Boonton 4540 Peak Power Meter User Manual - Manual (Page 47)

Boonton 4540 Series RF Power Meter
Getting Started 3-19
3.6.3 Statistical Mode
Certain signals are completely random and provide no event that can serve as a trigger for measurements. CDMA or OFDM
are common examples. The 4540 Series Statistical Mode was designed to provide measurements for these type of signals.
Statistical Mode is only effective when a peak power sensor is connected to the 4540. It is the best choice for analyzing
―noise-like‖ signals that are modulated in a random, non periodic fashion. Statistical mode yields information about the
probability of occurrence of various power levels without regard for when those power levels occurred. Many digitally
modulated spread-spectrum formats use a bandwidth coding techniques or many individual modulated carriers to distribute a
source’s digital information over a wide bandwidth, and temporally spread the data for improved robustness against
interference. When these techniques are used, it is difficult to predict when peak signal levels will occur. Analysis of millions
of data points gathered during a sustained measurement of several seconds or more can yield the statistical probabilities of
each signal level with a high degree of confidence. Statistical Mode is best of the following types of measurements:
Moderate signal level (above about -40dBm 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 just how infrequent those peaks are.
Complementary Cumulative Distribution Function (CCDF) The CCDF is the probability that the power is greater
than a specific power value. CCDF is non-increasing in y-axis and the maximum power sample lies at 0%.
In a non-statistical peak power measurement the peak-to-average ratio is the parameter which 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 which 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 show this information. Assume a full length run of one hour plus has been made and the CCDF is analyzed.
At CCDF = 0% is the maximum peak power which occurred during the entire run. At CCDF = 1% is the power level which
was exceeded only 1% of the time during the entire run.