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Advantages of Crest Factor measurement | disadvantages of Crest Factor measurement

This page describes Crest Factor (CF) Measurement used for amplifier linearity test. It mensions benefits or advantages of crest factor measurement and drawbacks or disadvantages of crest factor measurement.

What is Crest Factor measurement?

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

CF, Crest Factor Measurement test setup
Image Courtesy : Microwave Journal

As shown, directional coupler or power splitter is used to couple incident signal for measurement using wideband power sensor. The video bandwidth (VBW) of coupler/sensor should be greater than bandwidth of the noise signal. This is needed for undistorted measurements. The second measurement is carried out at the output of the amplifier (DUT). Attenuator is used at the output of DUT to keep the output in measurement range of the sensor used at the output.

Based on the above experiment, CFs (i.e. Crest Factors) of input and output are compared. The output CF less than input CF indicates that amplifier is in compression region. Using input average power and output average power, one can determine gain of the amplifier. Gain changes with average power does not provide accurate measurement of the amplifier's compression region. Hence CCDF curve is used to obtain more insight into performance of the amplifier.

Benefits or advantages of Crest Factor measurement

Following are the benefits or advantages of Crest Factor measurement:
➨This method clearly identifies 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. Hence this incurs lower cost on the user.
➨The method is simpler and less error prone where as spectrum analyzers are difficult to configure and difficult to interpret the results.
➨It offers higher accuracy as uncertainty of power sensor measurements is in (1/10)th of 1 dB where as in spectrum analyzers/signal generators it is on the order of 1-2 dB.

Drawbacks or disadvantages of Crest Factor measurement

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

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