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Amplifier Linearity Measurement Methods | IMD, NPR, CF measurement

This page covers Power Amplifier Linearity Measurement Methods. The linearity measurements such as IMD (intermodulation distortion) measurement, NPR (noise power ratio) measurement and CF (Crest Factor) measurement or peak-to-average power ratio measurement are described.

GaN technology is accelerating this days. The most popular GaN devices are wideband RF Power Amplifiers (PAs) used in 5G networks. 5G system requires very stringent linearity requirements. Traditional tools are not sufficient to predict real world measurement of amplifier linearity. Following methods are simpler, more accurate and less prone to errors compare to traditional tools.

Amplifiers are characterized by various parameters which include gain, Bandwidth or frequency response, power output, linearity, efficiency and noise figure.

Amplifier Linearity measurement method-1 | IMD measurement

Linearity : It is measured by measuring output power with the uniform increase of input power until amplifier enters into compression region.

One such parameter is P1dB, which measures point where output power does not follow input power linearly i.e. change of 1 dB input power gives less than 1 dB change in output power.

TOI, Third Order Intercept Point
Figure-1 : P1dB and IP3 curve

The measurement setup for IMD measurement is depicted in above block diagram. The setup consists of two signal generators whose output is combined using RF power combiner and fed to DUT (Device Under Test). The output is measured using RF Spectrum Analyzer.

IP3 measurement setup for IIP3 and OIP3
Figure-2 : IMD measurement setup

Other the desired outputs (at F1 and F2), the setup produces other products which are harmonics or IMD products of the two input signals (F1 and F2). The major consequencies will be due to 2f1+f2 and 2f1-f2 as they exists near to desired frequencies(f1,f2) in the frequency spectrum.

IMD products
Figure-3 : IMD products

Let us understand other parameter TOI (Third Order Intercept) Point. It is the point in which, each 1dB increase in input power results in 3dB increase in the output power. Hence slope of 3 is achieved as shown in the figure. Third order intercept point is result of third order IMD products viz. 2w1-w2 as well as 2w2-w1 as shown in the frequency spectrum of figure.

The table below mentions third order intermodulation output frequencies due to application of two input frequency channels (f1 and f2).

IMD products table
Table-1 : IMD products

The graph below mentions various points P1dB at input and output, TOI points at input (i.e. IIP3) and output (i.e. OIP3) etc.

Each P1dB increase results in 3dB increase in 3rd order IM (Intermodulation) power. TOI point is a figure of merit, which characterizes a RF receiver's tolerance subjected to multiple RF signals outside passband of interest. The TOI points on X-axis and Y-axis are designated as IIP3 and OIP3 respectively.
Refer IIP3 versus OIP3 which describes difference between IIP3 and OIP3.

Amplifier Linearity measurement method-2 | NPR measurement

NPR Measurement test setup
Image Courtesy : Microwave Journal

The figure depicts test setup for NPR (noise power ratio) measurement. As shown, in this method, AWGN generator is used to generate white noise. The noise is bandlimited by filter before being fed to DUT (i.e. Amplifier). The filter used is notch filter with notch greater than 50 dB below passband amplitude and width of approx. 1% or less than filtered noise BW.

The amplitude of the applied noise signal is increased to the point of nonlinearity. As we have discussed above, IMD products are result of mixing of multiple tones when amplifier is in nonlinear region. As we know noise signal is made of many tones, it is difficult to measure individual IMD products. Hence aggregate power of IMD products is measured at notch frequency using spectrum analyzer or VNA. As the products increases, depth of notch decreases.

The method is expensive as it requires high quality filter with sufficient notch depth to observe desired IMD products.

Amplifier Linearity measurement method-3 | Crest Factor measurement

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

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.

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.

For each power level, CCDF provides amount of time signal spends above the average power level. In other words, it provides probability of signal power above the average power.

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