BASEBANDDIGITALDATAdoc.docVIP

PAGE  PAGE 8 BASEBAND DIGITAL DATA Common form of base band signals to represent digital information bits are shown below. Voltage levels for uni-polar and bi-polar signals are also shown. BIT-RATE, BAUD-RATE BANDWIDTH denotes the duration of the 1 bit Hence Bit rate = bits per second All the forms of the base band signalling shown transfer data at the same bit rate. denotes the duration of the shortest signalling element. Baud rate is defined as the reciprocal of the duration of the shortest signalling element. i.e. Baud Rate = baud In general Baud Rate ≠ Bit Rate For NRZ : Baud Rate = Bit Rate ( = ) RZ : Baud Rate = 2 x Bit Rate ( = ) Bi-Phase: Baud Rate = 2 x Bit Rate ( = ) AMI: Baud Rate = Bit Rate A major consideration for base band signals is the minimum bandwidth and the upper and lower frequencies and respectively, in the signal. Considering bi-polar signal (i.e. –V or +V) NRZ The highest frequency occurs when the data is 1010101010……. i.e. This sequence produces a square wave with periodic time . From the Fourier series for a square wave, this waveform contains 1st, 3rd, 5th … (Odd) harmonics. If we pass this signal through a LPF then the maximum bandwidth would be 1/T Hz, i.e. to just allow the fundamental (1st harmonic) to pass. As illustrated in the diagram below, the data sequence 1010…… could then be completely covered. Hence the minimum channel bandwidth This corresponds to the ‘upper frequency’. The lower frequency limit is DC, for example if continuous 1’s were transmitted, the voltage is +V volts. Example: = 1 msec, Bit Rate = 1000 bits/sec Baud Rate = = 1000 Bauds Return to Zero Considering RZ signals, the max frequency occurs when continuous 1’s are transmitted. This produces a square wave with periodic time . Following the same reasoning as for NRZ If the sequence was continuous 0’s, the signal would be –V continuously, hence . Example Again = 1 msec, Bit R