CCDF (Trace Data)

(Not available with all measurement types)

CCDF shows the complementary, cumulative distribution function (CCDF Complementary Cumulative Distribution Function) for the selected input channel.

Overview

The complementary, cumulative distribution function (CCDF) is a statistical-power calculation and can be performed only on time-domain data. As its name suggests, CCDF is the complement of CDF Cumulative Distribution Function: The cumulative probability that a parameter will be less than a given value X., and is defined as follows:

CDF(K) = Probability(x £ K)
CCDF(K) = Probability(x ³ K)

CCDF provides better resolution than CDF for low probability signals, especially when log format is used for the y-axis.

The VSA plots CCDF using units of percent (%) for the y-axis and power (dB) for the x-axis. Power on the x-axis is relative to the average signal power, so 0 dB is the average power of the signal. Therefore, a marker readout of

Trace A Mkr 1: 2 dB 12 %

means there is a 12% probability that the signal power will be 2 dB or more above the average power.

Note the following when making CCDF measurements:

For best results, set the VSA's displayed frequency span to include all the energy of your signal. In other words, make sure the displayed frequency span includes the entire bandwidth of the measured signal.
The CCDF measurement does not restart:
- After a calibration
- After you continue a paused measurement
Many channel-specific changes such as changing the gate delay or input coupling restart the CCDF measurement on both channels
The CCDF trace shows the DATA? indicator when the average power drifts 8 to 10 dB from the average power measured in the first time record. For example, the CCDF trace would display DATA? if a transmitter signal was measured that was off when the CCDF measurement started but then powered on later in the measurement.
CCDF measurements are disabled during time averaging.

CCDF Calculation

Calculate the RMS value for all measured samples; this becomes the 0 dB point at the left end of the x-axis.
Normalize all samples to the RMS value in units of dB.
Determine which x-axis bin each sample belongs in between 0 and 20 dB.
Calculate the total number of samples that are greater than or equal to each x-axis bin and plot as a percent of the number of samples measured.

Samples Used in the Power Measurement

For the Vector and Analog Demod measurement types, the VSA computes CCDF using all samples in the current time record (all points in the active trace). Each successive time record adds additional samples to the CCDF measurement.

For other measurement types that provide the CCDF trace, the VSA computes CCDF using all samples specified within the measurement interval, which is specified on the Time tab of the measurement type properties dialog.

Restarting the CCDF Measurement

Selecting CCDF, restarting the measurement, or changing most measurement parameters restarts the CCDF measurement. For example, changing the range or center frequency resets the number of samples used in the CCDF measurement to zero and restarts the CCDF measurement.