S.A. Khodenkov1

1 Reshetnev Siberian State University of Science and Technology (Krasnoyarsk, Russia)

1 hsa1982sibsau@mail.ru

The presented research results are obtained using numerical electrodynamic analysis of 3D microstrip filter models. When modeling, ceramic substrates with a relative permittivity ε = 9,8 and a standard thickness h = 1 mm were used. At the same time, both central frequency of the passbands f0 = 2 GHz and the order, chosen as the sixth one was fixed to compare the frequency-selective properties of all the investigated microstrip filters objectively.

Let us consider the first resonator of a six-section microstrip bandpass filter. Such a resonator forms an irregular strip conductor placed on a dielectric substrate, consisting of a pair of long segments parallel to each other, grounded to the base from the free ends, connected to short segments on the opposite side, enabling the conductor to bend. On the opposite side, the segments are connected to each other through a wide segment.

In total, the first filter design has six microstrip resonators oppositely directed and electro-magnetically coupled to each other, determining its selective capabilities.

A large jump in the wave impedance of its microstrip lines sections, organized in this and in other filter designs considered in this work, makes it possible to significantly expand the high-frequency stopband of the microwave device.

One should note that the relative bandwidth Δf/f0 of the first considered filter can vary at least from 10% to 30%. At the same time, the passband of such frequency-selective device is formed by six resonances – one lowest oscillation mode from each resonator. Thus, the resonators used in the first design are single-mode ones.

The second investigated design of a microstrip bandpass filter differs from the one discussed above in six oppositely directed resonators additionally connected to each other in a conductive way using five strip jumper segments in the design. Due to this, the relative bandwidth Δf/f0 of the second filter can vary at least from 30% to 40%.

The third design of a microstrip bandpass filter has similar resonators, also connected to each other electromagnetically and additionally in a conductive way, but they are located on the dielectric substrate in a co-directional manner. This allows one to further significantly expand the band-width of the filter under the study. In this case, the relative width of its passband Δf/f0 can vary from 40% to 50%.

In the fourth design of a microstrip bandpass filter, adjacent extended segments connected to each other through a jumper segment were replaced by single segments also grounded to the base from the free end. As a result of this transformation of a strip conductor, a resonator of this design can be considered multimode – its six lowest oscillation modes participate in the formation of its passband, and a filter based on such a resonator can be considered broadband, which is confirmed by the value of the relative bandwidth Δf/f0, which can vary not less than from 50% to 60%.

While studying the first three designs, the square shape of the wide sections of strip conductors of the first resonators was used and, similarly, the square shape of the extreme wide sections of the strip conductor of a multimode resonator was used in the fourth design. At the same time, the length of the wide sections of the first, second and third single-mode resonators in six-section structures should be slightly different. The length of the wide internal sections of the strip conductor in a sixth-section filter based on a multimode resonator can be equal.

It is worth noting that the considered filters can be tuned when using extreme wide sections of strip conductors in rectangular-shaped designs, and the latter can be «stretched» both along the x and y axis. Although, the longest high-frequency stopband is observed in filters with the shape of these segments close to square, which allows us to recommend it as the most optimal ones.

At the same time, the presented devices have high frequency-selective properties, and extended stopbands in particular.

Therefore, the possibilities of implementing sixth-order microstrip filters with high frequency-selective properties and passbands with a relative width Δf/f0 varying from 10% to 60% are shown. The designs use single-mode and a multimode resonators with folded strip conductors. It is shown that additional conductive coupling between single-mode resonators makes it possible to expand the bandwidth of filters based on them significantly. In this case, the bandpass filter based on a multi-mode resonator has the widest passband.

Khodenkov S.A. Investigation of microstrip bandpass filters. Achievements of modern radioelectronics. 2023. V. 77. № 12. P. 50–59. DOI: https://doi.org/10.18127/j20700784-202312-07 [in Russian]

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