Patent for "Reflectionless Filters" Issued to CDL Research Engineer

Filters are used in virtually all electronic systems to define the operating frequency range and prevent unwanted, out-of-band energy from propagating to downstream components. This is important to improve dynamic range, suppress spurious signals and image bands, and limit interactions between high- and low-frequency components. In conventional filter structures, however, this is accomplished by reflecting the stop-band portion of the spectrum back to the source, which can sometimes cause problems of its own, usually through interaction with frequency-converting elements like multipliers or mixers. These issues are increasingly important as technology trends toward broader-bandwidth components and more tightly-integrated systems.

A recent patent issued to Matt Morgan, CDL Research Engineer, concerns a novel circuit structure comprising a "Reflectionless Filter," wherein the stop-band energy is absorbed internally rather than being reflected back to the source. In microwave terminology, the input impedance is theoretically constant at all frequencies (pass-band, stop-band, and transition-band).

This prevents the build-up of out-of-band standing-waves and eliminates interactions between components operating at different frequencies. It also makes them cascadable, so that small filtering elements may be placed at multiple points throughout the signal path to optimize sensitivity and dynamic range.

These filters, which can be low-pass, high-pass, band-pass, band-stop, and even multi-band, are very easy to implement and have a number of advantages besides the reflectionless property for which they were developed. Despite exhibiting a third-order, inverse-Chebyshev response, all elements of a single type -- resistor, inductor, or capacitor -- have the same value, no matter what the order of the filter. Not only does this simplify design, fabrication, and tuning, it limits the required elements to more nominal values which extend the applicable frequency range for a given technology. The filter response is also more stable with respect to component variation (say, with temperature) than conventional filters. Finally, despite containing resistive elements, the insertion loss of these filters in the pass-band is nominally zero, and is degraded less by low-Q components than their conventional filter counterparts.