Improved microstrip coupled-line bandpass filters for spurious responses suppression

In this paper, microstrip coupled-line bandpass filters using a improved SIR configuration are proposed. Conventional microstrip coupled-line filters are very popular for RF front ends because they can be fabricated easily. However, their large harmonic causes the shape of the passband to be asymmetric in the upper band and it worsens the skirt properties. By proper design, the harmonic of improved filters can be significantly suppressed through the use of improved SIR configurations. There are more than 20 dB suppression at 2f₀ and 3 f₀. This type of filter can be used to suppress the harmonic without any additional devices and regardless of the substrate.