Agilent Technologies E8267C PSG Manual de usuario Pagina 5

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Figure 3a.
Figure 3b.
When summing multiple frequencies, the phase relationships of the frequency components affect
the time-domain profile and peak-to-average characteristics of the composite signal. Figure 3a
shows the composite signal when all the tones have the same initial phase. Figure 3b shows the
composite signal when the tones have a random initial phase setting.
Although IMD is noticeably dependent of the phase relationships of the tones, IMD
test results from one phase set are not predictive of IMD test results from another phase
set based on phase relationships of the tones or peak-to-average ratio of the composite
signal; in other words, IMD test results are not strongly correlated to the phase relation-
ships of the tones in a statistical sense. Consequently, as the phase relationships of the
spectral components in the passband of the DUT vary over time, the nonlinear distortion
characteristics of the DUT vary in an unpredictable manner. As a result, testing with a
single phase set does not provide enough information to adequately characterize IMD.
Clearly, attempting to characterize a devices nonlinear distortion performance using all
the potential phase relationships that the spectral components in the passband of the
DUT may take on under real-world operating conditions (i.e. all the potential phase sets)
is impractical at best. So why not test using a phase set that results in a high crest factor
composite signal to simulate the worst-case device stress condition?
Σ
DUT
y = +1
0
y = 1
y
1
(t) = sin (ω
1
t)
y
1
(0) = 0
y
2
(t) = sin (ω
2
t)
y
2
(0) = 0
y
3
(t) = sin (ω
3
t)
y
3
(0) = 0
y
4
(t) = y
1
+ y
2
+ y
3
Time (t)
y = +1
0
y = 1
Time (t)
y = +1
0
y = 1
Time (t)
y = +3
0
y = –3
Time (t)
5
Σ
DUT
y = +1
0
y = 1
y
5
(t) = sin (ω
1
t + 21°)
y
5
(0) = .3584
y
6
(t) = sin (ω
2
t)
y
6
(0) = 0
y
7
(t) = sin (ω
3
t - 157.5°)
y
7
(0) = .3827
y
8
(t) = y
5
+ y
6
+ y
7
Time (t)
y = +1
0
y = 1
Time (t)
y = +1
0
y = 1
Time (t)
y = +3
0
y = 3
Time (t)
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