Technical Insights > RF + Microwave

Verify Wideband High-Power Amplifiers with Modulation Distortion Technique

2020-07-22  |  8 min read 

In wireless communications systems, RF power amplifiers (PA) play a critical role in the quality attained in the RF chain. Engineers maximize PA's linearity while maintaining high output levels. This balance brings more challenges to achieve extremely wide signal bandwidths at millimeter-wave frequencies.

Today, the common industry practice for RF PA characterization has two tactics of instrumentation for stimulus-response tests — a vector network analyzer (VNA) and a signal generator (SG) plus a signal analyzer (SA), as shown in Figure 1. A VNA characterizes RF PAs under a continuous-wave (CW) stimulus condition. While the signal bandwidths increase, the nonlinearity of a wideband PA cannot be characterized completely using a CW stimulus signal. You need to stimulate a real wideband test signal using a wideband vector SG and measure the PA's output responses using a wide analysis bandwidth SA. Besides, you need to move a device under test (DUT) between the two test systems to cover all the test items and calibrate the systems to make accurate and repeatable measurements.

Figure 1. Characterize RF wideband power amplifier performance

Table 1 illustrates the tactics and test items for distortion measurements using VNA and an SG plus an SG. To measure the nonlinearity of a wideband PA, you need to apply these two tactics in order to cover all the test items.

Table 1. Distortion measurements using a VNA and an SA plus an SG
  VNA SA plus SG
Distortion measurements
  • s parameters
  • Gain compression (P1 dB)
  • Third-order intermodulation (TOI) distortion
  • Noise figure measurements
  • Linearity with AM/AM and AM/PM (continuous wave)

What is the modulation distortion measurement?

Engineers are looking for a way to simplify the test setups and perform accurate, repeatable measurements. The new modulation distortion method uses a VNA and a vector signal generator (VSG), allowing complete and accurate characterization of an RF amplifier with a single connection. This method uses the architecture of the VNA to make distortion measurements at the input and output of the DUT, as shown in Figure 2. The setup continually measures the input and output signals. It also calculates the spectral correlation that enables the decomposition of the output spectrum into the linearly correlated and nonlinear distortion spectrum.

Modulation distortion measurement setups
Figure 2. Modulation distortion measurement setups

How the modulation distortion measurement works

When the input signal increases, the amplifier starts to behave in a nonlinear way. The output spectrum consists of the linear correlated and nonlinear distortion part. You can model the output signal spectrum as below:

Y(ƒ) = H(ƒ) X(ƒ) + D(ƒ)

Y(ƒ): spectrum of the output signal

H(ƒ): spectrum of the input signal

X(ƒ): complex transfer function

D(ƒ): Distortion part

The VNA measures the amplitude of the input spectrum |X(ƒ)|, the amplitude of the output spectrum |Y(ƒ)|, and the phase relationship of the tones to each other, j(Y(f)) - j(X(ƒ)). Calculation of the spectral correlation between the input and output spectrum can decompose the linear correlated and distortion spectrum that results from the DUT, as shown in Figure 3. The pink trace (the center diagram) is the linear part, and the green trace (the right diagram) is the distortion part.

Decompose the linear correlated and the nonlinear distortion spectrum
Figure 3. Decompose the linear correlated and the nonlinear distortion spectrum

ACP and NPR measurements

The nonlinear distortion spectrum includes in-channel and out-of-channel distortion. The ACP measurement is straightforward and is the ratio between the amount of spectral regrowth in an adjacent channel (the left and right side in the left diagram of Figure 3) to the power in the channel (the middle in the left diagram of Figure 3). The in-channel spectrum of the distortion part is what we characterized as the NPR, as shown in the right diagram of Figure 3.

The coherent measurement at the input and output makes the modulation distortion measurement independent of distortions from the VSG.

The coherent measurement at the input and output makes the modulation distortion measurement independent of distortions from the VSG. This means the VSG’s performance is not critical to the measurement results.

EVM measurements

Using the modulation distortion method, the VNA does not need to demodulate the signal to obtain the EVM results because it measures in the frequency domain. The total amount of distortion measured in the frequency domain equals the total amount of distortion in the time domain, as measured using the common demodulation EVM method. So, you do not need a wideband signal analyzer and knowledge of the parameters of the test signal; you can obtain EVM measurement results with the distortion part spectrum.

The total amount of distortion measured in the frequency domain equals the total amount of distortion in the time domain

Without demodulating the complex modulation schemes, you accelerate the EVM measurement speed. Besides, a VNA uses a narrowband receiver to capture the signal and sweeps local oscillator frequencies to stitch each frequency band (the bandwidth of the VNA’s receiver) for the entire frequency span. The narrowband receiver brings in less noise that provides a better dynamic range (SNR) and lower EVM noise floor then a wideband signal analyzer.

Deliver wider system dynamic range and lowest residual EVM

Traditional test setups, especially for 5G power amplifiers and beamformer integrated circuit design, verification, and production, are complex and can introduce a wide range of sources for potential errors. These include mismatch and cable loss, which compromise signal fidelity and reduce the accuracy and repeatability of measurements. The modulation distortion technique enables designers to accurately, repeatably, and quickly characterize the behavior of a device under a modulated wideband signal stimulus. It leverages state of the art calibration for the best accuracy, offers a single connection and a single touch for existing VNA measurements, and provides the lowest possible residual EVM available in the market today.