Load pull testing

Load pull analysis solutions

Nonlinear device characterization for active components

Active devices, such as power amplifiers (PA), are not inherently 50 Ohm components. They are designed to operate in an environment where the characteristic impedance is nominally 50 Ohms, but in reality, the impedance can vary widely. In other words, while the environment they are meant to operate in is typically standardized to 50 Ohms, the actual impedance seen by the amplifier can deviate from this value.

In order to achieve specific target specifications, such as minimizing power consumption or optimizing modulation performance, it is crucial to optimize the matching of components with the surrounding environment. This requires a test environment with variable impedances, as the performance of active devices can vary significantly depending on the impedances they are interfaced with in the target design. Load pull analysis is key to determining the optimal operating conditions, or “sweet spot,” for the final design.

In wireless systems, amplifiers often drive the RF signal into an antenna for transmission. These antennas show severe variance in their impedance around the nominal 50 Ohms. This varying impedance, seen by the amplifier, has unpredictable effects on amplifier performance and capabilities. It affects gain and efficiency, as well as modulation performance such as error vector magnitude (EVM).

Rohde & Schwarz is working with leading load pull companies to offer complete turnkey solutions for passive, active and hybrid load pull systems. Measurements can be conducted traditionally or via probing systems, depending on the accessibility of the device under test (DUT).

Your load pull testing challenges

Load pull measurements are applied in various test scenarios, and each comes with its own challenges. The scenarios can be divided into two categories: design optimization and modeling.

Design optimization challenges:

• Understanding the characteristics of different key performance indicators (KPI) across different impedances: The performance is typically shown in contour plots across a Smith chart for different impedances. Scenarios include continuous wave‑based (CW‑based) and modulated test cases based on the active device target usage. This information allows various target figures, such as maximum output power, lowest intermodulation, adjacent channel leakage ratio (ACLR) and EVM or best energy efficiency, to be optimized. Unfortunately, different KPIs have different sweet spots, so trade-offs must often be made when optimizing for the target application.
• Impedance optimization at second and third harmonic frequencies: Both impedance tuning at the fundamental frequency and impedance optimization at the second and third harmonic frequencies are required for best efficiency. This is especially true for higher power applications.
• Designing matching networks for PA transistors: This requires knowledge of the inherent device impedance at the input and output. Source pull at the input and load pull on the output help to identify the correct values.
• Verifying system level performance with different impedances at the RF frontend: This is done with a load pull measurement setup and often includes modulated performance verification with load pull testing. At the system level, when the radio frequency (RF) PA is connected to an antenna for wireless communication covering multiple frequency bands, the PA needs to handle the impedance variation of the antenna over the total used frequency range.

In modeling, the main challenge is creating reliable PA transistor models, as used in system-level simulation. For this, the transistors’ behavior must be described across many working conditions (e.g., frequency and level range), device settings (e.g., biasing) and different impedances on the input and output. Load pull analysis is typically used to get this information.

High-performance solutions for load pull testing

Load pull relies on the flexible variation of the impedance presented to the DUT, such as an RF power amplifier. This can be applied to the output as load pull or to the input as source pull, and the techniques remain the same. Load pull can be categorized based on approach and application:

• Passive load pull
• Active load pull
• Hybrid load pull
• Harmonic load pull
• Wideband modulated load pull