Satellite System Link Models
Essentially, a satellite system consists of three basic sections:
- The uplink
- The satellite transponder
- The downlink
1.The uplink model :
Fig. below shows the block diagram of a satellite earth station transmitter.
The primary component within the section of a satellite system is the earth station transmitter.
A typical earth station transmitter consists of an IF modulator, an IF to RF microwave up-converter, a high power amplifier (HPA).The IF modulator converts the input baseband signals to either an FM, a PSK or a QAM modulated intermediate frequency.The up-converter (mixer and BPF) converts the IF to an appropriate RF carrier frequency.The HPA provides adequate input sensitivity and output power to propagate the signal to the satellite transponder. The HPA’s commonly used are klystrons and TNT’s.
2.The satellite transponder:
Fig. below shows a simplified block diagram of a satellite transponder.
A transponder is a part of a satellite, which is a combination of transmitter and receiver. The main function of transponder is frequency translation and amplification. Based on the frequency translation process, there are three basic transponder configurations. These are single conversion transponder, double conversion transponder and regenerative transponder.
The uplink signal is received by the receiving antenna. The received signal is first band limited by Band Pass Filter (BPF), then it is routed to Low Noise Amplifier (LNA).
The amplified signal is then frequency translated by a mixer and an oscillator. Here only the frequency is translated from high-band up-link frequency to the low-band down link frequency. The mixer output (down link signal) is then applied to BPF then it is amplified by a High Power Amplifier (HP A). This down link signal is then transmitted to receiver earth station through a high power transmitting antenna.
3. The downlink :
Fig. below shows a block diagram of a typical earth station receiver.
An earth station receiver includes an input BPF, an LNA and an RF to IF down converter.
The BPF limits the input noise power to the LNA. The LNA is a highly sensitive, low noise device. The RF-to-IF down converter is a mixer, BPF combination which converts the received RF signal to an IF frequency.
The most common frequencies used for satellite communications are 6/4 and 14/12 GHz bands. The first number indicates the uplink (earthstation-to-transponder) frequency and the second number is downlink (transponder-to-earthstation) frequency. Since C band is most widely used, this band is becoming overcrowded. A typical C band transponder can carry 12 channels, each with a bandwidth of 36 MHz.
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