Enhancement of the Stopband Response of a Wideband Bandpass Filter by Cascading a Lowpass Filter to it
This paper presents the design of a wideband bandpass filter and discusses improving it's stopband characteristics by cascading a lowpass filter to it. The bandpass filter has a fractional bandwidth of over 100% coupled with a very steep rejection. The 3dB passband is 1GHz-3.5GHz, along with insertion loss requirements of 20dB at 0.8GHz and 40dB at 3.8GHz. Ripple in the passband should not exceed 0.1dB. Lines and quarter wavelength shorted stubs were used to realize the filter. On simulation of the design parameters of the filter, first spurious band appeared at twice the center frequency due to realization on an inhomogeneous medium such as microstrip .To eliminate these spurious harmonics additionally a lowpass filter was designed and cascaded to the actual bandpass filter. The paper explains how this lowpass filter is designed and the simulation results of the bandpass filter with and without the lowpass filter cascaded.
Keywords - Insertion Loss Method, Return Loss, Shorted Stub-Line, Microstrip, Low Pass Filter, Band Pass Filter