Write a short note on Satellite TV System.
1 Answer

Satellite TV Systems are planned to broadcast directly to home TV receivers, it takes place in the Ku (12-GHz) band. This service is known as direct broadcast satellite (DBS) service. There is some variation in the frequency bands assigned to different geographic regions.

  • The comparatively large satellite receiving dishes [ranging in diameter from about 1.83 m (6 ft) to about 3-m (10 ft) in some locations], which may be seen in some “backyards” are used to receive downlink TV signals at C band (4 GHz).
  • The major differences between the Ku-band and the C-band receive only systems lies in the frequency of operation of the outdoor unit and the fact that satellites intended for DBS have much higher equivalent isotropic radiated power (EIRP), As already mentioned C-band antennas are considerably larger than DBS antennas.
  • For clarity, only the Ku-band system is described here. Fig below shows the main units in a home terminal DBS TV receiving system. Although there will be variations from system to system, the diagram covers the basic concept for analog [frequency modulated (FM)] TV.
  • Direct-to-home digital TV, which is well on the way to replacing analog systems. However, the outdoor unit is similar for both systems. Block Diagram of Satellite TV System

Outdoor Unit:

  • This consists of a receiving antenna feeding directly into a low-noise amplifier/converter combination. A parabolic reflector is generally used, with the receiving horn mounted at the focus.
  • A common design is to have the focus directly in front of the reflector, a higher-gain receiving antenna is not needed because the DBS operate at a much higher EIRP.

  • The downlink frequency band of 12.2 to 12.7 GHz spans a range of 500 MHz, which accommodates 32 TV/FM channels, each of which is 24-MHz wide. Obviously, some overlap occurs between channels, but these are alternately polarized left-hand circular (LHC) and right-hand circular (RHC) or vertical/horizontal, to reduce interference to acceptable levels. This is referred to as polarization interleaving.

  • The receiving horn feeds into a low-noise converter (LNC) or possibly a combination unit consisting of a low-noise amplifier (LNA) followed by a converter. The combination is referred to as an LNB, for low-noise block. The LNB provides gain for the broadband 12-GHz signal and then converts the signal to a lower frequency range so that a low-cost coaxial cable can be used as feeder to the indoor unit.

  • The low-noise amplification must be provided at the cable input in order to maintain a satisfactory signal-to-noise ratio. An LNA at the indoor end of the cable would be of little use, because it would also amplify the cable thermal noise.

Indoor unit:

  • The signal fed to the indoor unit is normally a wideband signal covering the range 950 to 1450 MHz. This is amplified and passed to a tracking filter which selects the desired channel, as shown in Fig.

  • As previously mentioned, polarization interleaving is used, and only half the 32 channels will be present at the input of the indoor unit for any one setting of the antenna polarizer. This eases the job of the tracking filter, since alternate channels are well separated in frequency.

  • The selected channel is again down converted, this time from the 950-to 1450-MHz range to a fixed intermediate frequency, usually 70 MHz although other values in the very high frequency (VHF) range are also used.

  • The 70-MHz amplifier amplifies the signal up to the levels required for demodulation. A major difference between DBS TV and conventional TV is that with DBS, frequency modulation is used, whereas with conventional TV, amplitude modulation in the form of vestigial single sideband (VSSB) is used.

  • The 70-MHz, FM intermediate frequency (IF) carrier therefore must be demodulated, and the baseband information used to generate a VSSB signal which is fed into one of the VHF/UHF channels of a standard TV set.

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