User guide

Operation
PRBS Generator 10 Gb/s User Guide 27
Mark density is a measure of the number of logic 1's vs. total logic 1's and
logic 0's. Standard PRBS patterns consist of equal numbers of 1's and 0's
resulting in a mark density of 1/2.
The density of logic 1's and 0's can be qualitatively confirmed using a high
speed sampling scope. A mark density of 1/2 will result in an output eye
centered about 0 V. A mark density of 1/4 will result in an eye with a DC
offset approximately equal to (V pp/2 – V pp*MD) where MD is the mark
density of 1/2, 1/4 or 1/8. Note that measurements made on the
complimentary output will yield a similar DC offset but of opposite polarity.
Another method of qualitatively observing different mark densities is to
observe the relative screen intensity for the two logic states. For MD = 1/2,
the resulting eye will appear equally bright for logic 1's and 0's while an MD =
1/8 will yield a much brighter logic 0 vs. logic 1.
Waveform characteristics such as jitter, ampli
tude, rise and fall times are read
directly from the sampling oscilloscope's measurements. Nominal
measurements are all performed with a 10 GHz input clock with a (2
31
-1)
pattern and mark density set to 1/2
1
.
Table 4. N4970A pattern properties
Pattern Length Polynomial Tone Spacing @ 10 Gb/s ITU STD
2
7
-1 1 + X
6
+ X
7
78.7 MHz ITU-T V.29
2
10
-1 1 + X
7
+X
10
9.78 MHz CCITT O.1S2/ITU-T O.192
2
15
-1 1 + X
14
+X
15
305 kHz CCITT O.1S1/ITU-T O.151
2
23
-1 1 + X
18
+X
23
1.19 kHz CCITT O.1S1/ITU-T O.151
2
31
-1 1 + X
28
+X
31
4.67 Hz
1
Use of an external clock and precision timebase may be required to see the true jitter performance of the data
outputs.