I.Е. Burlakov1, S.D. Krekov2, D.Yu. Kotsan3, Ya.F. Bal’va4

1,2 Siberian federal university (Krasnoyarsk, Russia)

3 JSC «NPP «Radiosvyaz» (Krasnoyarsk, Russia)

4 L.V. Kirensky Institute of Physics SB RAS (Krasnoyarsk, Russia)

1 burlakovie@kirensky.ru, 2 sdk@kirensky.ru, 3 Denis.kocan28@gmail.com, 4 ya.f.balva@iph.krasn.ru

When designing the payload of modern satellite communications systems, the question very often arises of developing analog narrowband bandpass filters, which must have high selectivity and at the same time have low frequency response flatness throughout the entire passband. In such applications, it is usually acceptable for filters to have significant insertion loss, since this will later be compensated by the power amplifiers. It is also possible to have a high level of SWR, which can be restored using valves at the filter inputs. In this case, as a rule, it is required to ensure high miniaturization, low weight, manufacturability in production and low cost. The latest requirements can significantly limit the developer in choosing design solutions.

The disadvantage of popular synthesis methods, such as [2], is that they do not take into account the losses that inevitably occur in the materials from which the real filter is made. The frequency response of such filters has sharp edges. Meanwhile, the frequency response of a real filter always has round-shaped transition regions, and their magnitude is greater, the lower the intrinsic quality factor of the resonators of this filter.

In this article, as a method for synthesizing a filter taking into account losses, we consider the use of the frequency response predistortion method [5], which makes it possible to implement filters with a low frequency response even with a low intrinsic quality factor of the resonators by increasing the insertion losses and increasing the SWR.

When losses occur in the filter, its zeros and poles of reflection and transmission change their position in the frequency plane relative to the case without losses. Thus, the filter's poles are shifted to the right by an amount depending on its center frequency, bandwidth, and quality factor. The predistortion method is based on a preliminary shift of the filter poles by an amount and a further search for filter reflection zeros. This method assumes that the transmission zeros remain in the same places. In reality this is not the case, but in the passband of the filter this is acceptable due to the dominant influence of its poles [6].

As an example, we compared the characteristics of a 6th order filter with a central frequency of 1000 MHz, a pair of symmetrical zeros at frequencies: 993,5 MHz, 1006,5 MHz with traditional and predistorted settings. The behavior of the uneven frequency response of the synthesized filter on its effective quality factor is analyzed. The frequency response improved from 4,05 dB to 1,87 dB for
Qeff = 3000 and to 0,59 for Qeff = 8000, the slope steepness improved from 1,2394 to 5,748 for Qeff = 3000 and to 7,073 for
Qeff = 8000.

Burlakov I.Е., Krekov S.D., Kotsan D.Y., Bal’va Ya.F. Use of the predistortion method when designing filters with low frequency response unevenness. Achievements of modern radioelectronics. 2023. V. 77. № 12. P. 17–25. DOI: https://doi.org/10.18127/j20700784-202312-03 [in Russian]

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