Tunable RF Filters Research Articles

Dual-Bandstop Substrate-Integrated-Coaxial Tunable and Static RF Filters

This letter discusses the RF design and practical realization of dual-bandstop filters (BSFs) with compact size and two independently controlled rejection bands. They are based on miniaturized substrate-integrated-coaxial (SIC) resonant stages that are made from two capacitively loaded posts and a thru-line. Each of the stages creates two transmission zeros (TZs) that can be exploited for the realization of dual-bandstop RF filters with approximately half the size of conventional single-post series-cascaded SIC filter configurations. Further miniaturization is achieved by tuning the center frequency of the bands. For proof-of-concept validation purposes, three prototypes were designed, manufactured, and measured. They include a static (rejection bands centered at 4 and 4.5 GHz) and a tunable (between 3.6–4.6 GHz) dual-bandstop second-order RF filters and a third-order dual-bandstop RF filter with bands centered at 4 and 4.4 GHz.

Integrated graphene oxide resistive element in tunable RF filters

Adaptable communication systems are of great interest as they provide dynamic front end to accommodate the tunable spectrum management in advanced wireless systems. Memristor (acronym of memory resistor) is an emerging technology part of resistive RAM (RRAM) that has good potential for application in reconfigurable RF devices. The potentiality of using resistive switches for frequency tuning of high frequency RF filters is successfully explored in this article for the first time. Tunable RF filter is designed with detailed simulation using Ansys HFSS, and then correlated with measured results from experiment. As a proof of concept, a prototype of the tunable RF filter is fabricated by using a graphene oxide (GO) integrated with a conventional microstrip open stub notch filter. The resistor switching ability of the device is exploited for the frequency tuning. The resonating length of the notch filter is varied by changing the resistance of the active GO material between ‘HIGH’ (OFF) and ‘LOW’ (ON) resistance states. The measured results demonstrate the great potential of using RRAM in tunable RF devices. It also proves the possibility of tuning RF devices without any localized surface mount device (SMD) element or complex realization technique.

Tunable RF Filters Based on Liquid Crystal for Space Applications

On board of communication satellites, there are numerous RF filters in the payload used for guiding wanted signals as desired and for rejecting unwanted signals such as not to interfere with wanted signals. At present, satellites have a long lifetime (15 years at least for a typical geostationary communication satellite) and there is also time elapsing between its design and the start of in-orbit service (2 years at least). On the other hand, conventional RF filters for satellite applications are fixed frequency. Therefore, there is a growing demand for in-orbit tunable filters to be able to cope with changes in the market situation during that long time. Also, there are communication systems evolving that rely on tunable devices used for adaptive filtering or steering. This article reports on in-orbit tunable bandpass filters employing liquid crystal material and operating at 20 GHz, and especially on design aspects dealing with the harsh space environment, on the operating principles for commanding the filters on board the satellite, and on a qualification campaign passed successfully. A demonstrator unit is in manufacturing for in-orbit verification.

A varactor based tunable RF filter for multistandard wireless communication receivers

This paper presents the design of a tunable RF filter dedicated for multistandard operation in Software Defined Radio (SDR) receivers. This filter is based on the Dual-Behaviour Resonators (DBR) topology realized in microstrip technology with PTFE substrate and using SMV1405 varactors as tuning elements. It provides the independent control of its center frequency and bandwidth relative to the radio standards UMTS, Wi-Fi and LTE (Band N°7). The center frequency ranges from 2.142 to 2.65 GHz and the required bandwidth varies from 86 to 108 MHz. S-parameters simulations and measurements with non-linearity study are provided to evaluate the performance of the designed filter. A very good agreement between simulations and measurements is obtained and the designed filter shows acceptable performances in terms of insertion and return losses which are less than 6 dB and more than 14 dB, respectively for all bands. Moreover, linear properties regarding receiver requirements with a minimum 1-dB compression point (P1dB) of 11.4 dBm and a minimum third order intercept point (IP3) of 21.2 dBm are satisfying.

