Design of Low-Cost Coupled Line Narrow-Bandpass-Filter of 0.1 dB Equal-Ripple Response for GSM-LTE Applications

In the domain of GSM (global system for mobile communications) operations, ensuring distortion-free communication is crucial in frequency compact environments. Despite the challenges associated with achieving ultra-narrowband bandpass filters (FBW less than 0.1%) using High-Temperature Super-conductor (HTS) microstrip configurations, this work focuses on the design of a narrow-bandpass filter utilizing a copper conductor microstrip due to its widespread availability and cost-effectiveness. The designed filter is a fourth-order coupled line filter with an equal ripple response, achieving 0.1 dB attenuation. The compact design is both analytically solved and simulated using Advanced Design System (ADS) software for schematic and layout results, incorporating simulations based on the method of moments. The results demonstrate a return loss above 19 dB and an insertion loss below 1.49 dB at a narrow fractional bandwidth of 0.1%. Furthermore, at the 3 dB insertion loss point, the filter covers 0.63% (5 MHz) of bandwidth. The filter is appropriately matched with a 50 Ω standard transmission line. This design is deemed implementable for GSM lower frequency and LTE (Long-Term Evolution) 750 MHz operational bands, addressing the need for effective narrow-bandpass filters for congested spectrum environment.