Active Load Pull
One disadvantage of all passive processes with mechanical tuners is the ohmic loss in the tuner and the supply line to the probe, which prevent extremely low impedance values. As an example, amplifiers with a HF power of more than 10Watts may require load impedances as low as 1W . If, however, the losses already amount to more than 1W,, the amplifier cannot be systematically examined in this value range any more. This restriction can be overcome by means of active load pull systems. The basic principle is depicted in the illustration below:
The amplifier under examination produces a complex wave amplitude, a, at its output. The active load pull system produces a return wave amplitude, b, in the opposite direction. By adjusting the value and phase of wave b in relation to wave a, it is possible to create any required load impedance at the reference plane. The outlined principle is called a closed-loop process, since a decoupled part of the input wave is used for feedback. This process is very stable regarding undesirable oscillations; it forms the basic principle for the commercially available active load pull systems by Maury-Microwave. Since value and phase of the return wave are adjusted by electronics, and as there are no longer any mechanically moving components, these active load pull systems achieve an extremely short test time. The process is not limited to load pull at the output. With a similar circuitry at the input of the DUT, it is also possible to vary the input impedance (source pull). For stability evaluations, it is also possible to create negative impedances. In this case, the value of the wave amplitude b is higher than a.
One distinct advantage of this method is its easy expansion for load pull measurements with several harmonic frequencies. For this purpose, frequency multipliers are used to generate harmonic HF carrier signals, which are phase-synchronized with the fundamental wave. For each harmonic, a feedback circuit is implemented, allowing it to be fed back into the amplifier output with a variable value and phase. With the Maury-Microwave systems, it is possible to generate and use up to 6 active frequencies. It is up to the user to decide which part of the available frequencies is used for source pull and which part is used for load pull.
The photo shows a test system during a practical demonstration. In order to preserve the high flexibility of the basic system, external amplifiers are used for feeding back high HF powers. The depicted measurement system is equipped with 4 active frequencies in a frequency range of 0.7-40GHz. Power is only limited by the selection of couplers and external amplifiers. As an example, load pull measurements have been conducted on amplifiers with a HF power of up to 500Watts (Active Load Pull Surpasses 500 Watts, Application Note 5C-087, Maury-Microwave, November 2011).
The Active Load Pull Process with Modulated Signals
Communication systems work with complex modulated signals in order to transmit high data rates. Examples for today's modulation standards are 3G, 4G and 5G interfaces for mobile communications, 802.11ac for WLAN routers and 802.11p for the new automotive data interfaces. Modulation bandwidths of more than 100MHz are often required. It is very important to conduct testing with a modulated signal, in accordance with the product application. Due to high modulation bandwidths, the test results deviate significantly from those measured with CW signals. In addition, the appropriate frequency spectrum needs to be ensured which is required to fulfill government regulations for product approval. The active load pull systems by Maury-Microwave support load pull measurements with modulated signals which conform to all common standards. The illustration below shows the principle of a fully automated measuring station for 4 frequencies (fundamental wave fo and harmonic 2×fo at the input and the output of the probe):
The amplifier under test is fed with a complex modulated signal at the input. However, the amplified signal shows a modified spectral form at the output. In order to be able to feed back this exact signal, the spectral form of the output signal is measured first. This signal is then reconstructed and fed back as a return wave with variable value and phase. However, since the DUT reacts to load changes with a change of the spectrum, this control process needs to be repeated several times until there is a satisfactory consistence between the spectrum of the output signal and the spectrum of the reconstructed return signal.