Eric J. Naglich

Tuned to Resonance: Transfer-Function-Adaptive Filters in Evanescent-Mode Cavity-Resonator Technology

A number of promising technologies can be found today in the marketplace of reconfigurable filter ideas. They range from sub-mm-scale acoustic filters, lumped elements, two-dimensional resonators, and full three-dimensional solutions. From a system perspective, an equally diverse pool of communication, radar, electronic warfare, and sensing systems need reconfigurable filters. Despite a strong demand for such filters though, it is not easy to identify a technology that satisfies all requirements. While it is relatively straightforward to satisfy one or two important specifications such as low loss or high selectivity, it is often quite challenging to simultaneously satisfy all of them. For instance, this is particularly true when low power consumption, small form factor, and low loss become simultaneously critical decision factors. Several combinations of such factors can result in necessary design tradeoffs with no obvious solutions. Table 1 summarizes several common deciding factors in selecting a reconfigurable filter technology.

Quasi-Elliptic Multi-Band Filters With Center-Frequency and Bandwidth Tunability

A class of tunable multi-band bandpass filters (BPFs) that is based on a quasi-bandpass topology is presented. Unlike other approaches, it features a quasi-elliptic-type filtering response with controllable passbands in terms of center frequency and bandwidth. Additional transmission zeros (TZs) are produced through cross couplings that are introduced between adjacent quasi-bandpass sections through their non-resonating nodes (NRNs). These TZs are synchronously tuned with passband reconfiguration. The lack of direct interaction among the resonating nodes of this multi-band BPF architecture results in a transfer function with high tuning robustness. Furthermore, when compared to prior-art coupled-resonator multi-band BPF designs, a reduced number of inverters are needed when synthesizing a large number of transmission bands. For the first time, the coupling-matrix formalism of the engineered multi-band BPF is expounded. In addition, a microstrip dual-band BPF prototype with mechanically-variable capacitors is manufactured and characterized for experimental demonstration purposes.

Reflection-Mode Bandstop Filters With Minimum Through-Line Length

This paper presents a technique that enables the design of high-order microwave bandstop filters with a total through-line length of significantly less than one quarter-wavelength at the filter center frequency. The total through-line length using this technique can be zero regardless of the filter order in theory, but it is limited in practice by the finite line length required to obtain a desired coupling magnitude between the through line and a resonator. A fifth-order quasi-elliptic reflection-mode bandstop filter design is shown with total through-line length of less than 1/15th of a wavelength in suspended-stripline technology. The measured response has a 42-MHz 30-dB bandwidth at a center frequency of 3 GHz and four reflection zeros. The bandstop filter design technique described in this paper shows great promise for reducing the size and weight of microwave bandstop filters.

Short-Through-Line Bandstop Filters Using Dual-Coupled Resonators

A new approach using dual-coupled-resonator bandstop sections to realize microwave bandstop filters with arbitrarily short through-line length is proposed. In contrast to our recent work in 2015, this approach does not require the resonator-to-through-line couplings to be comprised of both electric and magnetic coupling, i.e., mixed coupling. An exact transformation from a conventional inline bandstop filter topology to a dual-coupled-resonator bandstop filter topology is presented. A design procedure is given for both all-dual-coupled-resonator designs and mixed (single-coupled and dual-coupled-resonator) designs. A fifth-order elliptic dual-coupled-resonator microstrip prototype is presented with a center frequency of 500 MHz and a through-line length of 6.35 cm, 17% the length of a conventional design. A fifth-order elliptic mixed-resonator microstrip prototype is presented with a center frequency of 500 MHz and a through-line length of 5.01 cm, 13.7% the length of a conventional design.

Microwave bandstop filters with minimum through-line length

The theory of bandstop filters with minimum through-line length is developed in this paper. In contrast to conventional similar-resonator bandstop filters that employ odd multiples of 90-degree impedance inverters between resonators, the proposed design technique enables filters with through-line length restricted to only that which is required to obtain a desired coupling coefficient magnitude between a resonator and the through-line. Both even- and odd-order designs are shown to verify the concept. A 4th-order microstrip prototype with 45 degrees of through-line length per resonator was fabricated, measured, and compared to two conventional designs. It has half the through-line length of conventional designs.

Power-dependent bandstop filters for frequency-selective limiting

The use of a power-dependent coupling structure that allows a cul-de-sac bandstop filter topology to continuously transform between a resonant all-pass response and a bandstop filter response with increasing input power is shown. In contrast to limiter devices that provide a wideband short circuit or ferrite resonance under high-power excitation, the concept presented in this paper provides the ability to design limiters with frequency-selectivity and without magnetic materials. For verification, a third-order power-dependent bandstop filter was designed and fabricated. It has a center frequency of 2.15 GHz, 3 dB bandwidth of 400 MHz, 1 dB limiting threshold of approximately 25 dBm, a response time of 10 ns, and provides over 16 dB of limiting.

Switched Allpass-to-Bandstop Absorptive Filters With Constant Group Delay

A new reconfigurable bandstop filter class is proposed, which has the unique property of possessing multiple insertion loss states with a common group delay state. These filters switch between a low-insertion-loss allpass state and a high-rejection bandstop state. The theory, synthesis, and design of these filters are presented. As a demonstration of the concept, a fourth-order constant-group-delay switched bandstop filter microstrip prototype was designed, built, and tested. The prototype has a measured bandstop-state center frequency of 1000 MHz, a 3-dB bandwidth of 119.15 MHz, a stopband rejection of 32.52 dB, a maximum allpass-state insertion loss of 4.08 dB, and a negligible group delay variation between the two states.

RF-Power-Activated and Signal-Tracking Tunable Bandstop Filters

Two new classes of RF-power-dependent tunable bandstop filters that share a common filter topology are presented in this paper. RF-power-activated tunable bandstop filters transition from a resonant-allpass response to a bandstop response in the presence of a high-power RF signal, allowing for optimum use of the spectrum when used to solve interference issues in a receiver. Signal-tracking tunable bandstop filters automatically tune their center frequencies to that of a high-power RF signal, eliminating the need for complex sensing and control systems. The theory and design as well as laboratory prototypes are presented for both. The RF-power-activated tunable bandstop filter prototype achieves a minimum allpass-state insertion loss of 2.5 dB and a bandstop-state attenuation of greater than 50 dB. The signal-tracking tunable bandstop filter prototype attenuates a high-power signal by greater than 33 dB over a frequency range of 251–304 MHz.