03-10-2012, 04:34 PM
analog transmission
analog transmission.ppt (Size: 342.5 KB / Downloads: 20)
ANALOG AND DIGITAL
Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.
Amplitude Modulation
A carrier signal is modulated only in amplitude value
The modulating signal is the envelope of the carrier
The required bandwidth is 2B, where B is the bandwidth of the modulating signal
Since on both sides of the carrier freq. fc, the spectrum is identical, we can discard one half, thus requiring a smaller bandwidth for transmission.
Phase Modulation (PM)
The modulating signal only changes the phase of the carrier signal.
The phase change manifests itself as a frequency change but the instantaneous frequency change is proportional to the derivative of the amplitude.
The bandwidth is higher than for AM.