Examples of earth stations
Satellite communication tutorial
Examples of earth stations
In this chapter, let's look at two examples of earth stations: Home TV system receive only and Community antenna TV system .
Receive-only home TV system
If the broadcast takes place directly to home TV receivers, then this type of service is called Direct Broadcast Satellite (DBS) service .
A mesh type reflector can be used to focus signals in a double horn . It has two separate exits. One output will receive C-band signals and another output will get Ku-band signals.
Television programming is derived primarily from first generation signals. These signals are transmitted by satellite to the main end stations of the C-band network. These signals are compressed and transmitted in digital form to the cable and DBS providers.
C band users can subscribe to pay TV channels. These subion services are cheaper compared to cable due to the availability of multi-source programming.
The block diagram of the DBS TV receiver is shown in below.
The outdoor unit mainly consists of receiving antenna and low noise converter (LNC) . Low Noise Converter (LNC) is nothing more than the combination of a low noise amplifier (LNA) followed by a converter. The receiving antenna is directly powered in the LNC.
In general the parabolic reflector is also used with the receiving horn antenna for better beam focusing.
Usually the signal sent to the indoor unit is a wideband signal. The frequency of this signalis between 950 MHz and 1450 MHz. In the indoor unit, this signal is amplified with the help of an amplifier .
The amplified signal is applied to a tracking filter and converter step-down. It selects the desi red channel and converts its frequency to an intermediate frequency (IF) of 70 MHz.
IF amplifier amplifies the strength of the signal in order to demodulate it correctly. The baseband (demodulated) signal is used to generate a Vestigial Single Side Band (VSSB) signal. This signal is fed into one of the VHF / UHF channels of a standard television.
Frequency modulation (FM) is used in DBS television. While amplitude modulation (AM) in the form of VSSB is used in conventional television. This is the major difference between DBS television and conventional television.
Community aerial television system
Aerial televisione community (CATV) The system uses a single outdoor unit and multiple power supplies. These power supplies are available separately for each direction of polarization. For this reason, all channels will be available simultaneously on the indoor receiver.
The block diagram of the indoor unit of CATV system is shown below.
In this case, there is no need for a separate receiver for each user. all carriers are demodulated in a common receiver-filter system. After that, the channels are combined into a multiplexed signal. This signal is then transmitted via cable to the subscribers (users).