Examples of Earth Stations
In this chapter, let us discuss about two examples of earth stations: Receive-only Home TV system and Community Antenna TV system.
Examples of Earth Stations -Receive Only Home TV System
If broadcasting takes place directly to home TV receivers, then that type of service is called as Direct Broadcast Satellite (DBS) service.
A mesh type reflector can be used for focusing the signals into a dual feed-horn. It is having two separate outputs. From one output will get C-band signals and from other output will get Ku-band signals.
Television programming mostly originates as first generation signals. These signals are transmitted through satellite to network main end stations in C band. These signals are compressed and transmitted in digital form to cable and DBS providers.
C-band users can subscribe to pay TV channels. These subscription services are cheaper when compared to cable because of the availability of multiple-source programming.
The block diagram of DBS TV receiver is shown in below figure.
Outdoor unit mainly consists of receiving antenna and Low Noise Converter (LNC). Low Noise Converter (LNC) is nothing but the combination of Low Noise Amplifier (LNA) followed by a converter. The receiving antenna is directly fed into LNC.
In general, the parabolic reflector is also used with the receiving horn antenna for more focusing of the beam.
In general, the signal fed to the indoor unit is a wideband signal. The frequency of this signal lies between 950 MHz and 1450 MHz. In indoor unit, this signal gets amplified by using an amplifier.
The amplified signal is applied to a tracking filter and down converter. It selects the desired channel and converts its frequency to an Intermediate Frequency (IF) of 70 MHz.
IF amplifier amplifies the signal strength in order to demodulate it properly. The baseband (demodulated) signal is used to generate a Vestigial Single Side Band (VSSB) signal. This signal is fed into one of VHF/UHF channels of a standard TV set.
Frequency Modulation (FM) is used in DBS TV. Whereas, Amplitude Modulation (AM) in the form of VSSB is used in conventional TV. This is the major difference between DBS TV and conventional TV.
Examples of Earth Stations -Community Antenna TV System
The Community Antenna TV (CATV) system uses a single outdoor unit and multiple feeds. These feeds are available separately for each sense of polarization. Due to this, all channels will be available at the indoor receiver, simultaneously.
The block diagram of indoor unit of CATV system is shown in below figure.
In this case, there is no need of separate receiver to each user. Because, all the carriers are demodulated in a common receiver-filter system. After that, the channels are combined into a multiplexed signal. This signal is then transmitted through a cable to the subscribers (users).