Technical Insights > RF + Microwave

Speed the design of 5G components with Modulation Distortion measurements

2021-01-28  |  11 min read 

In wireless communication systems, power amplifiers (PAs) play a critical role in determining the condition of the communication service in terms of signal quality and battery life. A key transmission component is a Front-End Module (FEM), and a PA is a subcomponent of a FEM. The PA is in the last stage of the transmission chain and generates the RF power transmitted to the antenna. PA designers seek to maximize bandwidth linearity while maintaining a high level of modulation efficiency. This balance is challenging to achieve across extremely wide signal bandwidths in the millimeter-wave (mm Wave) frequency spectrum. To measure the nonlinearity of a PA under a modulated stimulus condition, the industry traditionally uses Error Vector Magnitude (EVM) as a figure of merit (FOM) for in-band characteristics and adjacent channel power ratio (ACPR) for out-of-band characteristics.

Keysight is leveraging its strong legacy in RF Test & Measurement to introduce a new simplified test method for measuring bandwidth linearity and modulation efficiency on high frequency, wide signal bandwidth components. This new test method leverages the innovative Modulation Distortion measurement for characterizing nonlinearity of a power amplifier under a modulated stimulus condition. The test approach for a 5G PA is used in this example.

Traditional test methods and deficiencies for 5G

The common legacy practice for performing PA characterization consists of two test stations (See Figure 1).

Figure 1: Legacy Test Setup

The first station uses a vector network analyzer (VNA) to make basic characterization measurements such as S-parameters, gain compression, third order intercept point (IP3), and sometimes noise figure. The second station features a signal generator and a signal analyzer and is used to generate and measure EVM and ACPR measurements, the figures of merit (FOM) for measuring PA non-linearity. The device is first tested with the VNA and then brought to the signal generator/signal analyzer test station.

The high mmWave frequencies and wide bandwidths of 5G FR2 signals make EVM measurements for 5G PAs more difficult than in the past. Measuring EVM for a 5G device with the traditional method, for instance, requires you to first modulate the signal with a Signal Generator that has a specific scheme for 5G new radio (NR) that includes a preamble, pilot, and data. You then need to capture the waveform, demodulate it with the specific scheme, draw the constellation diagram, and measure the error between the ideal constellation and the measured one to determine the EVM.

Legacy Measurement Challenge Summary

  • The EVM measurement becomes more difficult because the EVM of DUT is close to the residual EVM (EVM of test system). This difficulty is caused by the imperfection of the generated signal and the wideband noise captured by wideband receivers, and the S/N ratio degradation with bandwidth increase.
  • Measurements are inaccurate due to lossy cables and mismatches in high frequency, and the actual signal applied to DUT is different from the ideal signal.
  • Signal generation requires very wide modulation bandwidth (ex. 5G FR2)
  • A complex test system is needed for the optimization of the specific power level at the signal analyzer to minimize the nonlinearity of the receiver while optimizing S/N ratio.
  • A higher test system cost (signal generator, signal analyzer and vector network analyzer).

What is Keysight’s new Modulation Distortion measurement approach?

  • Keysight has developed a new measurement approach (Modulation Distortion) to perform PA characterization that addresses the challenges of the traditional method. This approach leverages a High dynamic range and the lowest residual EVM to quickly measure very low EVM. The Modulation Distortion test setup provides all the VNA measurements as well as ACPR and EVM in a single test station using a VNA and a Signal Generator (See Figure 2).

Figure 2: Integrated Test Setup

Test Setup and Test Approach

In this test setup, an external vector signal generator is used as an external input to the VNA to generate a standards-compliant, modulated repetitive signal with a given CCDF (Complementary-Cumulative-Distribution-Function) and PSD (Power Spectral Density). A test plan would follow these steps:

  1. Create the measurement channel – Setup the DUT input and output ports, and the internal pathways to drive the test process. Individual parameters are established for Sweep, RF path, Modulate, and Measure.
  2. Calibrate the System – Calibrating the system is a two-step process. First, perform a standard VNA calibration. Second, perform a source calibration. The source calibration establishes the reference plane, which enables a vector corrected measurement and corrects the stimulus at the DUT.  By using this source correction feature to correct your signal to the desired waveform, you will achieve the best accuracy and reproducibility of your measurement in a modulated stimulus condition.
  3. Create the Test Signal – Intermodulation (IM) is a common legacy parameter for quantifying the nonlinearity of a power amplifier. The PA is first excited by a two-tone stimulus to establish an IM measurement. You can then measure the intermodulation tones on the high-side and low-side of these tones. These measurements establish the nonlinear distortion of the power amplifier under a two-tone stimulus condition.

Modulation Distortion offers an alternative measurement method that accurately reveals the performance of the device under test (DUT) under actual operational bias conditions. You can achieve this result by using a modulated waveform with a specific signal bandwidth to create a high number of tones that stimulate the DUT. The use of a significantly higher number of tones compared to the IM method leads to more accurate performance measurements. In the Modulation Distortion approach, we create a stimulus signal called the compact test signal. Compacting the test signal increases the measurement speed. The VNA firmware selects a slice of the original VSG waveform which represents the statistical characteristics of that waveform, then removes spectral leakage using a brick-wall filter. Although it only uses a slice of the waveform, the compact test signal’s frequency signature is the same as that of the parent signal. In determining the length of the compact test signal, there is a tradeoff between measurement accuracy and speed. Using a relatively longer test signal reduces the performance difference between the two signals, with only a slight impact on measurement speed.

4. Run the Test and Analyze Results - Stimulate the input of the DUT with the compact test signal from the modulated source. Measure the DUT output signal tone-by-tone (measuring the nonlinearity) using frequency domain analysis. The VNA then measures the amplitude of the input spectrum, the amplitude of the output spectrum, and the phase relationship of the tones relative to each other. Post-processing is then performed to decompose the signal into separate components – linear and distortion. Modulation Distortion measurements are derived from the distortion component and are summarized in this table:

Error Vector Magnitude

Adjacent Channel Power

Noise Power Ratio

Band Power

Equalized EVM of the DUT (non-linear contribution)

Upper and lower side bands of the input signal

Input NPR

Input Band Power

Un-equalized EVM of the DUT (Includes non-linear and linear distortion due to frequency dispersion)

Upper and lower side bands of the output signal

Output NPR

Output Band Power

 

Upper and lower side bands contribution of the DUT

NPR contribution of the DUT

Band Power Gain of the DUT (magnitude and phase)

This simplified test setup makes it easier to accurately characterize the distortion contribution of a PA, especially in wideband applications like 5G. The wide system dynamic range generates low residual EVM and the VNA calibration technique enables high signal fidelity at the device-under-test (DUT) input. This approach to measuring Modular Distortion delivers consistent measurement results while increasing measurement speed.

Why use the Keysight Modulation Distortion application (MOD) on the PNA-X Microwave Network Analyzer?

This new test approach to measure Modulation Distortion in the PNA-X leverages some critical techniques that are unique to Keysight that bring these key benefits:

  • Compact Test Signal Creation enables accurate measurement within a relatively short time frame.
  • VNA calibration and de-embedding applied to modulation analysis for accurate modulated measurements at mm Wave frequencies.
  • Isolation of the distortion and additive noise contributions while removing contributions from the input signal increases measurement accuracy.
  • High dynamic range and lowest residual EVM to quickly measure very low EVM.
  • Eliminating the signal analyzer simplifies and cost reduces high-power setups and switch matrices for EVM, NPR and ACPR measurements.

To find out more about making Modulation Distortion measurements on the PNA-X Microwave Network Analyzer, click on the Solutions link below to download the Application Note.