Keysight E8663D, Руководство пользователя

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Keysight E8357D/67D & E8663D PSG User’s Guide 135
Basic Digital Operation
Using Waveform Clipping
Spectral regrowth is a range of frequencies that develops on each side of the
carrier (similar to sidebands) and extends into the adjacent frequency bands
(see Figure 3-13 ). Consequently, spectral regrowth interferes with
communication in the adjacent bands. Clipping can provide a solution to this
problem.
Figure 3-13 Spectral Regrowth Interfering with Adjacent Band
How Clipping Reduces Peak–to–Average Power
You can reduce peak–to–average power, and consequently spectral regrowth,
by clipping the waveform to a selected percentage of its peak power. The PSG
vector signal generator provides two different methods of clipping: circular and
rectangular.
During
circular clipping, clipping is applied to the combined I and Q waveform
(|I + jQ|). Notice in Figure 3-14 that the clipping level is constant for all phases
of the vector representation and appears as a circle. During rectangular
clipping, clipping is applied to the I and Q waveforms separately (|I|, |Q|). Notice
in Figure 3-15 on page 136 that the clipping level is different for I and Q;
therefore, it appears as a rectangle in the vector representation. With either
method, the objective is to clip the waveform to a level that effectively reduces
spectral regrowth, but does not compromise the integrity of the signal. Figure
3-16 on page 137 uses two complementary cumulative distribution plots to
show the reduction in peak–to–average power that occurs after applying
circular clipping to a waveform.
The lower you set the clipping value, the lower the peak power that is passed
(or the more the signal is clipped). Often, the peaks can be clipped successfully
without substantially interfering with the rest of the waveform. Data that might
be lost in the clipping process is salvaged because of the error correction
inherent in the coded systems. If you clip too much of the waveform, however,
lost data is irrecoverable. You may have to try several clipping settings to find a
percentage that works well.