Fast Tunable Photonic Single-Bandpass RF Filter With Multiple Arbitrary Switching Flat-Top Passbands

A fast tunable single-bandpass photonic radiofrequency filter (PRF) with multiple arbitrary switching flat-top passbands is proposed and experimentally demonstrated. The PRF is comprised of a narrow, flat-top optical filter based on a fiber Bragg grating with multiple phase shifts, and a high-speed wavelength switch module by incorporating an optical carrier-suppressed single-sideband modulation unit and a direct digital synthesizer. The combination of the single flat-top passband and the high-speed digital controlled wavelength switch in the optical domain enables us to achieve a fast and arbitrarily tunable single passband in the microwave domain. In the experiments, the proposed PRF shows a single flat-top passband with a 3-dB bandwidth of 740 MHz, a rectangular shape factor around 2.27, and an out-of-band rejection ratio greater than 41 dB. In the application demonstrations, the multitone or single-tone continuous-wave RF signals with different frequency components are applied to the PRF and then the desired components or waveforms are temporally separated and successfully recovered, verifying the multiple switching passbands with arbitrarily manipulated center frequencies and numbers over the frequency range from 0 to 12 GHz at a switching speed as fast as 1.73 ns.

Tunable Multiband Microwave Photonic Filters

The increasing demand for multifunctional devices, the use of cognitive wireless technology to solve the frequency resource shortage problem, as well as the capabilities and operational flexibility necessary to meet ever-changing environment result in an urgent need of multiband wireless communications. Spectral filter is an essential part of any communication systems, and in the case of multiband wireless communications, tunable multiband RF filters are required for channel selection, noise/interference removal, and RF signal processing. Unfortunately, it is difficult for RF electronics to achieve both tunable and multiband spectral filtering. Recent advancements of microwave photonics have proven itself to be a promising candidate to solve various challenges in RF electronics including spectral filtering, however, the development of multiband microwave photonic filtering still faces lots of difficulties, due to the limited scalability and tunability of existing microwave photonic schemes. In this review paper, we first discuss the challenges that were facing by multiband microwave photonic filter, then we review recent techniques that have been developed to tackle the challenge and lead to promising developments of tunable microwave photonic multiband filters. The successful design and implementation of tunable microwave photonic multiband filter facilitate the vision of dynamic multiband wireless communications and radio frequency signal processing for commercial, defense, and civilian applications.

A High-Power Low-Loss Continuously Tunable Bandpass Filter With Transversely Biased Ferrite-Loaded Coaxial Resonators

This paper presents a technology for high-power low-loss continuously tunable RF filters demonstrated by the example of a two-pole coupled-resonator filter. The resonators are shortened coaxial cavities loaded with ferrite inserts, where an externally applied transverse dc magnetic bias controls the center frequency. The filter is analyzed by simulations that take into account the inhomogeneity of the bias field. A prototype is manufactured and tested, exhibiting a tuning span of 41% in which the unloaded Q factor varies in the range from 1223 to 2425. Good correspondance is observed between simulation and measurement results. Furthermore, the power-handling capability of the filter is discussed and the ionization breakdown limit under normal conditions is estimated to be 800 W. The filter's linearity is characterized by a standard two-tone test, where the level of the third-order passive intermodulation product is observed to be -53.1 dBm at an input fundamental tone level of 2 × 43 dBm.

Tunable Filters? Reality Check Foreseeable Trends in System Architecture for Tunable RF Filters

The perpetual challenge of multiband/multistandard RF solutions for smartphones is to pack additional bands into the same footprint while also improving performance from one generation to the next. Are tunable filters the answer? No, not anytime soon. This article reviews leading trends in the RF front end of smartphones and provides a summary of realistic requirements for a hypothetical tunable RF filter to cover multiple bands and thus displace both switches and regular filters/duplexers. It also reviews the changes in system architecture required to address long-term evolution (LTE) carrier aggregation (CA). Multiple bands in simultaneous operation on a single antenna result in a large number of additional requirements for filters and duplexers. Isolation loss and linearity are among the hardest to meet. With RF front ends about to become more complex than ever, are tunable filters in our future? Maybe, but it depends on how you define “tunable”.

