Examining the possibility of selectivity increasing of low-pass filter with linear phase response

Radio engineering, including TV systems and devices


Tikhomirov A. V.*, Omel'yanchuk E. V.**, Semenova A. Y.***

National Research University of Electronic Technology (MIET), 1, Shokin Square, Zelenograd, Moscow, 124498, Russia

*e-mail: radiotav@yandex.ru
**e-mail: omelia81@gmail.com
***e-mail: semenova.anastasia.y@gmail.com


The paper envisages the possibility of a low-pass filter sensitivity without its phase response deterioration.

At present, high-speed radio data transfer lines operating in perspective frequency bands (such as K-band), digital means for radio signal filtration, meeting the Radio Regulations requirement do not exist. In this case, additional analog filter is added to radio transmission path. This causes intersymbol interference (violating Nyquist condition). Intersymbol distortions occur herein, caused by non-linear character of phase responses of the applied LPFs. Thus, the necessity of data transfer speed increase of radio channel data transfer (and, correspondingly, the effectiveness of radio frequency spectrum implementation) determines the necessity of carrier shaping by the filter with maximum amplitude-frequency response slope.

The selectivity of the filters with group delay approximation is inferior to filters with amplitude response approximation. It to employing additional low-pass filters, deteriorating the group delay uniformity in the passband.

One can conclude that the filter calculation implies compromise between frequency response slope outside the passband and the group delay uniformity in the passband.

This problem can be solved by combining LPF with linear frequency response (such as Bessel, Gauss or Papulis filters) and notch sections with transfer function zeros outside the LPF passband to obtain the frequency response with greater slope without significant effect on phase response and, correspondingly, group delay. Implementation of notch elements with the rejection frequency selection irrespective of LPF cutoff frequency seems to be the most perspective technique.

On an example of implementing the fifth-order Bessel filter for filtering out spectral components of impulse signals, the authors prove that the proposed technique allows ensure gain in suppressing minor lobes of pulse signal, while preserving the group delay uniformity.


filter selectivity, linear phase response, low-pass filter, band-stop filter


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