CCDF_baisc concept

CCDF

(Vector Signal Analyzer application only,
Not available with Digital, W-CDMA and cdma2000 demodulation)

Menu Path: Trace > Data > Ch1 or Ch2 > CCDF

CCDF trace data displays the complementary, cumulative density function (CCDF) for the selected input channel.

Overview

The complementary, cumulative density 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, 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 analyzer 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 signal average power, so 0 dB is the average power of the signal. Therefore, a marker readout of

Trace A Marker 2 dB 12 %

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

CCDF Calculation:

1. Calculate the RMS value for all measured samples; this becomes the 0 dB point at the left end of the x-axis.

2. Normalize all samples to the RMS value in units of dB.

3. Determine which x-axis bin each sample belongs in between 0 and 20 dB.

4. 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 Demod Off and Analog demod modes, the analyzer 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 WLAN - OFDM and -DSSS demod modes, the analyzer computes CCDF using all samples specified within the measurement interval.

Restarting the Power 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.

Tips

Note the following when making CCDF measurements:

• For best results, set the analyzer'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 restart the CCDF measurement on both channels, such as changing the gate delay, or input coupling.

• The analyzer displays DATA? if the average power drifts 8 to 10 dB from the average power measured in the first time record. For example, the analyzer would display DATA? if you measured a transmitter signal that was off when the CCDF measurement started but then turned on later in the measurement.

• CCDF measurements are disabled during time averaging.