Agilent Technologies E4406A, Руководство пользователя

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120 Chapter 4
Making Measurements
Making the Power Stat CCDF Measurement
Making the Power Stat CCDF Measurement
Purpose
Many of the digitally modulated signals now look noise-like in the time
and frequency domain. This means that statistical measurements of
the signals can be a useful characterization. Power Complementary
Cumulative Distribution Function (CCDF) curves characterize the
higher level power statistics of a digitally modulated signal. The curves
can be useful in determining design parameters for digital
communications systems.
The power statistics CCDF measurement 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. These factors are all related to modulation
and signal parameters. External factors such as signal compression and
expansion by non-linear components, group delay distortion from
filtering, and power control within the observation interval also affect
the measurement.
Measurement Method
The power measured in power statistics CCDF curves is actually
instantaneous envelope power defined by the equation:
P = (I
2
+ Q
2
) / Z
o
(where I and Q are the quadrature voltage components of the waveform
and Z
o
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.
To make the power statistics CCDF measurement, the transmitter
tester 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.