Isolating different waveforms in today's crowded and dynamic wireless spectrum requires adaptable communication systems. With the backends of wireless systems becoming highly flexible due to advances in digital processing, there is a need for a non-static front end. This project addresses the remaining bottleneck for fully tunable wireless systems by developing low loss reconfigurable filters for radio communication. The funding for the tunable filter projects comes from the Defense Advanced Research Projects Agency (DARPA), the Office of Naval Research (ONR), BAE Systems, Northrup Grumman, and MIT Lincoln Labs.

Many of our most recent results are summarized below. The title of each section is the title of a paper that can be found on ieeexplore or Google scholar. References to papers giving further detail about these designs and others from the past can be found in the student pages listed at the end of this page. Please email the authors if you have any questions or would like to discuss collaboration.

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

The Field Programmable Filter Array (FPFA) is a filter concept involving a "sea" of resonators that can be dynamically tuned in frequency, coupling type, and coupling strength to achieve semi-arbitrary filter responses between semi-arbitrary ports of the device. To inititially prove the concept, a 4-resonator device was designed and fabricated that had multiple modes of operation. This set of 4 resonators was able to be used as two independent filters with over 30 dB isolation between channels, a 3-pole filter, a 4-pole filter, or a switched filter bank without a switch that provided up to 65 dB of isolation between channels. Due to widely tuning positive-to-negative coupling elements, the filter shapes were also able to be tuned between self-equalized and elliptic filter responses, among others, in order to trade off near-band isolation for group delay. Future versions of the FPFA will include more resonators and more filter shape reconfiguration capabilities.

Fig. 1. Concept of 4-resonator FPFA. a) two independent 2-pole filters. b) 3-pole filter. c) 4-pole filter. d) switchable filter band without a switch

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

A substrate integrated evanescent-mode cavity bandpass filter was made to tune from 0.98 GHz to 3.48 GHz, a tuning ratio of 3.55-to-1. The filter was tuned through the use of piezoelectric actuators, which require very low power to provide actuation. The filter had 1.6 to 3.6 dB of attenuation in the passband, and a fractional bandwidth of 0.85% to 1.15 % over the tuning range. Filters with wide tuning ranges show promise to be integral parts of cognitive radios.

Fig. 2. Measured performance of the 3.5-to-1 tuning bandpass filter

High-Q Fully Reconfigurable Tunable Bandpass Filters 

High-Q (up to 750), 2-pole bandpass filters were made with the ability to electronically tune its center frequency over a wide range and each of its coupling values. One filter was able to tune in center frequency from 0.8–1.43-GHz, and the other tuned from 3.0 to 5.6 GHz. Having each coupling structure tunable allowed for tunable group delay (switching between a maximally flat and Chebychev response, shown above) and tunable/controllable bandwidth, allowing for a constant 25 MHz from 0.8 to 1.43 GHz. The designs also showed an innovative way to use low-Q tuning elements while retaining high resonator Q.

Fig. 3. Measured performance of the fully reconfigurable bandpass filter

Tunable High-Q Narrow-Band Triplexer

A tunable, narrowband triplexer was made for isolation of signals at different frequencies coming from the same antenna port. Each filter in the triplexer was tunable over the frequency range of 1.7 GHz to 3.4 GHz with a fractional bandwidth of less than 1%. The individual filters are able to be placed within 60 MHz of each other without strongly affecting the individual filters. A 6-plexer was also made with similar operating characteristics, but this data was not published.

Fig. 4. Measured performance of the tunable triplexer

Reconfigurable-Order Bandpass Filter for Frequency Agile Systems

A reconfigurable-order filter was made which could switch its response from a 2nd order response to a 4th order response, allowing a dynamic tradeoff between insertion loss and selectivity. It used commercially available varactors to tune inter-resonator coupling values between a very small value to values relevant to filter design. The filter was also tunable from 2.77 GHz to 3.55 GHz.

Fig. 4. Measured performance of reconfigurable order filter

The Use of High Q Toroid Inductors for LTCC Integrated Tunable VHF Filters

Using the IDEAS lab in-house full LTCC capability, high Q inductors were designed and integrated into the substrate for integrated VHF filters. These inductors were completely integrated into the LTCC substrate, allowing for tunable capacitiors and other electronic components on the surfaces of the circuit board. The inductor quality factors achieved were on the order of 60-75, and filters were made that tune over the band of 100 MHz to 200 MHz.

