Measurements, Testing, and Calibration Advanced Measurement Topics Informational

What is the active load pull technique and how does it achieve time-variant load impedances?

The active load pull technique synthesizes arbitrary load impedances at the output of a power amplifier (PA) by injecting a coherent signal back into the PA's output port, creating an effective load impedance that can be varied continuously over the entire Smith chart, including impedances with reflection coefficients |Gamma_L| > 1 (active loads that supply power to the DUT). The principle is: the reflection coefficient seen by the DUT at its output is: Gamma_L = b2_injected / a2_from_DUT, where a2_from_DUT is the wave traveling from the DUT output and b2_injected is the wave injected back toward the DUT by the active load pull system. By controlling the amplitude and phase of b2_injected, any Gamma_L can be synthesized. The active load pull system consists of: a source of the injected signal (either a separate signal generator phase-locked to the input signal, or a loop that samples the DUT output and re-injects it with controlled gain and phase), a power amplifier in the injection path (to provide enough power to create the desired Gamma_L against the PA's output power), and a directional coupler or circulator at the DUT output (to separate the DUT output wave from the injected wave for measurement). Active load pull achieves what passive (mechanical tuner) load pull cannot: impedances on the edge of the Smith chart and beyond (|Gamma| > 0.9, up to |Gamma| > 1), harmonic load control (independent control of the load impedance at the fundamental, 2nd, and 3rd harmonics for waveform engineering), and time-variant impedances (the load impedance can be changed within a single RF cycle, enabling envelope load pull for characterizing PAE under modulated signal conditions).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Active Load Pull for Power Amplifier Design

Active load pull is the most powerful technique for characterizing and designing power amplifiers. It enables exploration of the PA's performance over the entire impedance space, including regions that passive tuners cannot reach, and provides the data needed for optimal matching network design.

System Architectures

Open-loop active load pull: A separate signal generator (phase-locked to the input source) drives a power amplifier that injects power into the DUT output through a directional coupler. The injected signal's amplitude and phase are set independently. Advantage: simple, stable. Disadvantage: slow (each impedance point requires manual adjustment)
Closed-loop active load pull: The DUT output signal is sampled, amplified, phase-shifted, and re-injected. The loop gain and phase determine the effective Gamma_L. Advantage: the impedance automatically tracks the DUT's operating frequency. Disadvantage: potential instability if loop gain > 1
Hybrid (passive + active): A passive tuner provides the bulk of the impedance transformation (near the target impedance), and a small active injection provides fine-tuning and extends the Smith chart coverage. Most practical for high-power PAs where the injection amplifier power would be excessive for pure active load pull

Active Load Pull Parameters

Common Questions

Frequently Asked Questions

Why can't passive tuners reach the edge of the Smith chart?

Passive tuners have losses in the tuner mechanism, cables, and connectors between the tuner and the DUT. These losses limit the maximum achievable |Gamma| at the DUT reference plane. A typical passive tuner achieves |Gamma| = 0.85-0.95 at low frequencies (< 6 GHz) and |Gamma| = 0.7-0.85 at mmW frequencies. For many PA technologies (especially GaN class-F or class-J amplifiers): the optimal load impedance has |Gamma| > 0.9, which passive tuners cannot reach.

What is harmonic load pull?

Harmonic load pull independently controls the load impedance at the fundamental frequency and at the harmonics (2f0, 3f0). The harmonic load impedances dramatically affect the PA's efficiency and output power. For class-F operation: the 2nd harmonic load should be a short circuit and the 3rd harmonic should be an open circuit, squaring the voltage waveform and maximizing efficiency. Active load pull can precisely set these harmonic impedances, enabling systematic exploration of the waveform engineering design space.

How fast can active load pull change the impedance?

Open-loop active load pull: impedance changes in milliseconds (limited by the signal generator's phase and amplitude settling time). Closed-loop: impedance changes in microseconds (limited by the loop bandwidth). For envelope load pull (modulating the impedance at the signal's envelope rate): the active injection can be modulated at MHz rates (matching the signal's instantaneous power variation). This enables real-time characterization of the PA's behavior under modulated signals, capturing memory effects that static load pull misses.

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