Large attenuation, compact, C-band, absorptive bandstop filter

Extremely deep notches can be realized using low order resonators

Communications Electronics

U.S. Navy engineers have invented an absorptive bandstop filter design for frequencies beyond UHF that is capable of creating large attenuation using resonators comprised of printed circuit and surface mount elements. The patented invention is available via licensing to companies that would make, use, or sell it commercially.

Schematic of the Navy’s Bridged-T absorptive bandstop filter.

Bandstop filters (BSF) are ubiquitous in signal processing. Most BSFs are reflective; so the stopband attenuation is limited to the quality factors that can be realized in the resonators.

Early absorptive/reflectionless BSFs were large in size due to the use of transmission lines. Later designs were miniaturized through the use of an equivalent lumped element model. For frequencies beyond UHF, the realization of these circuits can be difficult due to distributed parasitic and self-resonance effects from the numerous lumped elements.

The Navy-fabricated fixed frequency prototype and TRL calibration substrate is pictured.

The invention group has demonstrated an absorptive 4.1 GHz (C-band) bandstop filter with greater than 50 dB of rejection and the return loss is better than 6 dB over the whole band of operation. The total size of this prototype is 7 mm x 8.2 mm.

With the Navy inventive absorptive filter, two paths are introduced between the input and output ports and the signals are imposed to cancel each other by proper adjustment of the phase and amplitude. In this way, extremely deep notches can be realized using low order resonators. For phase cancellation, an impedance inverter with a bridged-T topology is used.

Measured and simulated S11 (return loss) and S21 (insertion loss).

In specific, the Navy BSF is comprised of a series bandpass resonator R1L1C1, a bandstop resonator R2L2C2, a high-pass filter comprised of two series capacitors. The resonators are designed to resonate at the absorption frequency.

The high-pass section then acts like a delay or a phase shift, which is adjusted for optimal cancellation. When the input signal is bridged between the two passive circuit paths and both the resistance and the reactance of the paths are equal, near-infinite attenuation can be achieved.

Numerical methods and parametric simulations are suitable to optimize the operation of the bandstop filter.

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