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Fundamentals of Digital Television Transmission phần 6

Dốc quá trình chuyển đổi từ dải thông để stopband có liên quan với những thay đổi nhanh chóng trong giai đoạn đối với tần số. Vì vậy, hoàn toàn phẳng trong ban nhạc giai đoạn phản ứng là không khả thi. Trong thực tế, một lượng nhỏ của biên độ và gợn giai đoạn phải được dung thứ trong suốt dải thông. | 130 TRANSMISSION LINE FOR DIGITAL TELEVISION The frequency response of coaxial lines is simply the slope of the attenuation versus frequency curve. Reference to Figures 6-1 and 6-2 shows that the response tilt of coaxial lines is dependent on frequency and the length of line. In general the response tilt is small. In mathematical terms the slope of the frequency response is the first derivative of the formula for attenuation with respect to frequency or da df D 2Af 1 2 C B dB 100 ft per megahertz From the foregoing graphs and this formula it is apparent that the response tilt is greatest at the lowest frequencies. For example 38-in. rigid line operating at U.S. channel 2 the response tilt is 0.0007 dB per 100 feet per megahertz. Even a 2000ft run would exhibit only 0.09 dB over 6 MHz. Thus for all practical purposes frequency-response tilt may be ignored in rigid coaxial lines. For those who wish to do so this amount of tilt could be preequalized. This could be accomplished using either analog IF equalizers or programmable digital equalizers using the equation above and the appropriate frequency line size and line length. Phase nonlinearity and group delay variations do not occur in matched coaxial lines. Since the operating mode is TEM the phase is linear with frequency and there is uniform group delay. If the line is mismatched however phase nonlinearity and group delay is present depending on the antenna reflection coefficient and the line length. Eilers has published an analysis of this effect.10 The group delay for a constant antenna VSWR of 1.05 1 r D 0.025 and lossless transmission line are shown in Figure 6-5. The group delay is periodic with ripple frequency and magnitude directly proportional to line length. For this example a maximum group delay of 100 ns is computed for a 2000-ft line length. The ripple frequency increases to 24 ripples across a 6-MHz band for a line length of 2000 ft. The group delay is a consequence of phase ripple due to the .