T9000
Preliminary Data Sheet
November 2000
ISDN Network Termination Node (NTN) Device
12 PWM Module (continued)
12.4 PWM Auto Mode Example (continued)
In the physical circuit implementation, the width of the pulse follows a 2.5%—97.5% modulation scheme and
depends on the granularity and tick values as described in the following equation:
n × PWV
-----------------------
ROUND 128 × 1 + 0.95 × sin 2π ×
216
----------------------------------------------------------------------------------------------------------------------------------------
W(n) = FLOOR
x tick
(16)
Granularity
Note that, in step 1 of the above procedure (equation 3), it is assumed that range and granularity have already
been chosen. It is useful to look at the effects of selecting different values for range and granularity in order to
guide the selection of these values for a particular application. Consider an example using real numbers.
Suppose it is desired to generate a sine wave of frequency Fs = 697 Hz. Table 103 shows the resulting values of
PWV, Ka, Fa, and frequency error (Ferr) for all possible values of range and granularity.
Table 103. PWM Sine Modulator Programming Example
Granularity
Range
32
PP (µs)
2.08
Ks (Fs = 697)
688.666
344.333
172.166
86.083
344.333
172.166
86.083
43.042
172.166
86.083
43.042
21.521
86.083
43.042
21.521
10.760
43.042
21.521
10.760
5.380
PWV
95
Ka
Fa
Ferr (%)
0.17
1
1
689.85
344.93
172.01
86.12
344.93
172.01
86.12
43.03
172.01
86.12
43.03
21.52
86.12
43.03
21.52
10.76
43.03
21.52
10.76
5.38
695.80
695.80
697.63
696.72
695.80
697.63
696.72
697.17
697.63
696.72
697.17
696.95
696.72
697.17
696.95
696.95
697.17
696.95
696.95
697.00
64
4.17
190
0.17
1
128
256
32
8.33
381
−0.09
0.04
1
16.67
4.17
761
2
190
0.17
2
64
8.33
381
−0.09
0.04
2
128
256
32
16.67
33.33
8.33
761
2
1523
381
−0.02
−0.09
0.04
4
4
64
16.67
33.33
66.67
16.67
33.33
66.67
133.33
33.33
66.67
133.33
266.67
761
4
128
256
32
1523
3045
761
−0.02
0.01
4
8
0.04
8
64
1523
3045
6090
1523
3045
6090
12181
−0.02
0.01
8
128
256
32
8
0.01
16
16
16
16
−0.02
0.01
64
128
256
0.01
0.00
One interesting result in Table 103 is that, for different combinations of granularity (G) and range (R) whose prod-
ucts are equal, the resulting parameters for the sine wave output in each case are identical. For example, the pairs
(G = 1, R = 256), (G = 2, R = 128), (G = 4, R = 64), and (G = 8, R = 32) all yield identical values of PWV, Ka, Fa,
and Ferr. However, note from equation 16 that larger values of granularity will produce more rounding error for a
given W(n).
98
Lucent Technologies Inc.
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