Abstract
A variable attenuator is one of the essential components in radio frequency (RF) systems, such as automatic gain control amplifiers and full-duplex systems. Variable attenuators based on microelectromechanical systems (MEMS) technology have several advantages over the semiconductor counterparts, including low power consumption and suppressed harmonics. Attenuation can be realized by disruption of signal propagation, which is induced by moving electrodes placed next to a signal line. In this work, the effect of the moving electrodes on the RF characteristics of the variable attenuators is studied via numerical simulation. It is observed that 10 \(\upmu\)m of moving electrode displacement can result in 18 dB of attenuation dynamic range at 20 GHz. The similar type of RF MEMS variable attenuators reported previously showed substantial nonlinearity in attenuation-voltage characteristics, which becomes a serious drawback for applications where high-precision attenuation management is required. The main objective of the current study is, therefore, to achieve high dB-linearity, by employing shaped-finger comb-drive actuators in the moving electrode displacement control. In addition, a nonlinear relationship between force and displacement in a clamped-clamped beam spring is taken into account for more accurate device modelling. Through finite element analysis, it is shown that an improvement by a factor of twelve can be obtained in dB-linearity by using a single-comb shaped-finger actuator, compared to standard straight-finger comb-drives. The study also shows that the dB-linearity can be further (2.2 times additionally) improved by utilizing dual-comb shaped-finger actuators.
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Abbreviations
- A :
-
Attenuation, equals to \(-S_{21}\)
- \(C_{bias}\) :
-
Unit 2-D capacitance, bias comb
- \(c_{dblin}\) :
-
Coefficient of dB-linearity
- \(C_{DRIVE}\) :
-
Unit 3-D capacitance, drive comb
- \(C_{drive}\) :
-
Unit 2-D capacitance, drive comb
- \(d_{bias}\) :
-
Finger gap profile, bias comb
- \(d_{drive}\) :
-
Finger gap profile, drive comb
- E :
-
Young’s modulus
- \(F_{elec}\) :
-
Electrostatic force
- \(F_{mech}\) :
-
Mechanical restoring force
- \(g_{me}\) :
-
Gap between a moving electrode and a signal line
- \(k_1\) :
-
Spring constant, linear term
- \(k_3\) :
-
Spring constant, cubic term
- \(l_{ol}\) :
-
As-fabricated overlap length between fingers
- \(S_{11}\) :
-
Reflected power at port 1, return loss
- \(S_{21}\) :
-
Power transferred from port 1 to port 2, insertion loss
- u :
-
Distance from the moving finger tip, drive comb
- V :
-
Applied voltage, drive comb
- v :
-
Distance from the moving finger tip, bias comb
- \(V_{bias}\) :
-
Applied bias voltage, bias comb
- x :
-
Beam center (moving electrode) displacement
- \(x_{i}\) :
-
Excluded zone length
- \(x_{fex}\) :
-
Far-side extension length
- \(x_{max}\) :
-
Maximum displacement range
- \(x_{nex}\) :
-
Near-side extension length
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Acknowledgements
This work was partially supported by the Research Fund of Abdullah Gül University (FOA-2016-49).
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Hah, D. RF MEMS variable attenuators with improved dB-linearity. Microsyst Technol 29, 311–320 (2023). https://doi.org/10.1007/s00542-023-05427-8
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DOI: https://doi.org/10.1007/s00542-023-05427-8