Active Load Pull
Active injection load pull, more commonly referred to as 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). The simple active tuning chain consists of a signal source, a variable phase shifter and a variable gain stage. Common signal generators, such as the Keysight MXG, have built-in amplitude and phase control of the injected signal and are ideal for active load pull..
Hybrid-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 achievable 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 a2 for the active load pull as well as the receivers to measure the applied and transmitted power.
Hybrid-Active Load Pull to 110 GHz
To date, active and hybrid-active load-pull have been limited to coaxial measurements in commercially available systems because of the impracticality of measuring and controlling the magnitudes and phases of the a and b waves using waveguide frequency extenders. To overcome this limitation, Maury Microwave has introduced custom low-loss couplers with integrated down-conversion, which extend VNA-based load-pull measurements to 110 GHz, and a source control unit enables accurate amplitude and phase control to 110 GHz. The custom couplers enable direct a and b wave measurements at the vector calibrated DUT reference plane, increasing system measurement accuracy and providing critical device performance— output power, power gain, input Γ and power-added efficiency (PAE) —for each input power level and load impedance state. The amplitude and phase controller with the frequency multiplier and step attenuator on the load loop enable 0.01 dB magnitude and 0.1 degree phase control of the a2 wave.
Active Load Pull to 1.1 THz
Performing device characterization measurements at sub-THz frequencies can be challenging . It’s difficult to achieve accurate and repeatable power-control at the DUT reference plane when using waveguide extenders between 110 GHz and 1.1 THz, as the extenders typically output a fixed power. However, controlling the power delivered to the input of the DUT is critical in setting the device’s operating conditions and characterizing its performance over power, such as the large-signal characteristic gain compression.
MMW-STUDIO mmW and Sub-THz characterization software, offered by Maury Microwave and Vertigo Technologies, is a software suite designed to work with a 4-port VNA using waveguide extender modules and add accurate and repeatable high-resolution power control. The software enables the direct measurement of vector corrected power at the DUT reference plane, as well as control over the power delivered to the DUT. Doing so allows engineers to perform gain compression power sweep measurements over the available level of powers, and to perform S-parameter measurements at any arbitrary power level. MMW-STUDIO LP is a software add-on, which when used in conjunction with a Vector Modulation Unit (VMU), enables control over the magnitude and phase of the signals delivered to the input and output of the DUT. This enables an engineer to set arbitrary impedances, or perform active load pull measurements, where the magnitude of reflection presented to the DUT is achieved by controlling the reflected a2 wave and fulfilling Γ=a2/b2.