RF MEMS Extended Tuning Range Varactor and Varactor Based True Time Delay Line Design

Abstract

MEMS varactors are one of the important passive MEMS devices. Their applications include use in VCOs, tunable impedance matching networks, tunable filters, phase shifters, and true time delay lines. The shunt capacitive structure has been employed in most of the conventional MEMS varactor designs because of its simplicity. However, the capacitance ratio of this conventional shunt capacitive MEMS varactor is limited to 1.5 because of the MEMS Pull-In effect, which happens when the deflection between the MEMS top and bottom metal plates increase beyond 1/3 of the airgap between the two metal plates. At that time, the top metal plate will quickly snap down. This effect is the major limitation in MEMS varactor designs and can cause nonlinearity and mechanically instability. In order to eliminate this Pull-In effect, the author employed the so-called MEMS extended tuning range structure. This structure utilizes a variable height top metal beam with separate actuation parts. The airgap between the center part of the top beam and the bottom plate has been designed to be less than 1/3 of the airgap between the top beam and the bottom actuation pads. When DC bias is applied to the actuation parts, the entire top beam will move down together. Consequently, before the Pull-In effect happens at the actuation parts, the center part has already traveled through its entire tuning range, which means that the capacitive ratio of this kind of MEMS varactor can go to infinity. A fabrication process employing a GaAs substrate has been designed based on surface micromachining technology. The maximum capacitance ratio of the designed MEMS extended tuning range varactor is 5.39 with a C max value of 167 fF. Based on this MEMS varactor design, a Ka-band MEMS varactor based distributed true time delay line has been designed. This distributed true time delay line includes a high impedance CPW transmission line with 70 Ω unloaded impedance at 28 GHz and eight MEMS extended tuning range varactors based on the varactor design periodically loaded on the CPW line. The testing results show that a 56 • phase delay variation has been achieved at 28 GHz. The measured insertion loss at 28 GHz is −1.07 dB at the up-state and −2.36 dB at the down-state. The measured return losses, S 11 and S 22 , are both below −15 dB at 28 GHz and below −10 dB over the entire tested frequency range of 5 GHz to 40 GHz.

Metamaterial Inspired Varactor-Tuned Antenna with Frequency Reconfigurability and Pattern Diversity

“…It can be rescaled to cover other needed bands [ 40 ]. Although it is not easy to extend the design to higher frequencies when normal varactors are used, by using micro-electro-mechanical-systems (MEMS), a varactor with very small capacitance can be realized [ 41 ], needed at 6 GHz and higher, as considered in 5G systems.…”

“…Note that the application of the p antenna is not limited to LTE. It can be rescaled to cover other needed bands [ hough it is not easy to extend the design to higher frequencies when normal varac used, by using micro-electro-mechanical-systems (MEMS), a varactor with very s pacitance can be realized [41], needed at 6 GHz and higher, as considered in 5G s The measured and simulated reflection coefficients are given in Figure 12. The measurements show that the resonant frequency tuning range for both ports is very wide: from 1.7 GHz to 2.2 GH, covering the 1.71-2.17 GHz LTE band (e.g., the 1.7 GHz AWS-1 band, the 1.8 GHz DCS band, the 1.9 GHz PCS band, and the 2.1 GHz IMT band).…”

Topology Synthesis of the Multi-Tapped Meander Delay Line using Monte Carlo Method

“…E.g. Wilson and Atkinson [6] for SAW delay lines designing use a method of the pulse characteristic which is realized in MathCAD® system, and concurrence of the simulated and measured characteristics of the delay lines have achieved within the limits of 20 %. The example of use of a method of equivalent circuits for delay line design is presented in [8].…”

“…For example active delay lines: lines which use delay of signals in digital logic elements [2], the active lines using reactive properties of networks elements (e.g. varactors) [6], ultrasonic delay lines [7]. Passive delay lines are also used broadly: delay lines created using lumped elements [8], optical delay lines [4], etc.…”

Switchable Distance-Based Impedance Matching Networks for a Tunable Hf System

“…The tunability of adaptive impedance matching modules is controlled by the following: the L network [19,20] consisting of a series LC and a parallel LC network; the capacitance array [21][22][23][24][25] configured by parallel switched capacitors; and the tuning transformer [26] based on the nonlinear properties of the ferromagnetic core material. However, these circuits have intolerably high loss and are not suitable for portable communication devices.…”