Improving the Second-harmonic Pass Band Rejection of Microstrip Side-coupled Filters

Filters are used in communication systems to reject unwanted signals from propagating through a network. These signals may be interferers from other systems or they may be generated by the nonlinear behaviour of components within the network. For instance, an amplifier driven into nonlinear operation will produce spectral components at harmonic frequencies.

Filters provide sufficient rejection at harmonic frequencies. Because of their low cost and easy manufacturability, side-coupled filters constructed from printed-circuit board (PCB)-based microstrip are widely used throughout the industry. One of the main problems of the side-coupled filter is its limited rejection of the second harmonic. This application note demonstrates a solution that mitigates this problem through the introduction of notch-filter elements that improve the overall rejection of side-coupled filters at the second harmonic without significantly affecting the filter-passband behaviour.