Transceiver
A Transceiver is a device which can be both used for transmission and receiving purposes. Some devices are also called transmitter-receiver, but they are different from transceiver as they don't share common circuitry or single housing whereas transmitter and receiver within a transceiver share common circuitry or a single housing. There are many types of transceivers: RF transceivers, fibre-optic transceivers, Ethernet transceivers, wireless (WAP) transceivers, and more but here we are mainly concerned with RF transceiver for satellite purposes.[1]
Our mobile phones, walkie-talkie etc. are examples of transceivers.
A modem is similar to a transceiver, in that it sends and receives a signal, but a modem uses modulation and demodulation. It modulates a signal being transmitted and demodulates a signal which is received. [2]
In a radio transceiver, the receiver is silenced while transmitting. An electronic switch allows the transmitter and receiver to be connected to the same antenna and prevents the transmitter output from damaging the receiver. With a transceiver of this kind, it is impossible to receive signals while transmitting. This mode is called half duplex. Transmission and reception often, but not always, are done on the same frequency.
Some transceivers are designed to allow reception of signals during transmission periods. This mode is known as full duplex and requires that the transmitter and receiver operate on substantially different frequencies, so the transmitted signal does not interfere with reception. Cellular and cordless telephone sets use this mode. Satellite communications networks often employ full-duplex transceivers at the surface-based subscriber points. The transmitted signal (transceiver-to-satellite) is called the uplink, and the received signal (satellite-to-transceiver) is called the downlink. [3]
General Satellite Transceiver
In general, we require that the data that needs to be transmitted (when using transceiver as a transmitter), can be packeted, which is the input to the transmitter where it is modulated using different modulation scheme, and we also have to amplify our signal so that it can reach the earth at a detectable gain.
Case Study - Pratham
In Pratham satellite, two types of information signals are transmitted: Beacon and Downlink.
The Beacon signal only contains hard-coded data: " VU2BUJ PRATHAM IIT BOMBAY STUDENT SATELLITE ". This message is encoded into the microcontroller on the beacon board. Beacon board generates a signal at 145.98 MHz using crystal, the Power Amplifier on the board is controlled using the micro-controller to generate CW signal (morse code signal).
This beacon board is an elementary example of a transmitter.
Next, we have Downlink board. The Downlink signal contains telemetry data. Hence it has to communicate with the On-Board Computer to gather the data which needs to be transmitted. This data is received in packeted form which is passed from On-board microcontroller (downlink board microcontroller) to the transceiver CC1101 (Texas Instruments). It modulates the signal using FSK modulation on the 437.455 MHz frequency. This signal then passes through Power Amplifier which amplifies the signal, and it is radiated through the monopole antenna.
SImilarly, the Uplink board has the transceiver CC1101. The signal received by the uplink antenna is passed through LNA (Low noise amplifier) to increase the strength of the signal. Here LNA output will be input to CC1101 which will validate the authenticity of the signal after de-modulating it and then it will pass the data further down for processing.
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