UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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PROPAGATION EFFECTS REFLECTION MEASUREMENT EXAMPLES (ECHOES) • 3.5 MHz approx. • The delay will be: 1/3.5 = 0.28 microsec. • 80 meters approx. 20 TX signal power difference [dB] Maximums [dB] Spectrum Ripple Minimums [dB] Relationship between Frequency Interval in MHz between two power dips in the spectrum and related delay in microseconds These are inverse operations: Delay = 1/ (frequency interval) (In the spectrum you can only see short delays from close obstacles). Total, peak-peak [dB] 0 +6 – – 1 +5.5 –19.3 24.8 3 +4.6 –10.7 15.3 10 +2.4 –3.3 5.7 20 +0.8 –0.9 1.7
Let us assume that the electromagnetic signals (light and electromagnetic radio waves, but also X-rays) propagate at the speed of light, in other words: • 300 meters per microsecond. • 3.3 microseconds to travel one kilometer. • 224 microseconds to travel 67 kilometers. Of course, everything comes from the speed of light of 300,000 kilometers per second. Obstacles, that act as mirrors, send to the receiving antenna a signal that is the same as the signal required, but this signal is delayed and attenuated. With an analog signal you will see a double image on the screen. If you have a large screen of about 50 cm, the shift is: as many µS as there are cm of displacement between the two images. Knowing this, sometimes you can derive the distance of the point of reflection. In DVB-T systems you cannot see the double image, but the spectrum display shows dips in the spectrum (ripple). This can be explained considering that the signals become unphased along the way and the delay varies with frequency, resulting that in some areas of the spectrum there are signal reinforcements in some areas signal drops. Signals are equal to each other, the spectrum goes close to zero in the gaps and to +6 dB in the peaks. We will show you how instruments can provide easy and accurate systems to obtain the amplitude and distance of the reflections (ECHOES). 21
Page 1 and 2: UNDERSTANDING DIGITAL TV • Radio
Page 3 and 4: SUMMARY 6 Radio frequency spectrum
Page 5 and 6: The difference between the two sign
Page 7 and 8: The function that allows you to fin
Page 9 and 10: The power of the digital signal is
Page 11 and 12: The modulation is the same type for
Page 13 and 14: We know that signals can be “poll
Page 15 and 16: The DVB-T signal has this behavious
Page 17 and 18: Here you can see a schematic block
Page 19: When it is said that a DVB-T system
Page 23 and 24: We will illustrate a laborious, but
Page 25 and 26: This chart is useful, even if it do
Page 27 and 28: Another situation where the antenna
Page 29 and 30: The back to front antenna ratio is
Page 31 and 32: The reflections caused by a mismatc
Page 33 and 34: Since the system is immune to refle
Page 35 and 36: Operation is based on being able to
Page 37 and 38: Nearly every instrument has an SFN
Page 39 and 40: Here are a number of rules to impro
Page 41 and 42: This picture shows an example of an
Page 43 and 44: As an example, we have superimposed
Page 45 and 46: The diagram clearly shows the trans
Page 47 and 48: To obtain minimum reception stabili
Page 49 and 50: These predictions are verified when
Page 51 and 52: The electromagnetic field is measur
Page 53 and 54: Even if the MER is a good indicator
Page 55 and 56: The diagram shows once again the th
Page 57 and 58: This table is calculated for system
Page 59 and 60: Micro-interruptions are not always
Page 61 and 62: So ripple (waves) in the spectrum p
Page 63 and 64: In Figure 3 and 4 you can see the s
Page 65 and 66: Figure 5 shows DVB-T signal ch.8 th
Page 67 and 68: Using this case study you can see h
Page 69 and 70: Directive 2004/108/EC requires that
Page 71 and 72:
The diagram shows the real case of
Page 73 and 74:
MPEG4 REAL TIME SPECTRUM 6-HOUR BA
Page 75 and 76:
mod. HD COMPACT STC THE MOST ADVANC
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UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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