An interdigital resonator approach for wideband filter applications in ridged-waveguide technology is presented. The interdigital arrangement of the ridged-waveguide resonators ensures stronger coupling between the resonators. As the coupling sections are consequently enlarged by the interdigital arrangement of the resonators, more feasible filter structures are possible at increasing frequencies. The approach itself can be easily implemented with conventional filter synthesis formulas, which is demonstrated by two 20 GHz examples with a bandwidth of 2 GHz and 100 MHz, respectively. The designed filters are subsequently compared to the standard implementation of ridged-waveguide filters.

The new communication standards of the fifth and sixth generations (5G/6G) will lead to very high data rates in the gigabit range, which will be in particular realized by the use of frequencies in the millimeter-wave regime. In order to separate different signals from each other, not only a good spurious passband suppression, but also wide passbands of the used filters are necessary. Conventional

A solution for a good spurious passband suppression with better geometric dimensions are single-ridged waveguide filters, which consist of ridged-waveguide resonators, coupled by rectangular hollow-waveguide sections below the cutoff frequency. Closely related to the concept of broadband combline filters

In this contribution, the beneficial size of ridged-waveguide filters is combined with the advantages of the interdigital arrangement of the resonators, known from the combline technology. The consequential stronger coupling between the resonators allows longer coupling sections, which is highly beneficial for the fabrication process of broadband filters with good spurious passband suppression. First the coupling of interdigital ridged-waveguides in comparison to the conventional constellation is investigated. The concept is then integrated into the filter synthesis. Subsequently, a broadband and a narrowband filter are computed in the interdigital technology and compared to the conventional ridged-waveguide filters, afterwards. Both interdigital filter versions possess a very good spurious passband suppression and the dimensions guarantee good producibility. This proves the benefit of an interdigital resonator arrangement for wideband filter structures.

The general equivalent circuit for bandpass filters in waveguide technology is presented in Fig.

Bandpass equivalent circuit.

Cross section of ridged-waveguide.

The next step is to use the shown equivalent circuit for the implementation of single ridged-waveguide filters. The cross section of a ridged-waveguide is depicted in Fig.

Length cut through ridged-waveguide filter model.

Conventional coupled resonators.

Scattering parameters of resonators.

Interdigital coupled resonators.

For reliable fabrication, the coupling between the resonators has to be increased. With stronger coupling, the evanescent, rectangular hollow-waveguide sections can be extended without losing the desired filter characteristic.
Simple methods for investigating the coupling between two resonators are measurement or simulation, respectively. If all spurious resonances are ignored, a single, uncoupled resonator possesses one resonance peak at the resonance frequency

Conventional prototype.

Input and output were realized through coaxial feeding pins, which were loosely coupled to the resonators, in order to reduce disturbing coupling effects. The resonators for both structures where designed for a resonance at 12 GHz and the coupling length

Measured transmission of the built prototypes.

Since the first measurements showed good results regarding the improved coupling, a more detailed comparison between the conventional and the interdigital coupling was carried out. With respect to the theoretical filter synthesis, the influence of the coupling length

Comparison of inverter impedances.

As it can be seen, especially short coupling sections deliver very drastic differences in the realized inverter impedance. For sections of around 1 mm, the impedance of the interdigital approach is more than two times the impedance of the conventional approach. With increasing values for

In order to compare the different filter approaches, a Chebyshev bandpass filter of order

Simulation of passband of wideband filter.

Broadband filter dimensions of the different approaches.

The boundaries for the

Simulation of spurious behaviour of wideband filter.

Figure

Simulation of passband of narrowband filter.

In the following, the dimensional differences for a 100 MHz narrowband filter are investigated. The scattering parameters of the passband are depicted in Fig.

Narrowband filter dimensions of the different approaches

For narrow passbands, the coupling between the resonators is significantly smaller than for its broadband counterpart. With regard to Fig.

In summary, this bandwidth comparison makes it clear that the use of interdigital resonators should only be used for wideband applications. For narrowband applications, the conventional method is slightly better and shows a larger insertion loss in the upper frequency regions.

Simulation of spurious behaviour of narrowband filter.

Ridged-waveguide filters are outstanding for bandpass filters with large midband frequencies as well as good spurious passband suppression. Due to the concept of these filters, large bandwidths lead to filter dimensions that are problematic for a reliable fabrication. In this contribution, the use of interdigital resonators within the technology of ridged-waveguide filters was investigated. With the goal of a more dependable fabrication, the presented resonator constellation leads to a stronger coupling between the resonators, which consequently guaranetees more suitable filter dimensions. This can be traced back to an increased length of the coupling structures. Regarding the theoretical synthesis of these filters, special attention was given to the impedance values of the realized impedance inverters. The presented concept was varified by measurements of two coupled-resonator prototypes, as well as the simulation of two conventional as well as interdigital ridged-waveguide filters with a midband frequency of 20 GHz with a bandwidth of 2 GHz and 100 MHz. The synthesized broadband interdigital filter exhibits the desired passband as well as spurious passband suppression, while it maintains filter dimensions, which are beneficial in terms of fabrication.

The underlying research data can be requested from the authors.

AP and JW conceived the presented idea. JW implemented the numerical algorithms for electromagnetic computation and filter synthesis. JW wrote the manuscript in consultation with AP, MGE and TFE. TFE supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

The contact author has declared that neither they nor their co-authors have any competing interests.

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This article is part of the special issue “Kleinheubacher Berichte 2021”.

This work was supported in part by the Bayerisches Staatsministerium für Wirtschaft, Energie und Technologie (StMWi) under grant ESB064/002.

This paper was edited by Romanus Dyczij-Edlinger and reviewed by two anonymous referees.