Juseop Lee

A Synthesis Method for Dual-Passband Microwave Filters

This paper describes a synthesis method for symmetric dual-passband microwave filters. The proposed method employs frequency transformation techniques for finding the locations of poles and zeros of a desired filter. This method can be used to design dual-passband filters with prescribed passbands and attenuation at stopbands directly without the need for any optimization processes. To validate the procedure a dual-passband stripline filter is designed and fabricated. The stripline dual-passband filter is designed with passbands at 3.90-3.95 and 4.05-4.10 GHz, and 30-dB attenuation at the stopband. This measured results show a good agreement with the theoretical ones. The frequency transformation for symmetric dual-passband filters is also extended to include asymmetric dual-passband responses. This flexible frequency transformation preserves the attenuation characteristics of the low-pass filter prototype. Examples are shown to discuss the flexibility of this transformation.

A dual-passband filter of canonical structure for satellite applications

Due to the complex arrangement of frequency plans and spatial coverages in modern satellite communication systems, channels that are noncontiguous in frequency may be amplified by a single power amplifier and transmitted to the ground through one beam. In this letter, a dual-mode canonical filter with dual-passband is presented. The filter adopts dual-mode technique for mass and volume reduction. Canonical structure is adopted for maximum zero realization. To validate the design technique, a six-pole dual-mode dual-passband filter of canonical structure for Ka-band (30/20GHz) satellite transponder is realized. The measured frequency response of the filter shows good agreement with the computed one.

Switchless Tunable Bandstop-to-All-Pass Reconfigurable Filter

The theory of a new type of bandstop-to-all-pass reconfigurable filter is developed in this work. A bandstop filter structure with both source-to-load and inter-resonator coupling is implemented. The synthesis equations are manipulated such that the signals in the filters resonators and source-to-load transmission line can be made to constructively or destructively interfere at the output port through tuning of the resonant frequency of the filters resonators. The relationship between resonator quality factor, filter bandwidth, and the all-pass response state is shown for the first time. The theory is proven through fabrication of a bandstop-to-all-pass filter with resonator unloaded quality factors greater than 500. Measured results show that the filter can continuously tune from insertion loss of 2.1 dB in the all-pass state to insertion loss of 69 dB in the bandstop state at the center frequency of the filter. Analog tuning of the attenuation level is also shown. The capability to switch from an all-pass to a variable-attenuation bandstop response enables a spectrally aware system to operate over wide bandwidths when interference levels are low and to dynamically add bandstop responses when interference affects its performance or signal equalization is required.

Design of Triple-Passband Microwave Filters Using Frequency Transformations

This paper introduces a synthesis method for triple-passband microwave filters. A frequency transformation is developed for finding the locations of poles and zeros of the triple-passband filter. The poles and zeros obtained as such are optimized to achieve a transfer function with a reduced number of transmission zeros in order to reduce the number of cross-couplings. Six- and 12-pole triple-passband filters are synthesized for validation of this proposed method. A 12-pole triple-passband filter is fabricated with a microstripline structure and shown to provide good agreement between synthesis and measurement results. Finally, the frequency transformation for asymmetric triple-passband filters is briefly discussed.

A Tunable Bandpass-to-Bandstop Reconfigurable Filter With Independent Bandwidths and Tunable Response Shape

The theory of bandpass-to-bandstop reconfigurable filters is developed in this work, and example filters are demonstrated. Designs are developed that allow for reconfigurable response shape and independent bandwidths in the bandpass and bandstop modes of the filter. These capabilities could potentially be useful in dynamic, open spectrums, concurrent transmit-receive systems, and multimode antenna array applications. The demonstrated filters were fabricated using substrate integrated evanescent-mode cavity resonators. The resonators are tuned using deformation of a copper membrane induced by a piezoelectric actuator. In measurement, a 1.06% 3-dB bandwidth bandpass filter with 2.6-dB passband insertion loss was switched to a 0.82% 3-dB S11 bandwidth bandstop filter with 45 dB of isolation.

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

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.

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

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.

Frequency Response Control in Frequency-Tunable Bandstop Filters

This letter presents a tunable bandstop filter with the capability of controlling the shape of the frequency response. The proposed filter structure can be tuned to have either a Butterworth or a Chebyshev response. The filter can also be tuned to have different center frequencies. Both the frequency response and the center frequency of the filter can be tuned by only adjusting the resonant frequency of each resonator without controlling inter-resonator coupling. Since the proposed filter structure has non-zero inter-resonator coupling, it can find its future application in reconfigurable filters which can be tuned to exhibit either a bandstop or a bandpass response.

An Analytic Design Method for Microstrip Tunable Filters

This paper describes an analytic design procedure for microstrip tunable filters. Step-impedance resonators are employed and loaded with varactors for achieving agility in the filter response. Fixed lumped capacitors are utilized as admittance inverters in order to minimize the number of varactors in the filter. An analytic approach for filter design makes it possible to achieve a tunable filter showing the same frequency response when the center frequency is adjusted. A two-pole microstrip tunable filter whose center frequency can be adjusted from 1.1 to 1.5 GHz is designed to demonstrate the validity of design theory. A prototype tunable filter operating from 2.1 to 2.7 GHz is also designed and measured. A good agreement between the measured and simulated results is shown. Finally, three- and four-pole tunable filters are designed to show straightforward application of the presented design method to higher order tunable filter design.

Extended Passband Bandstop Filter Cascade With Continuous 0.85–6.6-GHz Coverage

This paper presents a cascade of tunable bandstop filters with a wide spurious-free upper passband, which is completely spanned by the tuning range of the notch responses. A collection of resonators is shown to be able to provide bandstop filter responses over a 7.8 to 1 tuning range. By using spurious-free upper passband aperture-coupled cavity bandstop filters, multiple resonators, each with octave tuning, can cover a multioctave frequency range in a cascade. It is shown that the upper passband of this type of filter is limited by the reactance of the coupling apertures, which produce an unwanted in-band resonance unless designed properly. The details of this design process are explained and used to design a six-resonator bandstop filter cascade that is able to provide a bandstop filter response with up to 55 dB attenuation over the continuous band of 0.85-6.6 GHz. Through dynamic allocation of the cascade circuits transmission zeros, one-, two-, three-, and four-pole bandstop filter responses of variable bandwidth can be realized over different frequency ranges, offering numerous bandwidth-attenuation level tradeoff combinations.