Vikram Sekar

A 1.2–1.6-GHz Substrate-Integrated-Waveguide RF MEMS Tunable Filter

This paper presents a high-performance substrate-integrated-waveguide RF microelectromechanical systems (MEMS) tunable filter for 1.2-1.6-GHz frequency range. The proposed filter is developed using packaged RF MEMS switches and utilizes a two-layer structure that effectively isolates the cavity filter from the RF MEMS switch circuitry. The two-pole filter implemented on RT/Duroid 6010LM exhibits an insertion loss of 2.2-4.1 dB and a return loss better than 15 dB for all tuning states. The relative bandwidth of the filter is 3.7 ± 0.5% over the tuning range. The measured Qu of the filter is 93-132 over the tuning range, which is the best reported Q in filters using off-the-shelf RF MEMS switches on conventional printed circuit board substrates. In addition, an upper stopband rejection better than 28 dB is obtained up to 4.0 GHz by employing low-pass filters at the bandpass filter terminals at the cost of 0.7-1.0-dB increase in the insertion loss.

Miniaturized UWB Bandpass Filters With Notch Using Slow-Wave CPW Multiple-Mode Resonators

In this letter, a miniaturized, ultra-wideband (UWB) filter with notch using a slow-wave co-planar waveguide (CPW) multiple-mode resonator (MMR) is presented. By employing a slow-wave CPW MMR, the resonator length is reduced by 40% at mid-band frequency compared to the conventional CPW MMR. The proposed UWB filter employs stub-loaded microstrip-CPW transitions that improve the rejection skirt and defected ground structures (DGS) to provide better stopband rejection. A novel bridge structure is proposed to create a notch in the filter passband to reject WLAN interference. A passband from 3.1 to 10.6 GHz (110% bandwidth around 6.85 GHz) with a rejection notch at 5.65 GHz is achieved with insertion loss of 0.9 dB and return loss better than 10 dB. Using DGS, a rejection greater than 22 dB is achieved from 11 to 16 GHz while still having a 25% reduction in filter size compared to the conventional case. Design, simulation and measurement of filter prototypes are presented.

A Self-Sustained Microwave System for Dielectric-Constant Measurement of Lossy Organic Liquids

In this paper, dielectric constants of lossy organic liquids are measured using oscillation frequency shifts of a negative-resistance voltage-controlled oscillator (VCO). The design and working principle of the oscillator and the effect of material loss are presented in detail. The proposed method provides relatively large frequency shifts of 10-110 MHz for lossy test sample volumes of 50-200 μL whose dielectric constants are between 2-13 at 4.5 GHz, thereby allowing good resolution in dielectric-constant measurement. To make the system self-sustained, the VCO is used as part of a frequency synthesizer system for frequency-to-voltage conversion and digital extraction of the frequency shift using a unique detection algorithm. The dielectric constant of several organic liquids have been extracted to an accuracy better than 2% using sample volumes of 50-200 μL , and has excellent agreement with reported values. The applicability of this system for sensing dielectric mixtures has also been shown, and volume fraction estimation has been demonstrated to an accuracy of around 1%.

Tunable SIW Structures: Antennas, VCOs, and Filters

Due to their high quality factor (hundreds), high power handling, and good isolation, SIW structures are excellent candidates for a variety of microwave devices, including filters, antennas, VCOs, and isolators. However, their use has been hindered due to their narrow BW and high sensitivity to the fabrication process. Tuning the SIW-based microwave structures has the advantages of covering more bands, the possibility of post-fabrication fine-tuning, and less crosstalk sensitivity. However, the conventional tuning methods applicable to regular microwave structures are not effective for SIWs. Finding a way to tune these structures over a wide tuning range while maintaining their high Q seems to be very appealing and, at the same time, very challenging. To address all these points in more detail, the main focus of this article was on frequency-tunable/ reconfigurable SIW structures. The reported tuning methods applied to SIW structures so far are studied, and their pros and cons are discussed in detail. Also, separate libraries for SIW-based tunable filters, antennas, and VCOs are provided.

A Half-Mode Substrate-Integrated-Waveguide Tunable Filter Using Packaged RF MEMS Switches

A two-pole, half-mode substrate-integrated-waveguide (HMSIW) RF MEMS tunable filter with 28% tuning range around 1.4 GHz is presented. The filter occupies a total area of λg × 0.7λg @ 1.4 GHz and exhibits an insertion loss of 1.2-3.4 dB, return loss better than 11 dB and quality factor of 75-140 for all tuning states, while having excellent linearity. The filter maintains a constant absolute 1 dB bandwidth of 85 ±10 MHz over the filter tuning range. Circuit models for the tunable HMSIW resonator and a methodology for systematic filter design are also described in detail.