Fig. 5. LTCC integrated toroidal inductors in VHF filter design

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

Two tunable evanescent-mode cavity filters were made that use switch(es) to enable and disable source-to-load coupling. One filter used commercially available, solid-state switches in order to allow for the fastest possible switching between bandpass and bandstop responses. The other filter used a single switch from Radant MEMS in order to achieve superior bandpass filter isolation and lower bandstop filter passband loss. The response of the second filter is shown below in Fig. 6 for both the bandpass and bandstop states. The filter was tunable from 1.9 GHz to 3.8 GHz with as low as 2.4 dB loss in the bandpass passband as much as 54 dB isolation in the bandstop notch.

Fig. 6. Measured performance of a tunable bandpass-to-bandstop evanescent-mode cavity filter. a) bandpass response. b) bandstop response

Switchless Tunable Bandstop-to-All-Pass Reconfigurable Filter

A bandstop filter was made that had the capability to electronically tune in center frequency and attenuation level. The filter was tunable from 2.75 GHz to 3.1 GHz while providing 2.1 dB to 70 dB of attenuation. The theory for the effect of a finite design bandwidth and resonator quality factor was also introduced. This filter has the capability of tuning to a very low loss all-pass state, and it is expected to find its application in wide bandwidth systems which require signal equalization.

Fig. 7. Measured performance of a tunable bandstop-to-all-pass filter.

Bandpass–Bandstop Filter Cascade Performance Over Wide Frequency Tuning Ranges

A cascade of a tunable bandpass filter and a tunable bandstop filter was made that could provide up to 100 dB isolation between two frequencies of interest. While only tuning of the bandstop filter is shown below in Fig. 8, the bandpass filter is also fully tunable, allowing for up to 100 dB of isolation between two arbitrary frequencies. The theory of how to design such as cascade was also developed. More recent measurements with wider bandwidth bandstop filters showed 120 dB of isolation over a 21 MHz bandwidth at 2.8 GHz.

Fig. 8. Measured performance of a tunable bandpass-bandstop filter cascade. Both the passband and stopband are independantly tunable.

High-Q Tunable Bandstop Filters with Adaptable Bandwidth and Pole Allocation

In response to the need for wider bandwidth bandstop filters than the bandstop filters used in the bandpass-bandstop filter cascade shown in Fig. 8, a 4-pole bandstop filter was made that could dynamically trade equi-ripple isolation depth for wider bandwidth. This filter used a new loading post geometry that increased external coupling for a given coupling aperture size. It was also tunable from 2.4 GHz to 3.6 GHz and had the ability to split attenuation poles between various frequencies to optimize the response for the spectrum at hand. For example, the device could provide a 4-pole response, two 2-pole responses, or four 1-pole responses at different frequencies.

Fig. 9. Measured performace of adaptable bandwidth bandstop filter

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

A cascade of three 2-pole bandstop filters (6 total resonators) was made that could place a notch response anywhere in the continuous frequency range of 0.85 GHz to 6.6 GHz. One, two, three, and four pole responses were available over different parts of the tuning range. Frequency ranges with 4-pole coverage were specifically designed to be in bands with expected high interference. This work also produced a method for extending the upper passband of aperture-coupled evanescent-mode cavity filters to a 7.8 to 1 range, meaning that a bandstop filter at 1 GHz could have a clean passband up to 7.8 GHz.

Fig. 10. Measured 2-pole performace of 6 resonator bandstop filter cascade

Tunable Bandstop Filter with a 17-to-1 Upper Passband

In order to extend the upper passband of evanecent-mode cavity filters beyond the 7.8 to 1 ratio shown in the section above, a new external coupling structure was made to provide a bandstop filter response with minimal passband perturbation. This coupling structure routs the source-to-load transmission line of the filter through the cavity. The filter was tunable from 654 MHz to 1.6 GHz, and the passband extended beyond 11 GHz, leading to a 17 to 1 ratio between the lowest measured resonance and the 3 dB cutoff frequency of the upper passband.

Fig. 11. Measured performace of 17-to-1 upper passband bandstop filter