When working with modulated signals, for a well controlled linearity behavior of the DUT, the reflection coefficients offered to the DUT should ideally be constant (not vary versus frequency) within the modulation bandwidth at the fundamental, as well as in all related frequency bands at baseband and harmonic frequencies. This situation is approximated in real circuit implementations, where the matching networks are placed directly at the reference planes of the active device.
In conventional load-pull setups, however, the actual physical impedance is always located at some distance from the DUT, which is much larger than for any practical matching network. This distance, as well as any physical length within the tuning element itself (such as the position of the probe in mechanical tuners), yields very large electrical delays causing rapid phase changes of the reflection coefficients versus frequency.
It is clear that these large phase deviations represent nonrealistic circuit conditions and will cause measurement errors such as IM3 asymmetry, spectral re-growth and EVM degradation. In general, maintaining the reflection coefficients constant over frequency is getting more and more difficult with the increase in modulation bandwidth of communication signals, not only in practical circuits, but definitely in load pull measurement setups.
Wideband Impedance Control
To overcome the aforementioned problems of losses and electrical delay in conventional load-pull characterization systems, while being able to work with realistic wideband communication signals, a novel open-loop system was developed employing wideband signal generation and signal acquisition (see MT2000 product page). When the nonlinear DUT is excited with a user-defined modulated signal, it generates signals in the baseband, fundamental and higher harmonic frequency bands. By measuring the device reflection coefficient at every frequency, the waves to be injected are estimated at every iteration. When the required reflection coefficient versus frequency (at every controlled band) is achieved, the iteration has converged and the large-signal parameters (power added efficiency, output power, intermodulation distortion, etc.) are measured. To address these needs.
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Application Notes and Data Sheets
Mixed-Signal Active Load pull System - 0.3 to 40.0 GHz
Active Harmonic Load-Pull With Realistic Wideband Communications Signals
Active Harmonic Load-Pull for On-Wafer Out-of-Band Device Linearity Optimization
A Mixed-Signal Approach for High-Speed Fully controlled Multidimensional Load-Pull Parameters Sweep
Base-Band Impedance Control and Calibration for On-Wafer Linearity Measurements
A Mixed-Signal Load-Pull System for Base-Station applications
Mixed-signal Active Load Pull: The Fast Track to 3G and 4G Amplifiers
Tracing The Evolution Of Load-Pull Methods
Mixed-Signal Instrumentation for Large-Signal Device Characterization and Modelling