EM Simulation

The proposed stepped-impedance impedance-transforming broadband 180^o hybrid ring is designed at 3 GHz, with equal power division, 15dB return loss, and an impedance-transforming ratio of 1:3 (40Ω to 120Ω). The circuit is realized on a 15-mil Al2O3 substrate with a dielectric constant of 9.8. The simulation is done by Sonnet. Figure 4.9(a) and (b) show the layout of the proposed circuit. The design parameters and physical dimensions are listed in Table 4.2 and Table 4.3. The diameter of the bonding wires is set to be 1 mil.

In Figure 4.9(a), it is shown that the discontinuity effect of three junctions needs to

be concerned so that the length of each CPS/interdigital CPS is different from the initial value. These discontinuities are the T junctions, the hybrid CPS/interdigital CPS junctions, and the CPW/interdigital CPS junctions. Note that instead of the designed port impedance of 40Ω or 120Ω, a CPW with impedance of 50Ω is connected to the interdigital CPS at each I/O port. This is because the network analyzer for measurement is based on system impedance of 50Ω and probing with CPW. Thus, the reference plane is de-embedded to the interconnection of the CPW/interdigital CPS junctions, as shown in the top-right corner of Figure 4.9(a).

(a)

Figure 4.9: The circuit layout of the proposed stepped-impedance impedance- transforming broadband 180^o hybrid ring. (a) Total circuit. (b) λ/4 line section using hybrid CPS/interdigital CPS

Table 4.3 Parameters and physical dimensions of the proposed stepped-impedance

impedance-transforming broadband 180^o hybrid ring.

ZH(Ω) ZL(Ω) θH(deg) θL(deg) LH (mil) LL (mil)

λ/4, Z1 = 63.2Ω 175 22 17 17 67.4 50.6

λ/4, Zt1 = 43.5Ω 175 22 9.2 19.9 30 107.4

λ/4, Zt2 = 71.2Ω 175 22 21.1 14.6 87.4 53.8

The bandwidth is designed as 100% from 1.5 to 2.5GHz. Figure 4.10(a) and (b) show the simulated return loss and coupling of the proposed impedance-transforming 180^o hybrid ring. The amplitude balance of the coupling parameters S31 and S41 (S32

and S42) are perfect, and the return loss is better that 10dB from 1.49 to 4.49GHz in the passband. Figure 4.11 shows the return loss for all the ports of the proposed 180^o hybrid ring. It is shown that the return loss for each port in the passband is better than 10dB so that the input and output ports are matched to 40Ω and 120Ω, respectively.

The simulated broadband performance of the 180^o hybrid ring is shown in Figure 4.12.

Compared to the frequency response of the 180^o hybrid ring without stepped- impedance in Figure 4.4, it shows no spurious S31 and S41 passband up to 12GHz.

(a)

(b)

Figure 4.10: The simulated results of return loss and coupling of the proposed impedance-transforming broadband 180^o hybrid ring. (a) Out-of-phase operation. (b) In-phase operation.

Figure 4.11: Simulated return loss for all the ports of the proposed impedance- transforming broadband 180^o hybrid ring.

Figure 4.12: Simulated broadband performance of the proposed 180^o hybrid ring.

4.4 Fabrication and Measurements

The circuit is fabricated on Al2O3 substrate with 15-mil thickness and a dielectric constant of 9.8. The proposed 180^o hybrid ring is designed to operate at 3 GHz, with

equal power division (3dB), bandwidth from 1.5 to 4.5 GHz, and input/output impedance-transforming ratio of 1:3 (40Ω to 120Ω). The passband frequency response of the proposed circuit is shown in Figure 4.13(a) and (b). The measured return loss is better than 10 dB from 1.49 to 4.42 GHz (99%) for out-of-phase operation and from 1.53 to 4.43 GHz (97%) for in-phase operation. The measured isolation is better than 25dB in the passband. The amplitude and phase balance of the coupling parameters (S31 / S41 or S32 / S42) are shown in Figure 4.14(a) and (b). The amplitude and phase balance in the passband are less than 0.55dB and 4^o, respectively, for both in-phase and out-of-phase operation.

(a)

(b)

Figure 4.13: Measured and Simulated results of the proposed impedance- transforming broadband 180^o hybrid ring. (a) Out-of-phase operation. (b) In-phase operation.

(a)

(b)

Figure 4.14: Measured amplitude and phase balance of the proposed impedance- transforming broadband 180^o hybrid ring. (a) Out-of-phase operation. (b) In-phase operation.

Figure 4.15 shows the measured return loss for all the ports of the proposed 180^o hybrid ring. It is shown that the return loss for all the ports in the passband is better than 10dB, so it is verified to be well-matched to the system impedance of 40Ω or 120Ω. The measured broadband performance of the proposed stepped-impedance 180^o hybrid ring is also given in Figure 4.16. It presents no spurious passband up to 10GHz. Figure 4.17 shows the photograph of designed 180^o hybrid ring, of which

Figure 4.15: Measured and simulated results of return loss for all the ports of the proposed impedance-transforming broadband 180^o hybrid ring.

Figure 4.16: Measured broadband performance of the proposed impedance- transforming broadband 180^o hybrid ring.

Figure 4.17: Photograph of the proposed impedance-transforming broadband 180^o hybrid ring.

Chapter 5 Conclusion

In this study, we proposed two Ku band reflection-type phase shifters, in which four and two varactors are used, respectively. The measured performance covers 12.2-12.7 GHz and can be used for broadcasting satellite service (BSS). In chapter two, a 360^o phase shifter with four varactors has been presented. The phase shift of 360^o is achieved by the reflection load based on the parallel connection of two series tuned varactors. The reduction of insertion-loss variation is attributed to the impedance transformer between the reflection load and the 3-dB 90^o hybrid coupler.

Measured results have shown that maximum phase shift is larger than 360^o, the insertion loss is 4.8 1.6 dB, and return loss is better than 14 dB over 500 MHz. ±

In chapter three, a reflection-type phase shifter with two varactors is fabricated and used in a phased array. This work extends the maximum phase shift of the phase shifter from 52^o to 286^o by resonating the varactor with via inductance and impedance transformers. Theoretically, the insertion-loss variation may be compensated with a resistance parallel to the reflection load. However, the measured insertion-loss variation is improved by only 0.5 dB with the used of compensating resistance due to

the parasitic effect of the chip resistor at high frequency. Furthermore, we integrate the phase shifter into a 4-element phased array. The measurement results show that we can change the direction of the main beam by adjusting the control voltage of phase shifters.

In chapter four, the 180^o hybrid ring adding a unit element at each port has been designed to exhibit Chebyshev response and impedance transformation. We use hybrid CPS/interdigital CPS as stepped-impedance and ideal phase inverter for size reduction of 70% and wideband performance. The fabricated 180^o hybrid ring exhibits a wide bandwidth of almost 100%, and its amplitude and phase balance are less than 0.55dB and 4^o, respectively. For system impedance transformation of 40Ω and 120Ω, the four ports of the network are well-matched.

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