parameters is shown in Fig. 2.14. The Coupling (S31) in Fig. 2.14 is 15.53dB, which is very close to the specified value, and the center frequency is indeed at 7.5GHz.
2.4.2 Section 2 and Section 4 (Coupling= 7.54dB)
The Coupling of section 2 and section 4 should be 7.54dB, this cannot be imple-mented by the conventional parallel coupled line coupler due to physical limitation.
There are two different types of VIP coupler were proposed in [14]. A type-I VIP coupler is shown in Fig. 2.15. There are four metal strips of which two are on the main substrate and the other two are on the VIP substrate. The strips
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Figure 2.14: Simulated results of section 1 and section 5
Metal
Magnetic wall Electric wall
(a) (b)
Figure 2.15: Cross-sectional view of the type-I VIP coupler
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Metal
Magnetic wall Electric wall
(a) (b)
Figure 2.16: (a)Even- and (b)odd-mode equivalent circuits of the type-I VIP coupler
on the main and VIP substrates are connected at two ends of the coupler. The performance of the coupler is improved by adding dielectric blocks at both sides of the VIP substrate, which use the same material as the main substrate and VIP substrate. This coupler can implement a coupler with coupling from moderate to tight coupling.
The even- and odd-mode equivalent circuits of the type-I VIP coupler is shown in Fig. 2.16. It shows that the total equivalent capacitance of each equivalent circuit is the combination of every capacitance shown in the figure. This means that three degrees of freedom, which are the VIP metal height, the width of the strips on the main substrate, and the gap width on the main substrate, are available to choose the even- and odd-mode characteristic impedances. The total width of the coupler has to be chosen carefully so that the junctions connecting to other sections can be laid out with minimal discontinuity.
The even- and odd-mode characteristic impedances with respect to the VIP
}
Z0e
Z0o
Z0e
Z0o
Figure 2.17: Even- and odd-mode characteristic impedances versus VIP metal height (Hmetal) of the type-I VIP coupler with G = 28 mils
metal height and the width of the strips on the main substrate are extracted by the EM simulator Ansoft HFSS. Fig. 2.17 depicts the extracted data, in which the gap width between two strips on the main substrate 28 mils.
As given in Fig. 2.17, Hmetal = 7mil, W = 15mil, G = 28mil are chosen for the case if Z0e = 78.46Ω and Z0o = 31.86Ω. The simulated results of scattering parameters is shown in Fig. 2.18. The Coupling (S31) is 7.7dB at 7.5GHz, which is very close to the ideal value 7.54dB.
2.4.3 Section 3 (Coupling= 0.82dB)
The type-I VIP coupler can not implement the extremely tight-coupled center sec-tion even when W equals 0. To achieve a coupling value as tight as 0.8 dB, the type-II VIP coupler is proposed in [14], as shown in Fig. 2.19. The ground plane
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Figure 2.18: Simulated results of section 2 and section 4
of type-II VIP coupler in the main substrate changes to two metal strips. Utilizing this finite-extent ground plane, the VIP coupler can achieve a coupling tighter than 0.8 dB. Again, a dielectric block as the type-I is used to compensate the modal phase velocities.
The type-II VIP coupler also has three degrees of freedom to choose the and odd-mode characteristic impedance such as the type-I VIP coupler. The even-and odd-mode equivalent circuits of the type-II VIP coupler are shown in Fig. 2.20.
The larger the gap between the two strips on the ground plane, the smaller the equivalent capacitance. Thus, the characteristic impedance of even-mode can be controlled by the Wg. In addition, the width of ground strips also has influence on Z0e. The metal height on the VIP substrate can affect the odd-mode characteristic
Metal
Magnetic wall Electric wall
(a) (b)
Figure 2.19: Cross-sectional view of the type-II VIP coupler
Metal
Magnetic wall Electric wall
(a) (b)
Figure 2.20: (a)Even- and (b)odd-mode equivalent circuits of the type-II VIP cou-pler
}
Z0e
Z0o
Z0e
Z0o
Z0e
Z0o
Figure 2.21: Even- and odd-mode characteristic impedances versus Wg and Hmetal
of the type-II VIP coupler
impedance. A large value of height of the metal on the VIP substrate will make itself and the electric plane forming a giant parallel plate capacitor. This means that the odd-mode characteristic impedance will significant decrease if the metal height becomes larger.
The even- and odd-mode characteristic impedances with respect to the Wg and Hmetal of the type-II VIP coupler are shown in Fig. 2.21. According to this figure, the Wg, Hmetal, and Wgnd are chosen to be 208mil, 60mil, and 16mil, respectively.
The simulated result is shown in Fig. 2.22. The Coupling in Fig2.22 only has 0.02dB difference to the ideal value.
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Figure 2.22: Simulated results of section 3