B+K precision RFM3000 Series, Руководство пользователя, Страница: 24 / 58

Getting Started 24
Pulse mode is best for the following types of measurements:
• Moderate signal level (above about -40 dBm except when modulation is off).
• The signal is periodic.
• A time snapshot of a single event is needed (minimum single-shot time is 200 nanoseconds).
• Typical modulation and signal types: LTE, 5G, RADAR, SatCom, TCAS, Bluetooth, Wireless LAN.
3.2.3 Statistical Mode
Certain modulated signals are completely random and provide no event that can serve as a trigger for measurements.
CDMA or OFDM are common examples. The RFM3000’s Statistical mode was designed to provide measurements for
these types of signals.
Statistical mode is only available when using peak power sensor. It is the best choice for analyzing signals with a high
crest factor, that are noise-like with random or infrequent peaks, or 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.
In Statistical mode the instrument continuously samples the input signal and processes all of the samples to build power
histograms. Many digitally modulated spread-spectrum formats use 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 forthe following types of measurements:
• 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 (CCDF)
The statistical analysis of the current sample population is displayed using a normalized Complementary Cumulative
Distribution Function (CCDF) presentation shown in figure
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. 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. The statistical data may be presented as a table in Text
Display mode.
The CCDF is 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.