3 Spectrum Analyzer Mode
3.6 Power Stat CCDF Measurement
3.6 Power Stat CCDF Measurement
Many modern digitally-modulated signals look noise-like in the time and frequency
domain, requiring statistical measurement of these signals for meaningful
characterization and differentiation. The Power Statistics Complementary
Cumulative Distribution Function (CCDF) measurement displays curves to
characterize the higher-level power statistics of digitally modulated signals. The
curves can be useful in determining design parameters for digital communications
systems.
The Power Statistics CCDF measurement displays probability on the Y-Axis and
amplitude on the X-axis, for a display of the statistical amplitude distribution of a
signal. This distribution can be affected by many factors. For example, modulation
filtering, modulation format, combining the multiple signals at different frequencies,
number of active codes, and correlation between symbols on different codes with
spread spectrum systems will all affect measurement results. These factors are all
related to modulation and signal parameters. External factors such as signal
compression and expansion by nonlinear components, group delay distortion from
filtering, and power control within the observation interval also affect the
measurement.
The power measured in power statistics CCDF curves is actually instantaneous
envelope power defined by the equation:
P = (I
2
+ Q
2
) / Zo
where I & Q are the quadrature voltage components of the waveform, and Zo is the
characteristic impedance.
A CCDF curve is defined by how much time the waveform spends at or above a given
power level. The percent of time the signal spends at or above the level defines the
probability for that particular power level. For capturing a lower probability down to
0.0001%, this measurement is made in the single mode by pressing Single. To make
the power statistics CCDF measurement, the instrument uses digital signal
processing (DSP) to sample the input signal in the channel bandwidth. The Gaussian
distribution line as the band-limited Gaussian noise CCDF reference line, the user-
definable reference trace, and the currently measured trace can be displayed on a
semi-log graph. If the currently measured trace is above the user reference trace, it
means that the higher peak power levels against the average power are included in
the input signal.
Power Stat CCDF Measurement Commands
The general functionality of "CONFigure" on page 2753, "INITiate" on page 2754,
"FETCh" on page 2754, "MEASure" on page 2756, and "READ" on page 2755 are
described in the section SCPI Operation and Results Query in the topic
Spectrum Analyzer Mode User's &Programmer's Reference 1131
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