MEMS Devices

MEMS Phase Shifter
Microwave phase shifters are key elements of modern telecommunication systems such as phased array antennas and phase modulators.

Several are the commonly adopted circuit topologies: switched-line, loaded-line, reflect-line, switched-networks and distributed phase shifters (see figure 1).

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(a) Switched-line phase shifter: by using SPST or SPDT switches the input signal is switched between lines of different electrical lengths.

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(b) Loaded-line phase shifter: a transmission line is loaded with two impedances of different values selected by two switches.
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(c) Reflect-line phase shifter: it consists of a 3-dB coupler terminated with reconfigurable loads. The loads are designed in order to generate a constructive interference and the desired phase shift at the coupler working frequency.

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(d) Switched-networks phase shifter: a differential phase shift is achieved by switching the input signal between a low-pass and a high-pass filter. A 2 SPDT or a 4 SPST switch is generally adopted for the switching operation.

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(e) Distributed phase shifter: the working mechanism consists in modify the capacitance of a transmission line by using a reconfigurable load distributed along the line length (diodes, transistors or passive components such as capacitors and stubs or MEMS switches).

Figure 1: Some examples of phase shifter topologies [1].

They can be designed by using ferrite materials, PIN diodes, or FET switches [1]. Ferrite-based phase shifters guarantee excellent performance and can handle high RF power. Unluckily, they are also characterized by a large size, high fabrication costs and a DC power consumption [2]. PIN diodes or FET phase shifters are a good alternative requiring a lower DC power, lower cost designs and no manual tuning [3-4]. However, they generally exhibit considerable losses in the microwave frequencies. A promising solution to overcome this drawback is represented by the use of MEMS bridges as electronic switches. In fact, RF-MEMS phase shifters result in lower insertion loss, especially from 8 to 120 GHz. Moreover, since MEMS switches have very small up-state capacitances, phase shifters based on MEMS technology exhibit a wider bandwidth than conventional solid-state devices.

The main disadvantages of MEMS phase shifters are:

  • high switching times that limit their application to relatively slow scanning arrays;
  • power-handling capabilities of 10-50 mW, limiting the application of a MEMS phase shifter to arrays requiring a relatively low radiated power per element of the array;
  •  they occupy a wider amount of area then conventional solid-state phase shifters.

 

In the field of phase shifter’s design the research activity of the EML2  group is maily focused on the investigation of new strategies (such as the artificial transmission lines approach) guaranteeing improved performance as well as miniaturization.

In figure 2 are illustrated a loaded-line and a reflect-line phase shifter designed by our research group. The production process of this phase shifters is currently in progress at Fondazione Bruno Kessler of  Trento.

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Figure 2: Layout of a loaded-line and a reflect-line phase shifter designed at EML2.

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Figure 3: Measured differential phase-shift (∆φ=45°): a. for the loaded-line phase shifter, b. the reflect-line phase shifter

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Figure 4: Comparison between simulated and measured results. Reflection and transmission coefficient amplitudes: loaded-line phase shifter (a. and b.),   reflect-line phase shifter (c. and d.)
REFERENCES

[1] G. M. Rebeiz, “RF MEMS Theory, Design, and Technology”, New York: Wiley, 2003.
[2] F. Congedo, G. Monti, L. Tarricone, P. Farinelli, E. Chiuppesi, R. Sorrentino, J. Iannacci, V. Mulloni, B. Margesin, “Design and Realization of Loaded- and Reflect-Line X-band RF MEMS Phase Shifters”, 11th International Symposium on RF MEMS  and RF Microsystems, Otranto, Italy, 28-29-30 June 2010.
[3] F. Congedo, G. Monti, L. Tarricone, P. Farinelli, R. Sorrentino, J. Iannacci, V. Mulloni, B. Margesin, “MEMS-Based Frequency-Tunable Reflect-Line Phase Shifter”, Proceedings of the 12th International Symposium on RF MEMS and RF Microsystems, Athens, 27-29 June 2011.