The goal of load pull is to measure the response of a device as a function of load impedance. Considering our DUT as a two-port device, ΓL is nothing more than a2/b2, or the ratio between the reflected- and forward-traveling waves.
Active Load Pull
Active load pull relies on external sources to inject a signal into the output of the DUT, thereby creating a2. Because a2 is no longer limited to a fraction of the original reflected signal, as is the case with the traditional passive mechanical tuner, external amplifiers may be used to increase a2 nearly indefinitely so that ΓL can achieve unity (ΓL>1 is theoretically possible but has no practical consideration).
Hybrid Passive-Active Load Pull
Both traditional passive mechanical tuner systems and active injection load pull systems have their advantages and disadvantages. While mechanical tuners are simple, less expensive and can handle high power, there is no physical way to overcome the losses involved with the system that limit obtainable ΓL. While active load pull systems are extremely quick, capable of ΓL=1 and easily integrated for harmonic measurements on-wafer, high-power setups require more-expensive band-limited amplifiers.
It is possible to obtain the advantages of both systems while minimizing the disadvantages, using a technique referred to as hybrid load pull. Hybrid load pull refers to a combination of active and passive tuning in the same system. Traditional passive mechanical tuners can be used to reflect high power at the fundamental frequency allowing a much smaller active injection signal, using much smaller amplifiers, to overcome losses and achieve ΓL=1. Additionally, since the powers at harmonic frequencies are often well below the power of the fundamental signal, less-expensive wideband amplifiers may be used with active tuning to accomplish active harmonic load pull with ΓL,nf=1. In both cases, only a low power is required for active tuning.
Active load pull system overview and demonstration
Tour IVCAD Software Suite
IVCAD advanced measurement and modeling software, offered by Maury Microwave and AMCAD Engineering supports multiple load pull techniques including traditional load pull using external instrumentation, VNA-based load pull, active load pull and hybrid load pull. It performs DC-IV and pulsed-IV measurements and incorporates device modeling tools. Its modern visualization capabilities give users a greater ability to view, plot and graph measurement data in an intuitive manner.
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Application Notes and Data Sheets
On-Wafer, Large-Signal Transistor Characterization from 70–110 GHz Using an Optimized Load-Pull Technique
Overcoming the Challenges of On-Wafer Load Pull Measurements at Millimeter-Wave Frequencies for 5G Applications
Tracing The Evolution Of Load-Pull Methods
Using Maury ATS Software to Extend the Agilent PNA-X to Active Load Pull; An Introduction to Active Load Pull
Hybrid Load Pull
Agilent/Maury/AMCAD Webinar on Active and Hybrid-Active Load Pull Using the PNA-X