Integration of BST varactors with surface acoustic wave device by film transfer technology for tunable RF filters

This paper presents a film transfer process to integrate barium strontium titanate (BST) metal–insulator–metal (MIM) structures with surface acoustic wave (SAW) devices on a lithium niobate (LN) substrate. A high-quality BST film grown on a Si substrate above 650 °C was patterned into the MIM structures, and transferred to a LN substrate below 130 °C by Ar-plasma-activated Au–Au bonding and the Si lost wafer process. Simple test SAW devices with the transferred BST variable capacitors (VCs) were fabricated and characterized. The resonance frequency of a one-port SAW resonator with the VC connected in series changed from 999 to 1018 MHz, when a dc bias voltage of 3 V was applied to the VC. Although the observed frequency tuning range was smaller than expected due to the degradation of BST in the process, the experimental result demonstrated that a tunable SAW filter with the transferred BST VCs was feasible.

A 30-MHz–2.4-GHz CMOS Receiver With Integrated RF Filter and Dynamic-Range-Scalable Energy Detector for Cognitive Radio Systems

A 30-MHz-2.4-GHz complementary metal oxide semiconductor (CMOS) receiver with an integrated tunable RF filter and a dynamic-range-scalable energy detector for both white-space and interference-level sensing in cognitive radio systems is reported. The second-order RF filter has only two stacked transistors, and its use, in combination with a subsequent harmonic rejection mixer, results in wideband interference rejection. The energy detector with programmable rectifiers provides dynamic-range (DR) scalability, enabling shared use for white-space/interference-level detection and automatic gain control. A prototype chip, fabricated using 90-nm CMOS technology, achieved over 42-dB harmonic rejection including 7th-order component without any external device, a 67-dB gain, a 5-8-dB noise figure, a -11-dBm in-band third-order intercept point, and a +38-dBm second-order intercept point while drawing only 25-37 mA from a 1.2-V power supply. Multi-resolution DR-scalable spectrum sensing with a 0.2-30-MHz detection bandwidth, -83-dBm minimum sensitivity, and a 29-48-dB DR was demonstrated.

SDR front-end를 위한 Complex Bandpass Sampling

Bandpass sampling(BPS) 기술은 나이퀴스트(Nyquist) 샘플링 주파수보다 낮은 주파수를 사용하여 RF 대역의 신호를 직접 하향변환 할 수 있다는 장점을 가지고 있지만, 나이퀴스트 영역에서 self-image의 중복을 피하기 위해서는 샘플링 주파수의 선택에 제약이 따른다. 2개의 ADC를 사용하는 2차(second-order) BPS는 self-image를 제거하기 위한 신호처리가 추가 된다는 조건으로 샘플링 주파수의 선택이 자유롭다. 하지만 RF 대역이 바뀌면 신호처리를 위한 파라미타를 재구성해야 한다. 본 논문에서는 2차 BPS의 한 형태인 quadrature BPS의 구조를 가지면서, 재구성이 필요 없는 간단한 보상 필터만을 사용하여 임의 RF 대역으로부터 나이퀴스트 영역으로 하향 변환하는 complex BPS 기반의 SDR front-end에 대하여 기술한다. Bandpass sampling technique has an advantage that it uses lower sampling frequency than Nyquist criterion. But special care is required in choosing sampling frequency to avoid self-image overlapping in the first Nyquist region. Recently, the second-order BPS techniques which can suppress possible self-image by using an additional ADC and by employing digital signal processing have been proposed. This paper addresses a complex BPS based SDR front-end. Unlike general second-order BPS, it needs simple FIR filter to compensate delay in the second ADC. We show a method to find proper sampling frequencies to down convert RF signals selected by tunable RF filter operating in arbitrary frequency range.

