Hjalti H. Sigmarsson

High-

In this paper, the authors present a design technique that enables inter-resonator and external coupling control for high-quality-factor (Q) tunable bandpass filters. The design incorporates low-Q varactors as part of the inter-resonator and external coupling mechanisms without degrading the overall high Q of the original filter. Detailed design methodology and equations are presented to illustrate the concepts. A first-time demonstration of these concepts is presented for a widely tunable high-Q evanescent-mode cavity bandpass filter. The cavities are integrated in a low-loss substrate with commercially available piezoelectric actuators and solid-state varactors for frequency and bandwidth tuning. This technique allows for reduced bandwidth variation over large tuning ranges. As one example, a constant 25-MHz absolute-bandwidth filter in the 0.8-1.43-GHz tuning range with loss that is as low as 1.6 dB is presented as an example. The filter third-order intercept point is between 32.8 and 35.9 dBm over this tuning range. To further show the impact of the technique on high- Q filters, a filter Q that is as high as 750 is demonstrated in the range of 3-5.6 GHz, while using low-Q varactors (Q < 30 at 5 GHz for a 0.4-pF capacitance) to achieve more than 50% reduction in bandwidth variation over the tuning range.

Q

In the present work, a widely tunable high-Q air filled evanescent cavity bandpass filter is created in an LTCC substrate. A low loss Rogers Duroidreg flexible substrate forms the top of the filter, acting as a membrane for a tunable parasitic capacitor that allows variable frequency loading. A commercially available piezoelectric actuator is mounted on the Duroidreg substrate for precise electrical tuning of the filter center frequency. The filter is tuned from 2.71 to 4.03 GHz, with insertion losses ranging from 1.3 to 2.4 dB across the range for a 2.5% bandwidth filter. Secondarily, an exceptionally narrow band filter is fabricated to show the potential for using the actuators to fine tune the response to compensate for fabrication tolerances. While most traditional machining techniques would not allow for such narrow band filtering, the high-Q and the sensitive tuning combine to allow for near channel selection for a front-end receiver. For further analysis, a widely tunable resonator is also created with a 100% tunable frequency range, from 2.3 to 4.6 GHz. The resonator analysis gives unloaded quality factors ranging from 360 to 700 with a maximum frequency loading of 89%. This technique shows a lot of promise for tunable RF filtering applications.

Fully Reconfigurable Tunable Bandpass Filters

In this letter, the design and RF packaging of substrate integrated widely tunable filter is presented. The filter consists of two heavily loaded evanescent-mode cavities embedded into the substrate. The filter is actuated using two piezoelectric discs, which move thin, flexible membranes that form the top of the cavities. To demonstrate the wide tuning a filter is fabricated and measured to cover a very wide frequency range from 0.98 to 3.48 GHz (tuning ratio-3.55:1). The measured insertion loss is less than 3.57 dB for a 1.1% fractional bandwidth filter.

Highly Loaded Evanescent Cavities for Widely Tunable High-Q Filters

In this paper, we show a new bandstop filter circuit topology. Unlike conventional bandstop filter circuit topologies, the new circuit topology has inter-resonator coupling structures. The presence of these inter-resonator coupling structures enables convenient switching from a bandstop to a bandpass filter. Using the new bandstop filter topology, this paper demonstrates a design of a frequency-agile bandstop-to-bandpass switchable filter. The filter is composed of tunable substrate-integrated cavity resonators and can be switched to have either a bandstop or bandpass response. Switching is achieved by turning on and off switches placed within the filter structure. A prototype of the proposed design is fabricated and the concept is verified experimentally.

Substrate Integrated Evanescent-Mode Cavity Filter With a 3.5 to 1 Tuning Ratio

In this paper, we show a tunable inter-resonator coupling structure capable of varying the coupling coefficient between resonators from positive to negative values, including values that approach zero. The presented inter-resonator coupling structure can be tuned to generate large isolation between two resonators as well as the required coupling for filter responses. Using the inter-resonator coupling structure, this paper demonstrates the concept of a field-programmable filter array (FPFA). The proposed array is composed of tunable resonators and can have multiple functionalities by routing signals from input ports to output ports. Signal routing can be achieved by controlling the inter-resonator coupling with a wide tuning ratio of coupling coefficients. A unit cell of the proposed FPFA was fabricated to prove the proposed concept. It is shown that the unit cell can be adjusted to have filter array responses with two second-order bandpass responses, third-order bandpass responses, and fourth-order bandpass responses. The unit cell can also be operated as a switchable filter bank without a switch. All operation modes are verified by measurements. This paper also demonstrates, for the first time, a reconfigurable filter that can be tuned to have both elliptic and self-equalized responses using the presented inter-resonator coupling structure.

