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Sec. 3–5 Line Codes and Spectra 183
TABLE 3–5
Digital Word
THREE-BIT DAC CODE
Output Level, (a n ) i
000 +7
001 +5
010 +3
011 +1
100 -1
101 -3
110 -5
111 -7
1
R(0)= a
8
i=1
(a n ) i 2 P i = 21
where P i = 8 for all of the eight possible values. For k Z 0, R(k) = 0. Then, from
Eq. (3–36a), the PSD for w 2 (t) is
w2 (f) = |F(f)|2
T s
(21 + 0)
where the pulse width (or symbol width) is now T s = 3T b . For the rectangular pulse shape of
width 3T b , this becomes
w2 (f) = 63T b a sin 3pfT 2
b
b
3pfT b
for the case where = 3. Consequently, the first null bandwidth for this multilevel polar NRZ
signal is B null = 1(3T b ) = R3, or one-third the bandwidth of the input binary signal.
In general, for the case of L = 2 levels, the PSD of a multilevel polar NRZ signal with
rectangular pulse shapes is
multilevel NRZ (f) = Ka sin /pfT 2
b
b
/pfT b
where K is a constant and the null bandwidth is
B null = R>/
(3–53)
(3–54)
Example 3–11 PLOT THE PSD FOR A MULTILEVEL NRZ LINE CODE
Evaluate and plot the PSD for a Multilevel NRZ line code with R = 6 bitssec and L = 8 levels.
See Example3_11.m for the solution. Compare this result with Fig. 5–33.