**7. Design of multimodal reconfigurable microwave and mm-wave circuits**

This section presents two applications of the RF-MEMS presented in Section 5 when combined with the multimodal circuits presented in Section 2: 180° phase switches and bandpass filters. All of them were fabricated using the process described in Section 3.

#### **7.1. Phase switches**

A 180° phase switch is a circuit that shifts the phase between 0 and 180°. This is a multi-purpose element required in the microwave and millimeter-wave applications, such as high-sensitivity radiometers or electronic beam steering in phased-array antennas. The specifications for these systems are, in most cases, a broadband operation, a low power consumption, and small size. They can be implemented monolithically using HBT/HEMT-based MMICs or using MEMSbased solutions as shown subsequently.

**Figure 21(a)** shows a compact, uniplanar 180° phase switch fabricated on a quartz substrate [9]. It is based on two different back-to-back (BTB) CPW-to-slotline transitions [39] (symmetric and antisymmetric transitions, respectively), creating two phase paths with a relative transmission phase shift between them of 180°. Each path is selected using two single-pole-doublethrow (SPDT) switches. The SPDT consists of two ohmic-contact MEMS series switches

**Figure 21.** (a) Manufactured 180° phase switch. (b) Measured insertion loss (symmetric and antisymmetric paths) and measured phase shift between the two paths.

(described in Section 5.2.2) and a power divider. **Figure 21(b)** shows the measured performance of the circuit, featuring a 180° ± 5° phase shift between both states in a bandwidth of 35% (14–20 GHz), with an insertion loss smaller than 2 dB in both paths.

**Figure 22(a)** shows the photograph of a second compact, uniplanar 180° phase switch fabricated on a quartz substrate [10]. In this case, it is based on an air-bridged CPW cross [57]. The two CPW arms of the cross are loaded with capacitive-contact MEMS switches (described in Section 5.2.1). The two phase-switch states (0°/180°) are obtained by actuating the MEMS switches in opposite states (ON/OFF and OFF/ON), resulting in a multimodal interaction between the two CPW modes (even and odd) at the air-bridged cross. The CPW-to-slotline transition [39] at the input port and the CPW taper at the output port are included in order to enable the measurement of the circuit with a probe station. **Figure 22(b)** shows the measured results of phase shift between both states and insertion loss, featuring 180° +1.8°/−1° and ± 0.1 dB insertion-loss unbalance, respectively, in a very wide bandwidth (5–25 GHz). The measured insertion loss is better than 2 dB in 10–20-GHz frequency band.

#### **7.2. Uniplanar bandpass filters**

In **Table 2**, the values of the equivalent circuit elements obtained to fit measurement are listed. The capacitive switch features a high capacitance ratio (*CON*/*COFF* = 50.2). The ohmic switches feature low insertion loss (<1 dB) and high isolation (>20 dB) for f < 10 GHz (parallel switch)

This section presents two applications of the RF-MEMS presented in Section 5 when combined with the multimodal circuits presented in Section 2: 180° phase switches and bandpass

A 180° phase switch is a circuit that shifts the phase between 0 and 180°. This is a multi-purpose element required in the microwave and millimeter-wave applications, such as high-sensitivity radiometers or electronic beam steering in phased-array antennas. The specifications for these systems are, in most cases, a broadband operation, a low power consumption, and small size. They can be implemented monolithically using HBT/HEMT-based MMICs or using MEMS-

**Figure 21(a)** shows a compact, uniplanar 180° phase switch fabricated on a quartz substrate [9]. It is based on two different back-to-back (BTB) CPW-to-slotline transitions [39] (symmetric and antisymmetric transitions, respectively), creating two phase paths with a relative transmission phase shift between them of 180°. Each path is selected using two single-pole-doublethrow (SPDT) switches. The SPDT consists of two ohmic-contact MEMS series switches

**Figure 21.** (a) Manufactured 180° phase switch. (b) Measured insertion loss (symmetric and antisymmetric paths) and

**7. Design of multimodal reconfigurable microwave and mm-wave** 

filters. All of them were fabricated using the process described in Section 3.

and for f < 25 GHz (series switch).

138 MEMS Sensors - Design and Application

based solutions as shown subsequently.

measured phase shift between the two paths.