New Bandstop Filter Circuit Topology and Its Application to Design of a Bandstop-to-Bandpass Switchable Filter

A switchless bank of three tunable bandpass filters is demonstrated which can be used as a triplexer over a large tuning range. Three high Q narrow-band evanescent-mode cavity filters are integrated in a single substrate using an input feed-line network to achieve multi-band operation. Using shorted feed-lines for magnetic field coupling allows the out of band impedance to be dominated by the reactance of the feed-lines. The feed-lines are then designed to have high out of band impedance over the tuning range. A three-way switchless filter bank is presented in the 1.7 GHz–3.4 GHz range maintaining less than 4 dB loss for 14 MHz-31 MHz 3-dB bandwidth over most of the tuning range. The filter bank is fabricated in one low-loss Rogers TMM3 substrate using standard PCB processing, and commercially available 0.5mm thick piezoelectric actuators are used for the electrical tuning.

Tunable Inter-Resonator Coupling Structure With Positive and Negative Values and Its Application to the Field-Programmable Filter Array (FPFA)

This paper presents a tunable bandpass filter with a reconfigurable filter order. The proposed filter consists of an array of tunable resonators with variable inter-resonator coupling structures between adjacent cavities. This allows the frequency selectivity of the filter to be adjusted for the given spectral environment without any geometrical modifications to the filter structure. To demonstrate the concept a 2 − 2 resonator array is fabricated using substrate integrated evanescent-mode cavity resonators at 3.2 GHz. Each resonator has an adjustable frequency response and can be tuned from 2.77 to 3.55 GHz. The filter has two separate signal paths, either through two or four resonators. The reconfigurable inter-resonator coupling is implemented with floating vias inside a coupling iris. The vias are connected using variable capacitors and can be included or excluded from the iris by adjusting the capacitance. This structure shows the potential for the inclusion of reconfigurable-order filters in future agile radio systems.

Tunable high Q narrow-band triplexer

Upon the application of a magnetic field, ferromagnetic particles will self-align along the field lines of the applied field. Rods are thus formed that can grow until they are stopped by a substrate after application of a material and at the angle of the applied field. The dynamic growth of the columns is beneficial for implementing packages with complicated three-dimensional system integration. The growth of the rods, internal to the material, has the ability to adapt to the gap between pads or even an angle between the substrates. In this work, we demonstrate these advantages by showing multilayer interconnects as well as angled connections for three-dimensional vertical chip stacking concepts. Four layers of silicon on top of each other were integrated with multilayer interconnects and the RF performance is demonstrated. In addition, we implemented a novel three-dimensional packaging structure for a horizontal-to-vertical plane transition, with the performance evaluated for a variety of angles. Consequently, the Z-axis anisotropic conductive adhesive is shown to be a suitable solution to implement advanced three-dimensional integrated microwave applications.

Reconfigurable-order bandpass filter for frequency agile systems

In the present work, the authors present a design technique to create widely-tunable high Q narrow-band filters with bandwidth control by utilizing low Q varactors, while maintaining the high Q of the original filter. Detailed design methodology and equations are presented to illustrate the concept and a widely tunable high Q evanescent-mode cavity bandpass filter with controllable bandwidth is demonstrated as an example of this design technique. The filter is designed and fabricated in low-loss Rogers TMM3 substrate. A thin copper membrane is laminated on top of the substrate to allow frequency tuning, achieved using commercially available 0.38 mm thick piezoelectric actuators. The feed-lines with the impedance matching and the varactor based bandwidth control section are incorporated in parallel with the coupling iris on the back-side of the filter. A constant absolute bandwidth filter is also presented, which is tuned from 0.89 GHz to 1.47 GHz with constant 25 MHz (± 0.2 MHz) 3-dB bandwidth and less than 3 dB insertion loss over the entire tuning range. The extracted filter Q is in the range of 250–350, even though low Q varactors (Q ≪ 30@1 GHz) with capacitance in the 0.5 pF to 4 pF range are utilized for the bandwidth control.

Magnetically Aligned Anisotropic Conductive Adhesive for Microwave Applications

Dual-mode and/or dual-band microwave filters often employ high quality factor (Q ), physically large, and frequency static cavity resonators or low Q, compact, and tunable planar resonators. While each resonator type has advantages, choosing a dual-mode and/or dual-band resonator type is often limited by these extremes. In this paper, a new dual-mode and/or dual-band resonator is shown with Q (360-400) that is higher than that of planar resonators while being frequency tunable (6.7% tuning range) and compact relative to standard cavity resonators. In addition, both degenerate modes of the resonator are tunable using a single actuator. The resonator is used in a single-resonator two-pole filter design and a double-resonator dual-band filter design. An analytical model is developed and design techniques are given for both designs. Measured results confirm that the proposed resonator fits between the design spaces of established dual-mode and/or dual-band resonator types and could find application in systems that require a combination of relatively high Q, tuning capability, and ease of integration.