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BASEBAND DIGITAL DATAdoc

By Albert Smith,2014-07-04 21:32
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BASEBAND DIGITAL DATAdoc

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

    1

denotes the duration of the 1 bit B

    1 Hence Bit rate = bits per second B

    All the forms of the base band signalling shown transfer data at the same bit rate.

     denotes the duration of the shortest signalling element. E

    Baud rate is defined as the reciprocal of the duration of the shortest signalling element.

    1 i.e. Baud Rate = baud E

    In general Baud Rate ? Bit Rate

    For NRZ : Baud Rate = Bit Rate ( = ) BE

    B RZ : Baud Rate = 2 x Bit Rate ( = ) E2

    B Bi-Phase: Baud Rate = 2 x Bit Rate ( = ) E2

     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. ffLH

    Considering bi-polar signal (i.e. V or +V)

    NRZ

The highest frequency occurs when the data is 1010101010…….

    i.e.

    2

    . From the Fourier series This sequence produces a square wave with periodic time ;2E

    strdthfor a square wave, this waveform contains 1, 3, 5 … (Odd) harmonics.

    If we pass this signal through a LPF then the maximum bandwidth would be 1/T Hz, i.e. to

    stjust allow the fundamental (1 harmonic) to pass. As illustrated in the diagram below, the data sequence 1010…… could then be completely covered.

Hence the minimum channel bandwidth

    3

    BaudRate111 B;;;Since;BaudRateminT22EEThis corresponds to the ‘upper frequency’. The lower frequency limit is DC, for example fUif continuous 1’s were transmitted, the voltage is +V volts. Example:

     = 1 msec, Bit Rate = 1000 bits/sec B

    1 = 1000 Bauds Baud Rate = E

    BaudRate B;f;;500HzminU2

     f;'DC'L

Return to Zero

Considering RZ signals, the max frequency occurs when continuous 1’s are transmitted.

    4

    . Following the same reasoning as for This produces a square wave with periodic time ;2E

    NRZ

    BaudRate ;;BfminU2

    If the sequence was continuous 0’s, the signal would be –V continuously, hence . f;'DC'LExample

     Again = 1 msec, Bit Rate = 1000 bits/sec B

     = 0.5 msec E

     Therefore,

    1 Baud Rate = = 2000 Bauds E

    BaudRate B;f;;1000HzminU2

     f;'DC'L

    i.e. twice the bandwidth of NRZ Bi-Phase

    Maximum frequency occurs when continuous 1’s or 0’s transmitted.

    This is similar to RZ with

    5

    1 = 2 x Bit rate Baud Rate = E

    BaudRate ;;BfminU2

    The minimum frequency occurs when the sequence is 10101010…….

    e.g.

In this case = BE

     Baud Rate = Bit rate

    BaudRate ;;BfminL2

    Example:

     = 1 msec, Bit Rate = 1000 bits/sec B

     1000 Hz f;f;500HzUL

Comments:

    6

    RZ and NRZ require channel BW down to DC, i.e. DC coupled. This causes problems with drift and line capacitance. It is better to AC coupled lines.

    RZ is not preferred because it has twice the bandwidth of NRZ.

    Bi-phase may be AC coupled and is used for this reason and also because of other useful properties for e.g.

    There is transition every bit ? or ? which eases clock generation at the receiver - describes as

    self clocking.

    Bi-phase is actually a phase modulated signal; phase shift keying.

    The main disadvantage with bi-phase is that although the BW I same as NRZ, it extends up to twice the upper frequency of NRZ.

    Base band signal are not usually transmitted directly although bi-phase signals are used. Usually NRZ signals are used to modulate carrier to produce

     ASK Amplitude Shift Keying

     FSK Frequency Shift Keying

     PSK Phase Shift Keying

    These are discussed in later sections.

    7

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