Tunable RF Filters Fabricated Using MOCVD Deposited Graded Composition BST Films

Compositionally graded Barium Strontium Titanate (BST) thin films were deposited by MOCVD on r-plane sapphire substrates with patterned platinum electrodes. Tunable parallel plate capacitors were fabricated using these films by ion-milling and photolithography. The capacitors show a tunability of 36% for an electric field of + 13V at a measurement frequency of 1 MHz. These tunable capacitors were integrated with resonators fabricated on FR-4 substrates to form tunable RF filters. Characterization of the filters shows that the center frequency can be tuned from 3.710 GHz to 4.062 GHz by applying a tunable bias of 12 volts.

MOCVD Growth of Compositionally Graded BaxSr1-xTiO3 Thin Films

This paper describes an MOCVD technique for producing both monolithic and compositionally graded BaxSr1-xTiO3 (BST) thin films for tunable RF device applications. Specifically, we describe a “multiple injection flash evaporation” technique, which can easily produce BST thin films with a wide variety of compositional grading and dopant profiles. Preliminary results show the films to have excellent voltage standoff properties and tunability of dielectric permittivity. These materials are presently being used in the development of tunable RF filters for communication and radar applications.

Tunable and Reconfigurable Photonic Signal Processor With Programmable All-Optical Complex Coefficients

A new all-optical microwave photonic filter structure that can realize arbitrary programmable complex coefficients, multiple taps, and shape-invariant frequency tuning over the full free-spectral range (FSR) range is presented. It is based on a new optical RF phase shifter achieved by using a programmable wavelength processor (PWP) comprising an array of liquid crystal on silicon pixels. It manipulates the amplitude and phase of optical spectral components, which enables both single-sideband modulation and arbitrary complex coefficient function to be obtained simply by versatile programming the PWP only, without changing the rest of the structure. Experimental results demonstrate reconfigurable and tunable shape-invariant multitap RF filters with wideband tuning range over the full FSR by software programming of the complex coefficients.

Modal control of mechanically coupled NEMS arrays for tunable RF filters

A novel tuning strategy of nanoelectromechanical systems (NEMS)-based filters is proposed based on the modal control of mechanically coupled NEMS arrays. This is done by adjusting separately addressed distributed actuation and detection configurations proportionally to desired modal vectors. This control scheme enhances the global output signal, raising the power handling of the filter on all channels. Although the modal control of 1-D arrays exhibits narrow-band responses with adjustable resonance frequency, its application to 2-D arrays produces filters with both adjustable bandwidth and central frequency. One possible realization scheme is suggested by using electrostatically driven coupled NEMS arrays whose transduction gains are adjusted by changing the electrodes¿ bias voltages. Dispersion effects on both 1-D array and 2-D array frequency response are analytically expressed using eigenvalues perturbation theory. Based on these results, we show how to reduce their impact by appropriately choosing the coupling stiffness and the number of resonators.

A 5-GHz Wireless LAN Transmitter with Integrated Tunable High-Q RF Filter

A tunable, high-Q RF filter suitable for wireless transmitters has been implemented in a standard digital 0.18-mum CMOS technology. Active circuits are used to enhance the Q of two independent LC resonators, which are cascaded to form a wide bandwidth filter that can be tuned in both center frequency and bandwidth. A 5.145-GHz stagger-tuned filter with a 200-MHz bandwidth and 0.8 dB of ripple is demonstrated in an 802.11a sliding-IF transmitter. The transmitter provides spectral mask and EVM-compliant output power of -8.26 dBm for a 64-QAM OFDM signal. At lower output power, an EVM of -32.3 dB is achieved.

Optimized multistandard RF subsampling receiver architecture

This paper presents a novel subsampling-based down-conversion topology for multistandard radio receiver design. This receiver topology is based on two subsampling stages. The first stage has a fixed RF subsampling frequency; however, the IF sampling frequency of the second stage is variable and depends on the standard being considered. This approach overcomes various undesirable effects related to the sampling frequencies and noise aliasing. By optimizing the choice of the RF subsampling clock frequency, complete multistandard RF bands are down-converted to the same IF band. Quadrature baseband channel downconversion is achieved by a tunable IF sampling frequencies clock. A tunable band-pass RF filter and an IF band-pass filter are designed to perform anti-aliasing and limit wideband noise. A generic design methodology is proposed and validated through its application to a GSM, UMTS and IEEE-802.11g multistandard receiver. Simulation results of this receiver design example confirm the validation of proposed subsampling receiver topology and show the efficiency of the design methodology.

