Details of Signal Modulation in the C & M system


Before getting into the details of C & M system, let us see how to send a digital data say a movement command for the servo from UNIX WS to SCC thro' the Optical Fiber link.See the simple block diagram.FIG

The C & M system at CEB consists of a digital part,analog part and OF system and the C & M system at antenna shell consists of OF system,analog part and a digital part.I have already mentioned to you that our OF link is an analog link which can carry analog signals from few MHz to about 1 GHz.But the user commands generated in UNIX WS is a digital data and sent to the COMH as digital signals.So, the digital part multiplexes the digital command data(125 Kbits/sec),digitized analog voice (62.5 Kbits/sec) ,dialling information and aux. channels in to a single bit stream of 250 Kbits/sec and sent it to the analog part.The analog part, in turn, converts it into Frequency Shift Keying (FSK)signals of appropriate power and sent it to the OF system.The OF system converts the electrical signals to the optical signal @ 1310 nm.

The systems at the antenna end reverse this process and converts it to the digital signal for further processing by the respective subsystems like COMMUNICATION CONTROLLERS and MICRO-CONTROLLERS in ANTCOM and SCC.

What is meant by Frequency Shift Keying(FSK) ?


You would have learnt/heard already about

  • Amplitude Modulation where the amplitude of the carrier is varied by the amplitude of the modulating analog signal
  • Frequency Modulation where the frequency of the carrier is varied by the amplitude of the modulating analog signal.
  • Phase Modulation where the phase of the carrier is varied by the amplitude of the modulating signal.

    Frequency Shift Keying is a special type of modulation where the digital signals ("0" & "1") changes the frequency of the pseudo carrier to Frequency(MARK) or Frequency(SPACE).

    If "t" is one bit time,the bandwidth (BW)occupied by the FSK signal is calculated as

    (freq(mark)+1/t) - (freq(space) - 1\t) = freq(mark) - freq(space) + 2/t

    For example,in the Forward Link, freq(mark) = 19 MHz , freq(space) = 17 MHz and t = 4 microseconds which corresponds to a data rate of 250 Kbits/sec.

    Therefore , BW(fsk) = (19-17) + 2/4*10 ¯6 = 2.5 MHz.

    So, the C & M system requires a bandwidth of min. 2.5 MHz for faithful conversion of digital data into analog signals and viceversa.

    Next, we'll see the actual block diagram of the C & M system and discuss the signal flow thro' the system.

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