Thinking Outside the Band: Absorptive Filtering

TL;DR

This paper introduces a reflectionless filter cell that reduces out-of-band issues, offers high stability, and is simple to design and implement, improving system performance and flexibility across various applications.

Contribution

The article presents a novel reflectionless filter cell with a third-order Inverse Chebyshev response that is easy to design, cascade, and integrate, enhancing system stability and reducing out-of-band problems.

Findings

Achieves 14.47 dB peak stopband attenuation

Exhibits an order of magnitude greater amplitude and phase stability than conventional filters

Allows simple cascading for increased attenuation

Abstract

The reflectionless filter cell described in this article alleviates many system problems associated with excess out-of-band gain, impedance mismatches, and component interactions. The simplest filter cell exhibits a third-order Inverse Chebyshev response with 14.47 dB peak stopband attenuation, and several can be cascaded for additional attenuation as needed. They are simple to design and easy to use - for much like small fixed attenuators ("pads"), they can be placed anywhere in the signal path desired without fear of causing unwanted standing waves, and instead reducing them where they already exist beyond the intended frequency range. Compared to conventional filter topologies with similar cutoff frequencies, they exhibit an order of magnitude greater amplitude and phase stability, ensuring accurate and repeatable complex gain in calibrated systems. Finally, the component requirements are moderate in value and minimal in number, easing their implementation while improving design yield, and extending the applicability of a given component technology beyond the normal range, both above and below.