Active Load Pull & Hybrid-Active Load Pull

Introduction

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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, Γ_L is nothing more than a2/b2, or the ratio between the reflected- and forward-traveling waves.

Active Load Pull

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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 Γ_L can achieve unity (Γ_L>1 is theoretically possible but has no practical consideration).

Hybrid Passive-Active Load Pull

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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 Γ_L. While active load pull systems are extremely quick, capable of Γ_L=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 Γ_L=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 Γ_L,nf=1. In both cases, only a low power is required for active tuning.

Typical setup for Active Load Pull measurements using an Agilent PNA-X.


Vector-Receiver Load Pull with active fundamental impedance control


Vector-Receiver Load Pull with passive fundamental and active harmonic impedance control

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