Then, the filtered signal is recombined through the output hybrid coupler and fed back to the input hybrid coupler for recursive operations. The input signal is divided by the input hybrid coupler and filtered separately. The balanced recursive structure, shown in Figure 2b, was proposed to enhance the reflection characteristics and reliability in the case of filter failure. Therefore, a new K-band absorptive bandpass filter (ABPF) that can simultaneously satisfy the requirements for high-performance and reliable operation must be investigated. Moreover, the above-mentioned filters are all exposed to the degradation of the return characteristics when the output load is varied. For K-band filters, several structures based on microstrip line resonators, RF-MEMS and GaAs MMIC have been proposed, but they are all reflective in the stopbands. Although there have been previous studies on reflectionless filters based on lumped elements or microstrip resonators, these techniques need to be investigated more to be adopted for K-band applications since the self-resonance characteristics of lumped components and requirements for high Q-factor increase the difficulty of realization. That is, absorptive (reflectionless) filters are highly desirable to avoid possible problems caused by out-of-band reflection combined with nonlinear devices, such as intermodulation products, gain ripple, and instability. However, conventional filters are only matched in the passband and are fully reflective in the stopband and transition band, resulting in unwanted interferences in multi-channel operations of HTS radio frequency (RF) chains configured with complicated nonlinear devices. In an HTS system, multiple narrow spot-beams with high power are used, resulting in the requirement for high-performance and reliable filters without causing performance degradation of adjacent beam elements, as is conceptually shown in Figure 1. With the rapidly increased demand for high data rate services, high-throughput satellite (HTS) systems have recently been considered as the key technology in the satellite industry. Furthermore, the stopband showed the reflectionless characteristic with the return loss being better than 7 dB. The measured results showed a 3 dB passband at 280 MHz with the center frequency at 19.9 GHz and improved roll-off characteristics. The proposed architecture was verified by comparing it with the performance of the conventional two-stage cascaded BPF. Furthermore, since the same BPFs were reused, the electrical filtering order within the given physical BPF stages could be increased effectively. Using the proposed structure, stable return characteristics that were insensitive to the output load variation in the passband, a reduction in standing wave due to absorption in the stopband, and potentially high reliability could be achieved. The proposed structure was configured using two passive microwave hybrid couplers, two conventional bandpass filters (BPFs), and a recursive path control module consisting of a phase shifter and an optionally variable gain amplifier. Here \(n\) is the order of the filter.This article presents a new K-band absorptive bandpass filter (ABPF) based on a microwave balanced recursive architecture. \): Odd-order Chebyshev lowpass filter prototypes in the Cauer topology.
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