Wide piezoelectric tuning of ltcc bandpass filters

Abstract

This work treats design and fabrication issues associated with innovative tunable front-end components which combine two different ceramic technologies, namely multilayer ceramic circuit boards (low temperature cofired ceramics or LTCC) and piezoelectric actuator technology within a single device. The need for such components is particularly arising due to the increasing number of wireless services and associated frequency bands in the range between 0.5 and 2.5 GHz. This has led in the past to the concept of software-defined radio (SDR) which would provide a cost-efficient solution by treating signals digitally and software-controlled up to the highest possible frequencies and as close as possible towards the transmit antenna, while the final analogue section at the antenna comprises only few high-performing and frequencyagile, tunable components. However, as a consequence of demanding component specifications, SDR has not yet found noticeable application in consumer markets despite ongoing search for suitable device concepts and fabrication technologies.Similar to the known micro-electromechanical (MEMS) approaches for tuning and switching, this work presents a modified parallel plate capacitor with high-permittivity dielectric and piezoelectrically movable top electrode as a tuning element. Like in MEMS solutions, there is no tunable dielectric material required for tuning as for example paraelectric material, which would introduce additional losses. The proposed device therefore has the potential for a high quality factor. Contrasting MEMS, piezoelectric actuators exhibit proven reliability and lifetime. Also sticking of contact surfaces can be overcome by the actuator force. The size of the actuator in the order of several millimeters is not impedimental in the present context, since it compares well to the size of planar integrated filters in the frequency range mentioned.The vertical movement of the electrode opens an air gap above the dielectric film which allows for substantial lowering of the effective dielectric constant and capacitance. When applied as a shunt capacitor in a coupled microstrip lines LTCC bandpass filter, the center frequency of the filter is tuned from 1.1 GHz to 2.6 GHz (tunability of 135%) with 200 V control voltage and low insertion loss of value 4 dB (at zero-bias) to 2 dB (at the maximum bias). For a more compact size, one electrode of the piezoelectric element is simultaneously used as the center microstrip line of a filter employing three coupled lines. Its equivalent circuit has been used to explore the change of the capacitor parameters across the entire tuning range. The capacitor varied from 7 pF to 1.35 pF with a quality factor between 60 and 160. The quality factor could be improved by a factor of 7 when the metallization of the piezoelectric actuator, changes from 80 nm to 500 nm.This thesis discusses also the effects of tuning mechanism on the overall quality factor, return loss, insertion loss, and the relative bandwidth at the mid of the band as a function of ...

Piezoelectric Materials for Advanced Integrated RF Components

“…The piezoelectric cantilevers replace sections of interior coupled lines of a conventional untuned¯lter so that the tuning function consumes no extra space. This section summarizes the work that has been reported in [24].…”

Effects of dielectric substrates on piezoelectric transducer tunable filter

“…PET tunable microwave filters are novel and potential devices because PET has no connection to microwave circuits, and can be used in not only planar microstrip filters but also three‐dimensional structure filters such as evanescent‐mode cavity filters and dielectric resonator filters . Compared to the amount of published work on other tunable microstrip filters, the literature on PET tunable microstrip filters is only little .…”

Novel factor of merit for center‐frequency tunable bandpass filters comparison

“…Thus, tunable bandpass filters constitute an interesting solution to this problem since they can perform over all operating bands. For planar filters, the frequency tunability can be achieved by using various technologies like MEMS [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], ferromagnetic [16], ferroelectric [17][18][19], optical [20], piezoelectric [21,22], mechanical [23], or semiconductor [23][24][25][26][27][28][29][30][31][32] tuning elements.…”