Digital modulation:
Digital modulation in somewhat similar to the analog modulation except base band signal is of discrete amplitude level. For binary signal it has only two level, either high or logic 1 or low or logic 0. The modulation scheme is mainly three types.
Digital modulation in somewhat similar to the analog modulation except base band signal is of discrete amplitude level. For binary signal it has only two level, either high or logic 1 or low or logic 0. The modulation scheme is mainly three types.
1. ASK or Amplitude shift Key
2. FSK or Frequency shift key
3. PSK or Phase shift key
ASK or Amplitude shift Key:
When the carrier amplitude is varied in proportion to message signal m(t). We have the modulated carrier m(t)coswct where coswct is the carrier signal. As the information is an on-off signal the output is also an on-off signal where the carrier is present when information is 1 and carrier is absent when information is 0. Thus this modulation scheme is known as on-off keying (OOK) or amplitude shift key.
FSK or Frequency shift key:
When Data are transmitted by varying frequency of the carrier, we have the case of frequency shift key. In this modulation carrier has two predefined frequency wc1 and wc2. When information bit is 1 carrier with wc1 is transmitted i.e. coswc1 and When information bit is 0 carrier with wc0 is transmitted i.e. coswc0
PSK or Phase shift key:
The phase of the carrier is shifted for this modulation. If the base band signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(wct+П). If phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.
When the carrier amplitude is varied in proportion to message signal m(t). We have the modulated carrier m(t)coswct where coswct is the carrier signal. As the information is an on-off signal the output is also an on-off signal where the carrier is present when information is 1 and carrier is absent when information is 0. Thus this modulation scheme is known as on-off keying (OOK) or amplitude shift key.
FSK or Frequency shift key:
When Data are transmitted by varying frequency of the carrier, we have the case of frequency shift key. In this modulation carrier has two predefined frequency wc1 and wc2. When information bit is 1 carrier with wc1 is transmitted i.e. coswc1 and When information bit is 0 carrier with wc0 is transmitted i.e. coswc0
PSK or Phase shift key:
The phase of the carrier is shifted for this modulation. If the base band signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(wct+П). If phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.
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