Advanced RF power amplifier characterization: waveform engineering and time-domain load pull

Your task

Modern RF power amplifiers (especially GaN-based devices for 5G, 6G, radar, satellite and aerospace) are frequently operated near their nonlinear limits to maximize efficiency or increase output power. Traditional smallsignal S-parameters or scalar power measurements cannot fully capture device behavior.

When designing and validating amplifiers, engineers need to determine the optimum source and load impedance at the fundamental frequency and at harmonic frequencies. The harmonic terminations directly influence output power, efficiency, linearity, voltage swing, current waveform and device stress.

Conventional vector load-pull measurements provide valuable information about output power, gain, compression and efficiency relative to impedance.

Advanced waveform engineering also needs calibrated magnitude and phase information about the incident and reflected RF waves. Only with this information can RF voltage and current waveforms at the reference plane of the device under test (DUT) be reconstructed and used to understand the real operating mode of a device.

This information is important when optimizing highefficiency amplifier modes, validating nonlinear models or correlating simulated and measured device behavior. The challenge is to combine accurate phase-calibrated wave measurements, harmonic impedance control and a practical load-pull workflow in one measurement environment.

Rohde & Schwarz solution

Rohde & Schwarz and Focus Microwaves created a solution for phase-corrected vector load-pull and time-domain waveform characterization.

The setup combines an R&S®ZNA vector network analyzer with an R&S®ZN-ZCG comb generator (phase reference) and load-pull system from Focus Microwaves. The harmonic tuners from Focus Microwaves provide source and load impedance to the DUT with independent magnitude and phase control of the reflection coefficient at f0, 20 and 30, allowing each harmonic termination to be set and swept independently without disturbing the others. TheR&S®ZNA measures the calibrated incident and reflected waves, while the R&S®ZN-ZCG provides the phase reference required to align receiver phase information across fundamental and harmonic frequencies to create a practical measurement workflow for RF I/V waveform engineering. Engineers can characterize load-pull in the frequencydomain while reconstructing time-domain voltage and current waveforms at the DUT reference plane. The result is a deeper understanding of how impedance tuning can change actual device operations.

The state-of-the-art Focus Device Characterization Suite (FDCS) software supports the integration of the R&S®ZN-ZCG comb generator into the Focus Microwaves vector load-pull workflow. This allows the R&S®ZN-ZCG to act as the phase reference module for time-domain waveform measurements, together with the tuner calibration, wave calibration and load-pull measurement from Focus Microwaves.

Phase calibrated wave measurements

Stable phase measurements with a vector network analyzer (VNA) require a direct digital synthesizer (DDS) in the VNA and a phase coherence mode for calibrated phase relations on all receivers. The R&S®ZNA vector network analyzer provides both while the R&S®ZN-ZCG comb generator acts as the precision phase reference for the R&S®ZNA. The phase-characterized harmonic spectrum creates an easy one-step process for calibrating R&S®ZNA receivers across all internal frequency components.

This is essential for measuring the phase relationship between the fundamental and harmonic waves of a nonlinear DUT.

After calibration, the measured a-waves and b-waves are amplitude and phase adjusted and can be used in the FDCS time-domain workflow to derive RF voltage and current waveforms. Doing so requires direct receiver access for the R&S®ZNA, bidirectional couplers for the a-waves and b-waves, the R&S®ZN-ZCG comb generator and the generated vector correction file loaded in the FDCS wave load-pull project. The dynamic load line, waveform clipping, voltage-current overlap, harmonic content and the impact of different harmonic terminations can then be observed.

The additional waveform insight helps engineers move beyond traditional load-pull contours. Instead of merely identifying the impedance for maximum power or efficiency, engineers can understand why specific impedance levels work and how device waveforms are shaped.

Summary

The integration of the R&S®ZNA, R&S®ZN-ZCG and Focus Microwaves load-pull system provides a practical workflow for RF I/V waveform engineering and time-domain load-pull characterization.

Focus Microwaves load-pull tuners enable accurate impedance control at fundamental and harmonic frequencies, while the R&S®ZN-ZCG provides the phase reference needed for calibrated harmonic wave measurements.Engineers can reconstruct RF voltage and current waveforms at the DUT reference plane and link load-pull results directly to real amplifier behavior.

The solution extends classical frequency domain load-pull measurements into time domain, waveform-aware design workflows that let users optimize power, efficiency and linearity, while also helping validate high-efficiency amplifier modes and improve correlation between measurements and nonlinear simulations.