A new compact low-pass filter with broad stopband and sharp skirt characteristics

In this paper, we proposed a new compact microstrip low-pass filter with broad stopband and sharp skirt characteristics. The philosophy of the structure behind this novel microstrip low-pass filter is simple as it is composed of a pair of parallel coupled-line and an open-stub. With this configuration, a finite attenuation pole near the stopband cutoff frequency is available, and the notch frequency can be well controlled by adjusting the circuit parameters. The design formulas are derived by using an equivalent-circuit model of a parallel coupled transmission line. In order to validate the feasibility of the proposed design method, a 3-order Chebyshev low-pass filter with 0.01-dB ripple is designed, fabricated, and measured. Experimental results agree well with the simulation and analytical results.