Fundamentals of Digital Television Transmission phần 10

242 RADIO-WAVE PROPAGATION Raleigh, R085 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 −2 −4 −6 −8 −10 −12 −14 −16 −18 −20 −22 Peak to peak frequency response (dB) Figure 8-33. Tap energy versus response. Raleigh, R275 95.00 90.00 85.00 80.00 75.00 70.00 65.00 60.00 55.00 50.00 45.00 40.00 10 20 30 40 50 60 70 80 90 100 110 Distance (km) Calculated Measured Figure 8-34. Field strength versus distance. SUMMARY 243 Raleigh 110 100 90 80 70 60 50 40 30 10 20 30 40 50 60 70 80 90 100 110 Distance (km) FCC(50,10) Calculated FCC(50,50) FCC(50,90) R65 Figure 8-35. Comparison with FCC. The computed field strength is plotted along with predictions from FCC curves in Figure 8-35. The computed curve matches the FCC(50,50) curve best at 30 km and at long range. Up to 0.5 dB should be subtracted from the FCC curve to treat the Raleigh terrain properly. Measured data for R065 are repeated for comparison. Indoor antenna tests were performed at 36 sites. Three types of indoor antenna was tested: a loop, a single bowtie, and a dual bowtie over a ground plane. A usable signal with the indoor antennas was observed at all but three sites. At these sites, the median signal strength on the indoor antennas was lower than the outdoor measurements by 9.1, 6.8, and 11.1 dB, respectively. The loss in signal strength included the effect of height loss, building penetration loss, and a less directive receiving antenna. The equalizer tap energy was significantly higher than for the outdoor measurements. The average tap energy on the indoor antennas was about 6 dB compared to 15 dB on the outside antennas. This would indicated significantly higher multipath indoors. SUMMARY The factors that affect the propagation of digital television signals at VHF and UHF have been considered along with various means of estimating signal strength and frequency response. It is evident that the means do not exist to predict with 244 RADIO-WAVE PROPAGATION precision the field strength or frequency response at any location and time. This is due to the nature of the propagation environment. Free-space attenuation, ground reflections from a plane or spherical earth, refraction by an ideal atmosphere, and diffraction over spherical earth and well-defined obstacles lend themselves to precise calculations. However, the real world is much different. The effect of the earth’s rough surface, the temperature, humidity, and pressure variations of the atmosphere, and the locations, shapes, and reflection coefficients of natural and man-made obstacles are difficult to estimate. Nevertheless, it is important to understand the contribution of each of these factors. Understanding these factors is useful in assessing the difference between propagation at VHF and UHF. Both free-space attenuation and losses due to surface roughness are much higher for UHF. These losses are partially offset by the effect of ground reflections from smooth earth. In addition, diffraction losses are generally lower at UHF since fixed clearances are greater when measured in terms of Fresnel zone radii. Nevertheless, overall propagation losses are almost always greater for UHF. Fundamentals of Digital Television Transmission. Gerald W. Collins, PE Copyright  2001 John Wiley & Sons, Inc. ISBNs: 0-471-39199-9 (Hardback); 0-471-21376-4 (Electronic) 9 TEST AND MEASUREMENT FOR DIGITAL TELEVISION Although there are many tests and measurements for the transmission of digital television that are similar to those made for analog television, some are distinctly different. These will be the focus of this chapter. These tests include the measurement of power as well as linear and nonlinear distortions. Frequency measurements are also discussed. This discussion is not meant to be exhaustive. There are many tests that may be made in connection with the subsystems discussed in previous chapters. There are other tests that may be made at the systems level. The purpose of this chapter is to highlight a few of the key tests that may be used to characterize the RF performance of a digital television system. POWER MEASUREMENTS The measurement of power is fundamental to all digital TV transmission tests. Power output establishes the transmitter operating point and thus determines the level of nonlinear distortions at the source. The stress on high-power RF filters, transmission lines, and antennas is determined by incident and reflected peak and average power. At the receiver, the available signal power relative to noise and interference determines the availability of a viewable picture. Although the concept of power was discussed earlier, it is important that it be defined clearly as it relates to measurement. As noted earlier, both average and peak power are important to the transmission of digital TV. The average power must be known in relation to the dissipation and temperature rise in transmission equipment as well as the signal power available at the receiver. Average power refers to the product of the RMS signal voltage and current, integrated over the modulated signal bandwidth. Since the transmitted data stream is random in 245 246 TEST AND MEASUREMENT FOR DIGITAL TELEVISION nature, the average power is constant if the average is taken over a sufficiently long time. This is in contrast to the analog television signal, for which the average power varies with video content. Even though the average power is used to establish TPO, system ERP, and C/N, it is often desirable to measure peak power. Nonlinear distortions may lead to degraded system performance. This most often is due to overdrive somewhere within the system; the ability to measure peak power is a valuable tool for troubleshooting. The peak power must also be known in relation to the rating of transmission components. The peaks of the RF envelope are determined statistically by the random pattern of the data and the bandlimiting of the system. Thus the peak power levels must be described by both their magnitude and the percent of time they occur.1 For these statistics, the peak envelope power (PEP) is defined as the average power contained in a continuous sine wave with peak amplitude equal to the signal peak. Thus the PEP for a digital TV signal is defined in the same manner as for analog TV. The contrast is in the regular recurring peaks of the analog sync pulses at a constant amplitude versus the random occurrence of the digital peaks at random amplitudes. It is customary to state the peak power relative to the average power. Usually, this is a logarithmic ratio and is given in decibels. Since the peak power is statistical in nature, the peak-to-average power ratio is often presented in the form of a cumulative distribution function (CDF). This is a concept borrowed from the mathematics of probability that permits the description of the relative frequency of occurrence (probability) of a particular peak power level (the random variable). The RF power is sampled at regular intervals, and the power level measured at each interval is collected in one of many incremental ranges or “bins.” The number of times the measured level falls into a particular bin relative to the total number of measurements is computed for each bin and may be plotted as a histogram. Thus the histogram is a record of the frequency at which a particular incremental power range is measured. When properly constructed with sufficiently small power increments and a large number of measurements, the histogram approximates a probability distribution function (PDF). The probability of the peak-to-average power ratio exceeding a particular level is the usual parameter of interest to the engineer. This may be determined from the CDF, which is obtained by integrating the PDF from the maximum peak to average ratio down to unity. The peak and average powers are equal approximately 50% of the time; as the peak-to-average power ratio increases, the frequency of occurrence approaches but never becomes zero. A typical CDF for the 8 VSB signal is as shown in Figure 2-7. A variety of instruments are used to measure power. Some of these measure only average power. Others are capable of measuring peak power, from which 1 G. Sgrignoli, “Measuring Peak/Average Power Ratio of the Zenith/AT&T DSC-HDTV Signal with a Vector Signal Analyzer,” IEEE Trans. Broadcast., Vol. 39, No. 2, June 1993, pp. 255–264. ... - tailieumienphi.vn