Microstrip lowpass filter with flexible roll-off rates

This paper presents the design of a compact, ultra-sharp roll-off lowpass filter using stepped impedance modified T-type with stepped impedance stub (SMTS). The position of the transmission zero obtained due to the SMTS transmission line can be varied by changing the impedances and electrical lengths of the stepped impedance shunt stub (SIS) of SMTS and therefore, different roll-off rates can be obtained. The design equations used to obtain the parameters of the proposed SMTS structure are derived using the even and odd mode analysis and this enables the designers to calculate the parameters for a specific cut-off frequency (fC). Two SISs are connected, one at the input and another at the output side of SMTS to achieve good stopband characteristics. Finally, a lowpass filter with fC = 1.24 GHz and having a transition band of 0.11 GHz from 1.24 to 1.35 GHz with corresponding attenuation levels of 3-40 dB (roll-off rate is 336.36 dB/GHz) is designed, fabricated and tested. Good agreement is found between the simulated and experimental results.