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Load-pull tuning has long been a vital tool in analyzing and characterizing active devices, such as RF/microwave power transistors, to changes in the source and load impedances presented to the device. Typically, load-pull tuners change the impedances to the active device while additional measurement equipment—like a vector network analyzer (VNA)—is used to measure device characteristics, such as amplitude, phase, and scattering (S) parameters.

By tuning the impedances during measurements, the optimum source and load impedances to the device under test (DUT) can be found for the creation of impedance-matching circuitry. This enables transitions from the device impedances to the characteristic impedance (typically 50 Ω) of the circuit or system in which the active device will be used.  

Fig. 1

Over the years, load-pull technology has evolved from its humble beginnings to more elaborate and sophisticated tools and techniques. Many load-pull tuners developed with broadband capabilities and intended for creating controlled impedances at fundamental frequencies, or F0, have also created uncontrolled impedances at harmonic frequencies of the fundamental, such as 2F0, 3F0, and so on. Most engineers working with load-pull testing have worked with such an uncontrolled harmonic test environment for at least a 15-year period.

Efforts to control this test environment led to the development of harmonic load-pull solutions, using a fundamental-frequency test tone and one or more harmonic frequencies of that fundamental frequency. The use of harmonic load-pull testing has often helped to increase the power-added efficiency (PAE) of an amplifier or active device, or else reduce the error vector magnitude (EVM) of a power amplifier being operated in backed-off bias conditions.

Fig. 2

Early attempts to develop harmonic load-pull tuners typically inserted narrowband frequency discriminators, such as diplexers or triplexers, between a DUT and a wideband impedance tuner (Fig. 1). While such an approach can provide some control over the harmonic frequencies, it is typically limited in its power range and bandwidth and susceptible to spurious signal products.1

By 1998, a simple solution had been proposed for load-pull harmonic tuning,2 which was “productized” in the form of the Programmable Harmonic Rejection Tuner (PHT) from Focus Microwaves in 2000. Figure 2 shows a load-pull setup using a harmonic-rejection tuner on the output of the DUT.

Fig. 3These harmonic tuners are inserted between a wideband tuner and the DUT. They use a set of remote-controlled open-stub quarter-wavelength resonators at the first- and second-harmonic frequencies (2F0 and 3F0). They slide on top of the center conductor of the airline used for the wideband load-pull tuner and can be tuned back and forth with little ill effect on the fundamental-frequency impedance, Γ(F0).

The approach used in the PHT system can provide full 360-deg. phase control at high values of Γ on the order of 0.98, critical for optimizing the PAE of a power amplifier. The PHT systems have two important characteristics: their resonators reflect all harmonic power back to the DUT so that the wideband load-pull tuner acts as a pure fundamental-frequency tuner, and they are 50-Ω-matched at low frequencies to minimize spurious oscillations.

Fig. 4

Figure 3 shows a PHT and its internal mechanism for changing resonators. Figure 4 depicts load-pull contours from a wideband tuner (the deformation due to uncontrolled harmonic impedance can be seen). Figure 5 shows load-pull contours measured after a PHT has been inserted to maintain constant harmonic impedances.

Fig. 5

The PHT systems enabled analysis of high-Γ load-pull contours for many PHT users, providing data under controlled-impedance test environments.2,3 Still, these systems suffered frequency-band limitations and the amplitude of Γ could not be controlled, leading many customers to desire more wideband tuning and full control of Γ at harmonic frequencies.

For versatility, the Multi-Purpose Tuner (MPT) was introduced in 2004 (United States patent No. 7,135,941), with the first units appearing on the commercial market in 2006. The MPT is essentially a cascade of three wideband, single-probe tuners, all covering the same bandwidth and contained within a single instrument housing. The number of probes determines the possible tuner settings (Fig. 6).

Fig. 6In the case of two cascaded probes, a few million different tuner settings are possible, allowing high-gamma tuning and two-harmonic-frequency tuning. With three cascaded probes, billions of possible probe combinations are possible, allowing fundamental-frequency [Γ(F0)] and as many as three harmonic reflection factors [Γ(2F0), Γ(3F0)] to be synthesized simultaneously and independently.

The MPT concept apparently works for all frequencies and impedances within a given tuning range and within the common frequency bandwidths of all probes. A two-probe impedance tuner can cover the entire Smith chart for two frequencies, and can also tune three frequencies independently, but it cannot do so over the entire Smith chart. There are areas of the Smith chart, inside the |Γ| range of the probes, which harmonic tuning simply cannot reach.

Given the limitations of the two-probe load-pull systems, integrating three probes within a single load-pull system housing (using the same airline) made sense as part of the evolution of load-pull technology. Such as approach, implemented in three-probe MPT systems, eliminates adapter loss and residual reflections associated with assemblies of three cascaded wideband tuners.

A system with the three integrated probes must be calibrated within a limited period of time, and developing such a system requires the development of efficient tuning algorithms using the calibration data. Unfortunately, calibrating each of the billions of possible probe positions could possibly take years. Part of patent No. 7,135,941 is a calibration method. It involves de-embedding of the initialized impedance tuner—a concept not immediately obvious, and one which enables calibrations of MPT systems in just minutes.

The second challenge in designing an effective three-probe harmonic load-pull system (efficient tuning and high harmonic isolation) required several years of development and many engineering iterations, as is often the case for original designs (patent Nos. 8,629,742, 13/915,160, and 12,929,643, pending). Admittedly, the first harmonic load-pull tuning operation in 2004 with a three-probe MPT system took more than 30 min, a task which today requires barely more than two seconds.

Fig. 7

Achieving high tuning accuracy and high harmonic tuning accuracy can be extremely difficult for a system housing a cascade of three independent, highly reflective probes and the same airline. But after almost 10 years of work on this three-probe concept at Focus, the MPT system approach provides excellent tuning accuracy of typically 50 dB or more and similar harmonic tuning isolation (Fig. 7).

Gaining these excellent performance levels required ongoing improvements in software algorithms and mechanical precision. While a single-probe tuner may have possibly 10 problems or issues for a developer, a three-probe MPT system with its greater complexity can present as many as 1000 issues to overcome.

Fig. 8The validity of a four-probe harmonic load-pull tuning approach was explored by designing a tuner named “Quattro” for operation from 1.8 to 10.0 GHz. It is capable of independently tuning across frequencies from the fundamental, F0, to the fourth harmonic at 4F0. In measurements made with this four-probe tuner, all points of a load-pull pattern at a particular fundamental frequency, F0—such as the 2 GHz in Fig. 8 (the black dots)—can be tuned while Γ(2F0) to Γ( 4F0) remain fixed (the colored dots in Fig. 8).

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