Group Delay Variations in Microwave Filters and Equalization Methodologies

Abstract

In modern telecommunication systems, a constant group delay is necessary in order to avoid signal distortion. Unfortunately, a group delay variation is unavoidable. For example, a band-pass filter in microwave systems will create a group delay variation near the edge of pass band, i.e., in transition areas of the filter. According to simulations of ideal filter components (Q is infinite) in ADS, it shows a 5th-order Elliptic band-pass filter with stopband rejection of 20 dB and pass-band frequency from 7.5 GHz to 12.5 GHz would introduce a maximum group delay variation of 3.4 ns. System simulations in MATLAB show that it would cost extra power of 0.8 dB and 1.6 dB for 4QAM and 16QAM modulation respectively, to achieve the same performance (bit-error-rate) as the system without any group delay variation. Therefore, it is necessary to develop so-called group delay equalizers to compensate the variation of the group delay. Analog group delay equalizers are realized based on two methodologies: 1) allpass networks, which are used to generate a positive group delay with desired features; 2) a negative group delay circuit, which is used to compensate group delay variation originating from filters. Unfortunately, the existing negative group delay circuit suffers from a large attenuation in the frequency range where negative group delay occurs. In this thesis, a novel group delay equalizer circuit topology is proposed, to overcome the drawback of large attenuation. It is demonstrated that the proposed negative group delay circuit has a flat amplitude response with a variation less than 0.6 dB, keeping a feature of negative group delay. As example, the proposed equalizer is used to compensate the group delay variation originating from a low-pass filter with the pass-band from DC to 15 GHz. The simulation result exhibits that the group delay variation of the filter is reduced from 0.6 ns to 0.2 ns while the amplitude response is reduced less than 0.53 dB due to losses of the equalizer. For band-pass filters, one more stage of the proposed equalizer is needed in order to compensate the group delay variation in both sides of filters.