Response Diversity of Stub-Loaded Ring Bandpass Filter Based on Commensurate Line Element: Single- and Dual-Band Applications

Abstract

Response diversity of a commensurate line bandpass filter is described and discussed. It is found that a stub-loaded ring topology can generate different bandpass responses in a single band or two symmetric bands, depending on the line admittances in the topology. For a single band, the response can be designed to be an equiripple delay, Chebyshev in magnitude, or elliptic in magnitude. For dual bands, the outband attenuation curve can be adjusted with almost unchanged in-band matching. A generalized synthesis algorithm, supporting both the single-and dual-band applications, is given to calculate the admittance of each line element, according to the prescribed in-band specifications, e.g., ripple factor and cutoff frequencies. Further study reveals that the admittance of the line segment between the stubs is the key element dominating the response type in the single-band case. When the admittance is positive, zero, and negative, the response is an equiripple delay, Chebyshev in magnitude, and elliptic in magnitude, respectively. To validate the synthesis algorithm, four filter sets with different responses are given and simulated through a transmission line model. For circuit verification, a microstrip filter with a single-band elliptic response is designed, fabricated, and measured, of which the negative admittance line is realized by a coupled line section with shorted ends. The experimental results show that the filter achieves elliptic response successfully. The measured 3-dB fractional bandwidth is 86.6% centered at 0.97 GHz, within which the minimal insertion loss is 0.88 dB at 0.81 GHz. Furthermore, two transmission zeroes beside the passband are observed clearly at 0.31 and 1.59 GHz, offering a sharp rejection in the stopband. INDEX TERMS Bandpass filter, wideband filter, filter synthesis, transmission zero, dual-band.

Equiripple design of low‐pass filter using stub‐loaded Z‐shaped coupled‐line section

“…The design procedures are similar to that of order seven. In our previous works [7][8][9], the NE equals the number of unknown impedances; therefore, one can predict the response by using the chosen F ideal before the step of equation solving. However, this benefit is invalid here due to the complicated expressions of k i in (1).…”

“…Coupled lines can be widely found in microwave-distributed circuits [1], such as filter [2][3][4][5][6][7][8][9], coupler [10][11][12], phase shifter [13,14] and impedance transformer [15][16][17], due to its controllable coupling strength and ease of planar implementation [1]. In practical circuits, coupled lines appear generally as 'sections', which can be categorized by the number of ports, connecting topology, and ending condition [18].…”

“…Figure 1 illustrates the three mostly used CLSs for a filtering application. The first two can be widely found in bandpass filter designs from narrowband [2] to ultra-wideband [3,[6][7][8][9]. The third is well known as the Schiffman unit [13], which can be used for phase equalization [14] or coupling enhancement [12].…”

A Varactor-Based Very Compact Tunable Filter with Wide Tuning Range for 4G and Sub-6 GHz 5G Communications

“…As part of MW filters, microstrip (planar) filters also play an important role in RF front-end systems of the current and future wireless applications because of size, cost, weight and hardware realization benefits. In order to obtain different characteristics, several planar filters with specific performance have been studied and implemented, such as low-pass filters (LPFs) [ 10 ], bandpass filters (BPFs) [ 11 ], wideband BPFs [ 12 , 13 ], high-pass filters (HPFs) [ 14 ], balanced BPFs [ 15 , 16 , 17 ], band-stop filters [ 18 ] and multi-band BPFs [ 19 , 20 ].…”

Synthesis Design of Equal-Ripple and Quasi-Elliptic Wideband BPFs With Independently Reconfigurable Lower Passband Edge

“…One of the effective methods is to replace single-section circuited stubs by their multisection counterparts [6]. Besides, TZs also can be generated by exploiting two transmission paths [7]- [8], introducing I/O cross couplings [9], and using shunt short/open circuits at designated TZ frequencies [10]- [11]. In addition, other methods for introducing TZs include using the bandgap structure [12] and intrinsic zeros of a special multi-mode resonator [13]- [15].…”