Research On Chebyshev Bandpass Filter With Vertical Transition Structure

With the rapid development of 5G communication and internet time,filters play an increasingly important role in electronic information communication system.How to design a bandpass filter with high performance,low cost,multi-function,and easy integration has always been a hot issue in academic research.This dissertation focuses on the development trend of miniaturization,multi-band,ultra-wideband,anti-interference,and wide stopband,based on the microstrip-slotline vertical transition structure,six kinds of bandpass filters with different filtering characteristics are designed.The main innovations are as follows:1.Aiming at the problems of miniaturization design and the multi-band communication interferences of the filter,and a triple-passband filter based on the multilayer coupled structure is proposed.By utilizing the structural advantages of the multilayer dielectric board,the circuit size can be greatly reduced,and the space utilization rate of filter is improved.Using the spatial cross coupling of the half-wavelength open-loop resonators which make up the high passband,two finite-frequency transmission zeros are introduced at the both sides of the passband to improve the selectivity of filter.At the same time,the cross arrangement of quarter-wavelength short-circuited resonators constituting the lowpass band is used to form a zero-feeding hybrid coupled structure and generate another two transmission zeros.It not only effectively improves the passband isolation,but also makes the passband cut-off skirts steeper.The middle passband is composed of the stub-loaded stepped-impedance resonators.By adjusting the impedance ratio,the center frequency can be adjusted independently,which has high design flexibility and freedom.2.Aiming at the problems of out-of-band signal interferences and cut-off skirts of the filter is not steep,and a broadband bandpass filter with harmonic suppression is proposed.A broadband bandpass filter with six poles is realized by cascading two microstrip-slotline-microstrip vertical transitions.Without increasing the circuit size,by adjusting the electrical lengthθ₁ of the quarter-wavelength L-shaped open-ended slotline,combining with the technology of spurlines which etching on the feeding lines,the upper stopband can be widened to 3.86f₀ and the out-of-band harmonic suppression level can be improved to-20.63 d B.It greatly improves the out-of-band suppression level and greatly enhances the anti-interference ability of filter.In addition,the interdigital coupling capacitor on the top of filter successfully introduces two zeros at both sides of the passband,which effectively improves the slope of cut-off skirts and makes the filter have better selectivity.In order to further investigate the broadband filtering characteristics of microstrip-slotline-microstrip vertical transition,the L-shaped quarter-wavelength slotline resonators are replaced by the U-shaped half-wavelength slotline,by the microstrip connecting line on the bottom to construct a new broadband bandpass filter.Based on the equivalent circuits of odd-and even-mode,the locations of in-band six poles are obtained.According to transmission line network of proposed filter,the bandpass filter is synthesized in terms of prescribed specifications,and then the characteristic impedances of different transmission lines can be calculated,the initial physical size of designed model can be quickly determined.3.Aiming at the problem of in-band signal interferences of wideband filter,and a wideband Chebyshev equal-ripple responses bandpass filter with narrow notched band is proposed.Adopting four symmetrical microstrip-slotline vertical transitions to establish a wideband bandpass filter.Utilizing synthesis method based on transmission line network,in terms of fractional bandwidth and in-band equal-ripple factor,the initial values of physical model design parameters can be calculated.After fine-tuning,the final size of model is determined so as to achieve broadband Chebyshev equal-ripple responses.By embedding a half-wavelength slotline resonator in the middle of CPW connecting line,a narrow notched band with adjustable center frequency is introduced in the middle of passband,which can effectively suppress the interference signal in passband,so that the filter has stronger anti-interference ability.This structure takes the advantages of the hybrid transmission lines to achieve vertical transitions without via holes from the top to the bottom,and it effectively increases the freedom and flexibility of filter design.Compared with the traditional method of using metal via holes to connect the interlayer signal,it not only simplifies the process and saves the processing costs,but also effectively eliminates the parasitic coupling effects caused by metal via holes and reduces the loss of signal transmission.4.Aiming at the problem of poor frequency selectivity and the upper stopband harmonic suppression of the filter,and a Chebyshev responses bandpass filter with high selectivity and wide stopband is proposed.Through the source-load coupling effect of the top-layer microstrip,the finite-frequency transmission zeros are introduced at both sides of the passband,which effectively improve the out-of-band frequency selectivity and make the cut-off skirts of filter steeper.Taking advantage of the easy integration of CPW connecting line,two groups of lowpass filters with different center frequencies are embedded without increasing the circuit size.Making full use of frequency cut-off characteristic of the lowpass filter,two additional finite-frequency transmission zeros are introduced into the upper stopband,which not only effectively suppresse the out-of-band parasitic harmonics,but also further expand the bandwidth of upper stopband of the filter.At the same time,in order to ensure that the in-band Chebyshev equal-ripple responses are not affected,a synthesis method is carried out based on the equivalent circuit of lowpass filter to keep the same in-band Chebyshev equal-ripple response as the original filtering passband.5.Aiming at the problems of low common-mode signal suppression level and poor upper stopband suppression performances of balanced filter,a wide stopband Chebyshev responses balanced filter with high common-mode suppression is proposed.Using the inherent common-mode suppression characteristics of slotline resonator,the common-mode signal is fully suppressed over whole frequency range,and the design of balanced filtering circuit is simplified.Utilizing the synthesis approach with microstrip-slotline vertical transition structure,the Chebyshev equal-ripple responses with in-band four transmission poles are realized.In order to further improve harmonic suppression characteristics and expand the upper stopband bandwidth,two groups of defected ground structures are etched on the ground plane below the microstrip feeding line to form a lowpass filter.Making full use of the frequency cutoff characteristic of lowpass filter,a finite-frequency transmission zero is successfully introduced in upper stopband of differential-mode passband,which effectively enhances the suppression level of out-of-band parasitic harmonics.