# How to Characterize RF Distortion – Part 2: Making Distortion Measurements

In an earlier post, “How to Characterize RF Distortion – Part 1: CCDF Measurements,” I discussed how to evaluate waveform designs and statistical analysis of the signal power levels with complementary cumulative distribution function (CCDF) power curves. This is an essential step in characterizing the power distributions of digital modulation signals.

In this post, I will discuss common RF distortion types and their measurements in the frequency domain.

## RF Distortion Measurements

Distortion is the alteration of the original waveform. For a linear device, the input and output frequencies are the same; there are no additional frequencies created. The output signal only has amplitude and phase change. For a nonlinear device, the output may have a frequency shift or additional frequencies. Nonlinear distortion is typically unwanted and R&D engineers strive to minimize it. There are three major types of nonlinear distortion measurements in the frequency domain — harmonic, intermodulation distortion, and adjacent channel power (ACP) measurements.

### Harmonic distortion

The amplitude transfer characteristics of a circuit or device cannot precisely track the input signal. The amplitude shifts generate higher frequency components at integer multiples of the input signal. The high frequency components are harmonic distortion.

The most straightforward method for measuring harmonic distortion is to use a continuous wave (CW) tone as an input signal, and measure the output signal with a signal analyzer; see Figure 1. A device under test (DUT) might be an RF amplifier or mixer.

The signal analyzer sets to zero-span, which enables a time-domain power measurement to measure the power level at the fundamental and harmonic frequencies as shown to the right of Figure 2. Problems arise when the DUT has RF distortion product levels that approach the internally generated distortion product levels of the signal analyzer, from the sixth to tenth harmonics in this example. For more information on optimizing dynamic range for distortion measurements, please refer to the application note, “Optimizing Dynamic Range for Distortion Measurements.”

### Third-order intermodulation distortion

Two-tone, third-order intermodulation (TOI) distortion is a common test for RF distortion measurements. When two or more signals are present in a non-linear system, they can interact and create additional components at the sum and difference frequencies of the original frequencies, and at sums and differences of multiples of those frequencies. Figure 3 below shows the two-tone third-order Intermodulation measurement setup. The DUT could be an amplifier or a mixer.

Figure 4 illustrates TOI measurements with a signal analyzer. The two test tones are at frequencies 995 MHz and 1005 MHz. The third-order intermodulation products occur at frequencies 985 MHz and 1015 MHz and the TOI measurement results are 31.7 dBm (lower) and 32.2 dBm (upper).

For production testing, a vector signal generator alone can be used to generate two test tones using the internal baseband generator to save costs. Keysight offers an advanced correction routine which can suppress RF distortion products generated by the signal generator itself or an external pre-amplifier. To learn about how to configure distortion-free two-tone and multitone test signals, download the technical overview “N7621B Signal Studio for Multitone Distortion.”