Tunable bandstop filter that eliminates unwanted remixing at the input port

Increased design flexibility and ease of implementation


U.S. Navy engineers have invented an easy to implement, fully tunable reflectionless bandstop filter design using L-Resonators. The patented invention is available via license agreement to companies that would make, use, or sell it commercially.

Four-stage design of a fully tunable reflectionless bandstop filter.

Bandstop filters (BSF) are ubiquitous in signal processing. Most BSFs are reflective; they reflect the unwanted portion of the signal back towards its input port; this leads to the unwanted portion remixing at the input port – undesirable in almost all applications.

L-Resonators–due to their ease of design and implementation–are often used in BSFs at RF frequencies. Until now, BSFs based on L-Resonators were all reflective.

The invention group has demonstrated a fully tunable C-Band (3.96 – 4.4 GHz) reflectionless bandstop filter using L-Resonators with greater than 20 dB of in-band rejection and a return loss of better than 8 dB across the full tuning range.

The Navy-fabricated prototype of the 4-stage design.

The Navy invention uses a tunable quarter-wave L-Resonator with a resistor and capacitor to create a reflectionless band-stop filter. The quarter-wave resonator is tuned to the frequency that the signal should be filtered at using a varactor diode and voltage supply.

Once tuned, the quarter-wave resonator is able to reflect the portion of the input signal that is to be filtered, leading to an attenuation of the input signal at the tuned resonant frequency. The capacitor and resistor that are connected to the quarter-wave resonator are able to absorb and dissipate the reflected portion of the input signal, preventing it from remixing at the input port.

Measured 3-dB Fractional Bandwidth, Resonant Frequency, and Bias Voltage.

In order to achieve a reflectionless filter, the resistance value of the resistor (R) cannot be chosen arbitrarily. In order to determine R, a simulation of the BSF is created. Once created, the simulation is run for varying values of frequency and resistance to generate results in the context of insertion and return losses.

Multiple quarter-wave L-Resonators may be used in series for multiple filtering passes in order to reach the desired amount of attenuation.

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