Filters used in communication systems must reject unwanted signals from propagating through a network. One instance of these unwanted signals occurs when driving an amplifier into nonlinear operation, thereby producing spectral components at harmonic frequencies. Although microstrip side-coupled filters are widely utilized, they provide limited second-harmonic rejection.
In a four-page application note titled, "Improving the Second-harmonic Passband Rejection of Microstrip Side-coupled Filters," National Instruments demonstrates a solution to improve the second-harmonic rejection of this filter class. This technique incorporates notch elements into the design of a side-coupled filter without significantly affecting its passband response.
The application note first presents a side-coupled filter that’s built from several sections of coupled transmission lines. Each section in this filter is offset from the previous one, leading to a wide footprint. The NI AWR Design Environment filter synthesis tool is used to create the initial filter design based on user-defined characteristics, such as passband frequencies, passband ripple, and stopband rejection.
To achieve a narrower footprint, transmission lines are added between the coupled-line pairs in the design example. This, in turn, allows the filter to be housed in a narrower waveguide, eliminating the possibility of second-harmonic waveguide-mode propagation. To implement notch filtering, open-circuited microstrip stubs are added to the design. These stubs are a quarter-wavelength long at the second harmonic of the passband’s center frequency, enabling the unwanted second harmonic to be rejected.
Thanks to the narrower footprint, the filter can be housed in a WR-22 waveguide. The app note presents the filter’s frequency response both with and without the waveguide. The results demonstrate greater than 50-dB second-harmonic rejection. In addition, coax connectors are modeled using the Analyst three-dimensional (3D) finite-element method (FEM) simulator in the NI AWR design environment. The coax connecters are then incorporated into a complete simulation model, which is presented along with the simulation results. Lastly, for comparison purposes, the same filter is simulated inside a WR-42 waveguide. In this case, the second-harmonic rejection is 30 dB worse due to the waveguide effect.
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