Group Delay Variations in Wideband Transmission Lines: Analysis and Improvement
A. R. Eskandari1, L. Mohammadi2 

1A. R. Eskandari, Lecturer, Dept. of Electrical Engineering, Islamic Azad University East Tehran Branch, Tehran, Iran.
2L. Mohammadi, Faculty of Iran Telecommunication Research Center (ITRC), End of North Kargar, Tehran, Iran.

Manuscript received on August 18, 2011. | Revised Manuscript received on August 24, 2011. | Manuscript published on September 05, 2011. | PP: 122-128 | Volume-1 Issue-4, September 2011. | Retrieval Number: D0101081411/2011©BEIESP
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Abstract: Although poorly studied in the literature, Group Delay Variations (GDV) versus frequency is an essential factor which causes distortion and degradation in wideband satellite signals specially when using phase modulation and high data rates. In this paper, transmission line is analyzed as a dispersive medium and some kinds of coaxial cables such as RG58U, RG59U, RG213 and ECOFLEX15 are compared as GDV parameter point of view. Then the effect of reflections from discontinuities and impedance mismatches at transmission lines, on GDV quantity, is investigated by suggesting a novel network model of transmission line with discontinuity or impedance mismatch, and extracting a new formula for GDV. Graphical data are presented based upon the formula developed, and the simulation results are also given by AWR software which confirms the theory and formula. At last, based on the developed formula, some calculations will be carried out both to predict the values of GDV parameter and to compensate it. In this paper the frequency range of 100-1000 MHz is selected. The main reason of this selection is due to the practical application of coaxial cables for transmitting wideband satellite signals in remote sensing ground stations from down-converter to modem at IF frequencies such as: 140, 375, 720 MHz, etc. In addition, the introduced model and formula are generalizable to upper frequency bands.
Keywords: Group delay variations, transmission lines, coaxial cables, dispersion, discontinuity, mismatch.