ANALOG AND DIGITAL COMMUNICATION SYSTEMS

  Applications of different frequency spectrum?  ELF     EXTREME LOW FREQUENCY VF        VOICE FREQUENCY VLF      VERY LOW FREQUENCY LF         LOW FREQUENCY MF        MEDIUM FREQUENCY HF         HIGH FREQUENCY VHF      VERY HIGH FREQUENCY UHF      ULTRA HIGH FREQUENCY SHF       SUPER HIGH FREQUENCY EHF      EXTREME HIGH FREQUENCY

MODULATION :      THE PROCESS OF CHANGING SOME CHARACTERISTICS OF CARRIER SIGNAL (AMPLITUDE  / FREQUENCY / PHASE)   IN ACCORDANCE WITH THE INSTANTANEOUS AMPLITUDE OF THE MESSAGE SIGNAL IS KNOWN AS MODULATION . NEED FOR MODULATION : I) REDUCE THE HEIGHT OF ANTENNA II) FOR MULTIPLEXING III) FOR EASY RADIATION IV) AVOID MIXING OF SIGNALS V) REDUCTION OF NOISE .

CLASSIFICATION OF  VARIOUS MODULATION   TECHNIQUES : MODULATION TECHNIQUES

 BASE BAND SIGNAL : The message signal which is coming form an information source is called base band signal it is a low frequency signal it is the output of information source it combines with the carrier to generate modulated signal CARRIER SIGNAL : The signal which is used to carry message signal is called carrier signal it is a high frequency signal it is the output of oscillator it combines with the message to generate modulated signal MODULATED SIGNAL : The output of modulator is called modulated signal it is a high frequency signal it contains both message information and carrier properties it is the output of modulator

  MODULATION :      THE PROCESS OF CHANGING SOME CHARACTERISTICS OF CARRIER SIGNAL  (AMPLITUDE  / FREQUENCY / PHASE)   IN ACCORDANCE WITH THE INSTANTANEOUS AMPLITUDE OF THE MESSAGE SIGNAL IS KNOWN AS  MODULATION . AMPLITUDE MODULATION :- THE PROCESS OF CHANGING AMPLITUDE OF CARRIER SIGNAL  IN ACCORDANCE WITH THE INSTANTANEOUS AMPLITUDE OF THE MESSAGE SIGNAL IS KNOWN AS AMPLITUDE   MODULATION . FREQUENCY MODULATION :- THE PROCESS OF CHANGING FREQUENCY OF CARRIER SIGNAL  IN ACCORDANCE WITH THE INSTANTANEOUS AMPLITUDE OF THE MESSAGE SIGNAL IS KNOWN AS  FREQUENCY   MODULATION . PHASE MODULATION :- THE PROCESS OF CHANGING PHASE OF CARRIER SIGNAL  IN ACCORDANCE WITH THE INSTANTANEOUS AMPLITUDE OF THE MESSAGE SIGNAL IS KNOWN AS  PHASE   MODULATION .

   Modulation index :- It is defined as the ration of maximum amplitude of message signal to  maximum amplitude of carrier signal .                    M= Vm / Vc           M = modulation index           Vm = maximum amplitude of message signal         Vc = maximum amplitude of carrier signal

  Modulation index :- It is defined as the ration of maximum amplitude of message signal to maximum amplitude of carrier signal .                    M= Vm / Vc           M = modulation index           Vm = maximum amplitude of message signal         Vc = maximum amplitude of carrier signal Effects of over modulation? if M>1 it is called as over modulation ,due to this some problems will occurs in transmission . transmission quality will be poor envelop will occur in receiver(negative peak clipped off) excessive bandwidth required instability in carrier voltage,frequency interference with neighboring station signal may occur