Crest Factor Measurement: Advantages and Disadvantages

This page describes Crest Factor (CF) Measurement, a method used for amplifier linearity testing. It outlines the benefits and drawbacks of Crest Factor measurement.

What is Crest Factor Measurement?

The Crest Factor is the ratio of peak power to average power. The figure below depicts a test setup for CF measurement. Like NPR measurement, this method also uses a band-limited noise signal as input to the amplifier.

Image Courtesy: Microwave Journal

As shown, a directional coupler or power splitter is used to couple the incident signal for measurement using a wideband power sensor. The video bandwidth (VBW) of the coupler/sensor should be greater than the bandwidth of the noise signal for undistorted measurements.

The second measurement is carried out at the output of the amplifier (DUT). An attenuator is used at the output of the DUT to keep the output within the measurement range of the sensor used at the output.

Based on the experiment, Crest Factors (CFs) of the input and output are compared. An output CF less than the input CF indicates that the amplifier is in the compression region.

Using the input average power and output average power, one can determine the gain of the amplifier. However, gain changes with average power do not provide an accurate measurement of the amplifier’s compression region. Hence, the CCDF (Complementary Cumulative Distribution Function) curve is used to obtain more insight into the amplifier’s performance.

Advantages of Crest Factor Measurement

Following are the benefits of Crest Factor measurement:

• This method clearly identifies the amount of signal compression in wideband OFDM signals used in 5G and Wifi communications. OFDM signals have higher crest factors. This is not possible with average power measurements, as they do not reflect signal impairments occurring in such signals.
• CF measurement method uses low-cost noise sources and wideband USB peak power sensors, resulting in lower costs for the user.
• The method is simpler and less error-prone, whereas spectrum analyzers can be difficult to configure and their results can be difficult to interpret.
• It offers higher accuracy, as the uncertainty of power sensor measurements is in the order of (1/10)th of 1 dB, whereas in spectrum analyzers/signal generators it is on the order of 1-2 dB.

Disadvantages of Crest Factor Measurement

Following are the drawbacks of Crest Factor measurement:

• The CF method provides a single value based on a calculation from the maximum single peak power value of total measurement samples. Hence, the CCDF curve is used, which shows the amount of time a signal spends above the average power level. This CCDF probability curve is used by engineers to study the behavior of different classes of amplifiers.
• Gain changes with average power measurements do not provide an accurate indication of amplifier linearity. This is observed in multi-tone signals such as OFDM/OFDMA, in which the average power gain reduction is significantly smaller than compression.