A novel compact dual-band half-mode substrate integrated waveguide bandpass filter

This paper presents a novel dual-band bandpass filter (BPF) using half-mode substrate integrated waveguide (HMSIW) technology for the first time. The proposed filter is at least six times smaller than its conventional substrate integrated waveguide (SIW) counterpart with similar filter specifications. A three-pole dual-band Chebyshev BPF with center frequencies of 1.05 GHz and 1.3 GHz is designed and implemented on RT/Duroid 6010LM substrate. The measured insertion losses are 1.7 and 1.8 dB and return loss is better than 12 dB for each passband. A notch at 1.14 GHz with rejection level of 50 dB provides excellent frequency separation between filter passbands. A transmission zero at 1.681 GHz provides a rejection level greater than 70 dB in close vicinity of the higher filter passband. Absence of even order resonances gives a rejection level greater than 40 dB from 1.45–2.71 GHz. To our knowledge, this filter is the state-of-the-art in dual-band BPFs using SIW technology.

A novel approach for dielectric constant measurement using microwave oscillators

In this paper, planar microwave oscillators are used to measure the dielectric constant of organic liquids for the first time. Dielectric constant of an unknown material is calculated based on the change in oscillation frequency caused by the material-under-test (MUT). A split-ring resonator (SRR) is chosen as the sensing element due its small area and high confinement of electric fields which makes it sensitive to permittivity changes of the MUT above the SRR. A C-band microwave oscillator prototype is fabricated on RT/Duroid 5880 substrate and calibrated using ethanol and methanol as reference materials. The dielectric constant of small quantities (< 20µL) of acetic acid, xylene, isobutanol and ethyl acetate are extracted from measurement of oscillation frequency shift and show good agreement with previously reported values.

Miniaturized half-mode substrate integrated waveguide bandpass filters using cross-shaped fractals

This paper presents a novel miniaturization technique for half-mode substrate integrated waveguide (HMSIW) bandpass filters using cross-shaped fractal structures. The overall filter area is reduced by 28% and 37% compared to conventional HMSIW bandpass filters when the first and second iterations of the cross-shaped fractals are used, respectively. In addition, the miniaturized filters exhibit lower insertion losses because the measured resonator unloaded quality factor increases from 221 for a conventional HMSIW resonator, to 258 and 264 for the first and second fractal iterations, respectively. To make the filter compact, a novel capacitive coupling mechanism is introduced in this work that results in improved upper stopband rejection due to a transmission zero above the filter passband. Two-pole, 4.5% Chebyshev bandpass filter prototypes with center frequency of 1.15 GHz are designed and fabricated on Rogers RT/Duroid 6010LM substrate. The filters have measured insertion losses <1 dB and return losses better than 11 dB, and are very suitable for compact wireless communication systems.

Pole-Perturbation Theory for Nonlinear Noise Analysis of All-Pole RF MEMS Tunable Filters

This paper presents a theoretical approach to predict the effect of nonlinear noise mechanisms in all-pole RF microelectromechanical systems (MEMS) tunable filters. It is shown that both nonlinearity and noise can be expressed as perturbations of poles of the filter transfer function. Perturbations in the bandpass filter are mapped into its equivalent ladder network as perturbations in the prototype element values. Closed-form equations are derived to calculate pole-perturbations in Butterworth and Chebyshev filters using prototype perturbations. The proposed method is then used to calculate the effect of nonlinear noise phenomena due to Brownian motion in RF MEMS tunable filters for different input power levels. As a result, the filter phase noise is calculated as a function of input power, tuning state, fractional bandwidth, filter order, and frequency offset. The effect of filter nonidealities and their implications on phase noise are discussed. Finally, it is shown that signal-to-noise ratio degradation due to filter phase noise is most significant in MEMS tunable filters with low bandwidth, high order, and high quality factor.

A review of frequency synthesizer-based microwave chemical sensors for dielectric detection of organic liquids

This paper presents a review of recently reported chemical sensors for dielectric detection at microwave frequencies. The sensors are based on a sensing element exposed to materials under test (MUT) as a part of a VCO inside a frequency synthesizer loop. The permittivity of the MUT is translated into a change in the free running frequency of the VCO and the DC control voltage of the loop which is read out using an Analog-to-Digital converter for further processing. Two different on-board and on-chip prototypes are presented and compared.