Non-contact electrostatic microactuator using slit structures: theory and a preliminary test

A new type of electrostatic microactuator with pull-in and pull-out behaviors has been presented based on nonlinear electrostatic force acting on a slit structure. Theory for the microactuator has also been developed to understand the nonlinear behaviors of the microactuator when a voltage is applied to the microactuator. Using dimensional analysis and finite element analysis (FEA), the nonlinear electrostatic force was investigated and was formulated in a simple closed-form equation whose coefficients are functions of the geometry of the microactuator. Afterwards, a force balance equation of the microactuator was set up and then an equation for the height of the movable structure at a voltage was derived. Theoretical analysis shows that without mechanical contact the microactuator suffers from a pull-out as well as a pull-in phenomenon. When the movable grating is spaced by a distance larger than a minimum distance, the pull-in and pull-out phenomena occur at the corresponding pull-in and pull-out voltages, respectively. In a preliminary experiment using a microactuator made of a 1.5 µm thick polysilicon, the pull-in voltage is measured to be 19 V. As a new type of electrostatic microactuator, microactuators using slit structures have potential applications in micro- and nano-systems such as actuators with a large travel range, micromirrors, variable capacitors, tunable RF filters and microaccelerometers.

High- κ dielectrically transduced MEMS thickness shear mode resonators and tunable channel-select RF filters

Electrically coupled, high quality factor ( Q), tunable channel-select ladder filters comprised of dielectrically transduced thickness shear mode resonators are presented using integrated circuit compatible bulk micromachining technology. The filters are fabricated on the 3.2 μm thick device layer of heavily doped SOI wafers with a 20 nm thick hafnium dioxide film sandwiched between the polysilicon electrodes and the silicon device layer. The ladder filter consists of shunt and series resonators operating in the half-wave thickness shear vibration mode. Dielectric transduction provides a k 2 reduction in motional impedance relative to air-gap electrostatic transduction. Each constituent resonator of the filter can be excited at above 810 MHz resonant frequency with Q of 7800 in air and a motional impedance ( R X ) of 59 Ω. The ladder filter demonstrates a center frequency tuning range 8–817 MHz and an adjustable bandwidth from 600 kHz to 2.8 MHz, while maintaining an insertion loss <4 dB, stop-band rejection >30 dB and pass-band ripple <2 dB. By having a tunability feature, RF MEMS filters can accommodate various signal waveforms with bandwidth range 0.1–5 MHz. In addition, errors due to fabrication can be compensated and capacitive loading in receiver architecture can be minimized.

A CMOS TV tuner/demodulator IC with digital image rejection

Superheterodyne TV tuners have been implemented in discrete forms using tunable RF and SAW IF filters. Integrating TV tuners in CMOS technology without them is a challenging task to cope with technical issues such as harmonic mixing and image. The image rejection in low- or zero-IF systems has been limited to 30-40 dB by analog imperfections such as I/Q path gain and phase mismatches. A single-chip low-IF TV tuner solution is proposed so that the image can be suppressed digitally using an image cancellation technique based on a complex one-tap LMS signal decorrelation algorithm. Programmable digital filtering and video/sound demodulation make a multistandard TV tuner feasible in the 48-860 MHz VHF/UHF band. The chip has a maximum gain of 63 dB and an input automatic gain control (AGC) range from -15 to 25 dB with 0.85-dB steps. It achieves an image and IF rejection of 60 dB, a peak carrier-to-noise ratio (CNR) of 55 dB, and a peak sound signal-to-noise ratio (SNR) of 44 dB without frequency modulation (FM) de-emphasis. The prototype occupies 6/spl times/6 mm/sup 2/ in 0.25-/spl mu/m CMOS and consumes 1 W at 2.5 V.