S.А. Babunko – Ph.D. (Eng.), Associate Professor, Head of Laboratory,

JSC SPE «Salut-27» (Nizhny Novgorod)

E-mail: babunkosa@salut27.ru

Yu.G. Belov – Dr.Sc. (Eng.), Professor,

Nizhny Novgorod State Technical University n.a. R.E. Alekseev

E-mail: bel266@nntu.ru

In this paper, we consider two DBPF designs on strip lines with coupled half-wave resonators. These are relatively simple designs that allow to get a narrow bandwidth for the differential signal. The first with predominantly electrical coupling of resonators. Implementing a second-order DBPF based on this topology does not provide good common-mode signal suppression.

The second design of a DBPF with predominantly magnetic coupling of resonators. For a given level of resonator coupling, magnetic coupling increases the gap between the resonators. This gap is sufficient for effective interaction of resonators in the differential mode. 

Mathematical modeling of both DBPF structures in the CST Studio package and experimental study of manufactured samples were performed. The joint review made it possible to give a comparative assessment of their characteristics, to determine their advantages and disadvantages.

DBPF with electrically coupled resonators.

The center frequency of the filter bandwidth for the differential signal is determined by the length of the resonator's strip conductor, which must be close to half the wavelength in the corresponding strip line. The value of the loaded q-factor of the resonators is determined by the degree of coupling of the resonators with the input and output strip lines. The input resistance is determined by the distance of the connection point of the input (output) strip lines to the loop resonator relative to the zero potential line, and the value of the coupling coefficient is determined by the width of the gap between the resonators.

Using the full-wave simulator CST Studio, the dependence of the loaded q-factor and the coupling coefficient between resonators on the width of the gap between the resonators was calculated. The designed filter was manufactured and its characteristics were measured using a 4-port vector circuit analyzer. The frequency dependencies obtained using mathematical modeling are in good agreement with the experimental data up to the frequencies of 8…9 GHz. This indicates the adequacy of the mathematical model of the filter built in CST Studio. 

The designed DBPF with an electrical connection of resonators has a fairly low level of insertion loss (about 0,8 dB) at the central  frequency of the bandwidth for the differential signal, but poor suppression of the common-mode signal (about 14 dB). 

DBPF with magnetically coupled resonators.

The main difference is that the coupling of the resonators with each other is magnetic, and with the input and output strip lines –  capacitive. The value of the connection to external circuits depends on the width of the gap between the supply line and the resonator. If the width of the conductor of this line can be selected within a wide range, then the choice of gap sizes and link length has a significant impact on the filter characteristics. The dependence of the coupling coefficient of the resonators on the width of the gap between them was calculated. 

A filter with the calculated geometric dimensions was manufactured and its transmission characteristics were measured in the same way as a filter with an electrical circuit connection. The same characteristics were obtained using mathematical modeling. The results of the calculation and experiment are in good agreement up to the frequencies of 6 GHz. 

Comparison of this filter with the previous one shows that in this design, the common-mode signal suppression has increased significantly: the measured value at the central frequency of the filter was 40 dB (an improvement of 26 dB). The bandwidth loss on the differential signal is quite small (1,28 dB), although slightly higher than the previous filter.

Babunko S.A., Belov Yu.G. Differential microwave band-pass filters with increased common-mode interference suppression. Achievements of modern radioelectronics. 2020. V. 74. № 4–5. P. 39–47. DOI: 10.18127/j20700784202004-03. [in Russian]

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