Rigorous Design Approach for Bandpass Filter With Specified Wide Spurious-Free Upper Stopband

This paper presents a rigorous approach to design a bandpass filter with a wide spurious-free upper stopband. One of the unique features of the presented design method is that a spurious-free upper stopband range can be determined in advance. The presented design approach requires no coupling tests heavily relying on time-consuming electromagnetic simulations. For illustration, we have designed, fabricated, and measured a 4th-order microstrip Chebyshev filter. The second passband was defined to be at 8f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> (f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> : center frequency of the primary passband) so that the filter has a spurious-free region from f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> to 8f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf>.