PMX40 RF Power Meter – INSTRUCTION MANUAL
3-11
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Moderate signal level (above about -40 dBm except when modulation is off).
•
Noise-like digitally modulated signals such as CDMA (and all its extensions) or OFDM when
probability information is helpful in analyzing the signal.
•
Any signal with random, infrequent peaks, when you need to know the peak-to-average
ratio or Crest Factor and just
how infrequent
those peaks are.
Complementary Cumulative Distribution Function
The statistical analysis of the current sample population is displayed using a normalized
Complementary Cumulative Distribution Function (CCDF) presentation shown in Figure 3-7. The CCDF
is the probability of occurrence of a range of peak-to-average power ratios on a log-log scale. CCDF is
non-increasing in y-axis and the maximum power sample lies at 0%. A cursor allows measurement of
power or percentage at a user-defined point on the CCDF. As with all other graphical displays, the
trace can be easily scaled and zoomed, or the statistical data may be presented as a table in Text
Display mode.
Figure 3-7. CCDF Plot
The CCDF has become a useful tool for analyzing communication signals that have a Gaussian-like
distribution (CDMA, OFDM) where signal compression can be observed at rarely occurring peaks. It is
most often presented graphically using a log-log format where the x-axis represents the relative offset in
dB from the average power level and the log-scaled y-axis is the percent probability that power will
exceed the x-axis value.
In a non-statistical peak power measurement, the peak-to-average ratio is the parameter that describes
the headroom required in linear amplifiers to prevent clipping or compressing the modulated carrier.
The meaning of this ratio is easy to visualize in the case of simple modulation in which there is close
correspondence between the modulating waveform and the carrier envelope. When this
correspondence is not present, the peak-to-average ratio alone does not provide adequate information.
It is necessary to know what fraction of time the power is above (or below) particular levels. For
example, some digital modulation schemes produce narrow and relatively infrequent power peaks that
can be compressed with minimal effect. The peak-to-average ratio alone would not reveal anything
about the fractional time occurrence of the peaks, but the CCDF clearly shows this information. In
Figure 3-8, assume a full length run of one hour plus has been made and the CCDF is analyzed. At 0% is
the maximum peak power that occurred during the entire run. At 1% is the power level that was
exceeded only 1% of the time during the entire run.