**circuits**

**7.1. Phase switches**

**Figure 23(a)** shows a second-order bandwidth-reconfigurable bandpass filter, which was fabricated on a quartz substrate (*h* = 500 μm) [11]. The *λ<sup>o</sup> /2* slotline resonators are coupled by a slotline short-circuit (*K*12). The filter features multimodal immittance inverters (MIIs) based on CPW-to-slotline transitions [37, 58] which are embedded in the input and output slotline resonators. As shown in **Figure 23(b)**, two cantilever-type ohmic-contact MEMS switches (described in Section 5.1) are used to enable reconfigurable MIIs. When actuated, the switches modify the input and output coupling of the filter (*K*01 and *K*23), resulting in a change in the filter's fractional bandwidth (FBW). To keep the central frequency constant, another cantilever MEMS switch (also shown in **Figure 23(b)**) is integrated in *K*12 and actuated simultaneously. **Figure 23(c)** shows the filter measured results. It features two FBW states of 0.082 (when the inner switches are actuated) and 0.043 (when the outer switches and the impedance inverter switch are actuated), while maintaining a constant center frequency (18.9 GHz).

**Figure 24(a)** shows a second-order bandpass filter fabricated on a 5-KΩ-cm high-resistivity silicon substrate (ε<sup>r</sup> = 11.9, *h* = 200 μm), which uses switchable air bridges or SABs [6] (shown in **Figure 24(b)**) similar to the SAB of **Figure 14(a)**. The SABs are described in Section 5.2.3. Like

**Figure 22.** (a) Manufactured 180° phase switch. (b) Measured insertion loss for the two states and phase shift between the two states.

filter measured results. It features two operating frequencies, *f*

**8. Conclusion**

showing excellent performances.

\*, David Girbau2

**Acknowledgements**

AEI, Spain.

**Author details**

Adrián Contreras1

Benno Margesin5

Lluis Pradell1

a constant fractional bandwidth (FBW) of 14%. The measured filter IL is 4.6 dB in both states.

RF-MEMS Switches Designed for High-Performance Uniplanar Microwave and mm-Wave Circuits

In this chapter, the design considerations of RF-MEMS switches aimed to be integrated in uniplanar multimodal reconfigurable circuits for operation in the microwave and mm-wave bands have been presented. Different configurations of series and parallel switches featuring ohmic and capacitive contact have been discussed in detail, including the analysis of mechanical topologies which minimize the initial deformation due to residual stress. The fabrication process has been described, which provides the required flexibility to integrate the MEMS switches into higher-level RF communication systems. The switch RF behavior has been modeled using suitable equivalent circuits for both (ON/OFF) switch states. The models can easily be embedded into complex multimodal environments which enable multipurpose, compact designs. The fabricated switches have been experimentally characterized in terms of hysteresis, RF isolation (OFF state), and RF insertion loss (ON state), demonstrating an excellent behavior. As practical examples of the application into communication systems, some of the proposed switches have successfully been integrated into 0°/180° phase switches and into reconfigurable filters with either center frequency or fractional-bandwidth reconfiguration,

This work was supported by the Spanish MEC under Projects TEC2013-48102-C2-1/2-P, TEC2016-78028-C3-1-P and grant BES-2011-051305, the Mexican CONACYT under fellowship 410742, and the Unidad de Excelencia Maria de Maeztu MDM-2016-0600 financed by the

, Jasmina Casals-Terré<sup>4</sup>

, Julio Heredia1

1 Department of TSC, Technical University of Catalonia (UPC), Barcelona, Catalonia, Spain

, Antonio Lázaro2

, Flavio Giacomozzi5

,

and

, Miquel Ribó3

2 Department of EEEA, Rovira i Virgili University, Tarragona, Catalonia, Spain

3 Department of ET, La Salle-Ramon Llull University, Barcelona, Catalonia, Spain

, Marco Antonio Llamas1

\*Address all correspondence to: pradell@tsc.upc.edu

4 Department of EM, UPC, Barcelona, Catalonia, Spain

5 Fondazione Bruno Kessler, Trento, Italy

<sup>0</sup> = 12 GHz and *f*

http://dx.doi.org/10.5772/intechopen.76445

<sup>0</sup> = 13 GHz, with

141

**Figure 23.** (a) Second-order bandwidth-reconfigurable bandpass filter. (b) Detail of reconfigurable MIIs and reconfigurable inductive coupling *K*12. (c) Filter measured insertion loss and return loss.

**Figure 24.** (a) Second-order frequency-reconfigurable bandpass filter. (b) Detail of reconfigurable MIIs and SAB. (c) Filter measured insertion loss and return loss.

the previous example, the filter is fed by reconfigurable MII structures with embedded CPWto-slotline transitions but uses quarter-wavelength resonators coupled by a slotline gap. When actuated, the SABs reduce the resonators' effective length, which in turn increases the resonance frequency. As a result, the filter center frequency is shifted up. **Figure 24(c)** shows the filter measured results. It features two operating frequencies, *f* <sup>0</sup> = 12 GHz and *f* <sup>0</sup> = 13 GHz, with a constant fractional bandwidth (FBW) of 14%. The measured filter IL is 4.6 dB in both states.
