关于谐波Load Pull技术的综述

116 June 2013

1527-3342/13/$31.00?2013IEEE Digital Object Identifier 10.1109/MMM.2013.2248631

Andrea Ferrero (andrea.ferrero@polito.it) and Marco Pirola are with the Electronic and

Telecommunication Department, Politecnico di Torino, Turin, Italy.

Date of publication: 7 May 2013

Harmonic

Load-Pull Techniques

T

he characterization of microwave devices under nonlinear conditions is fundamental for device technology development, to improve device large signal model accuracy and reliability,

as well as an essential tool to support the design of power amplifiers (PAs). Since the input and output device terminations at fundamental and harmonics frequencies play a crucial role to set the device behaviors and performances, load-pull (LP) measurements are today one of the promi-nent characterization solutions at RF, micro-, and millimeter waves. I n this article, a concise but comprehensive overview of the most important LP techniques available today is presented and pros and cons of the several approaches are high-lighted and discussed.

Introduction

The term LP refers to the evaluation of a figure of merit (FOM) of a device under test (DUT), e.g., a GaN pseudomorphic HEMT rather than a com-plete power module, when the input and/or out-put loading conditions are changed. From the analysis of the measurement data, the loads able to provide the desired FOM can be identified straightforwardly and then synthesized in the design phase. LP measurements have become a de facto high-frequency characterization standard for several microwave and RF applications. They not only support PA designers but also provide feed-back for process amelioration and useful insights to nonlinear model developers for model improve-ments and refinements. For example, the design of PA matching networks can be accomplished in a

Andrea Ferrero and Marco Pirola

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single round with a high degree of success if proper LP data are available.

Since 1972, when Cusack et al. [1] introduced the first computer-assisted mechanical tuner, measure-ment systems and control software have evolved seamlessly into integrated hardware–software plat-forms suitable for the characterization of nonlinear DUTs. The manual or automatic test sets that measure

DUT performances as power, gain, and effi-ciency of the early days evolved to advanced LP systems, for example enabling the real-time monitoring of time-domain waveforms [2].Once the bench is properly calibrated, no matter how sophisticated it is, from single-frequency power-meter-based LPs to the most recent active differential four-port equipment, the obtained measurements give an effective insight of the actual nonlinear behavior under different loading [3]–[6]. For example, Figure 1 shows the effect of second harmonic load on the power-added efficiency of an HEMT transistor, while Figure 2 shows a more sophisticated example of harmonic differential mode load map with the corre-sponding time domain waveforms.

However, LP systems provide informa-tion strictly on the conditions under which the DUT is measured since interpolation and extrapolation are not possible. In other words, LP data are usable only at the operative condi-tions where they have been carried out. This empirical framework represents, at the same time, the good and bad aspects of LP tech-niques when compared to nonlinear device models results. Nonlinear models potentially offer much broader applicability and flex-ibility, moreover, they are fully integrated in computer aided design (CAD) tools and can operate under, at least in principle, various device operative conditions. However, to pro-vide accurate and useful data, models must undergo careful tuning and tweaking that at the end reduce their applicability range [7].Today, wireless and RF applications are pushing the edge of technological limits and, for successful designs, the traditional LP mea-surements, simply conceived to provide the best loading conditions, ensuring the desired performances, are no longer enough. I t has been demonstrated [8] that the terminations at harmonic frequencies, and not only the load at fundamental tone, are crucial for the maxi-mization of some PA FOMs (e.g., efficiency

and linearity). Moreover, for certain applications, like ultrafast bus, the differential PA driver is the preferred choice, hence the most favorable common and differ-ential loads must be identified.

From these considerations, it is clear there is a need for advanced LP setups, which span from high-power mechanical tuners with separate harmonic controls, to active differential LPs with separate control of com-mon and differential terminations [9]–[11].

Modern Load-Pull Systems

The purpose of an LP system is to measure DUT per-formances under different load or source impedances. An LP test bench can be conveniently seen as being formed by two distinct and defined sections:

?the measurement part, which acquires the DUT performances

?the loading section, which synthesized the required terminations.

t is important to keep the two aspects clearly divided since DUT performance evaluation is a typical microwave measurement application, while load syn-thesis and control is specific to the LP measurement field.

LP setups must provide experimental data, at least, on

?bias and frequency operative conditions ?input and output power levels

?load reflection coefficients in magnitude and phase.

Figure 1. Power-added efficiency (%) at 2 dB output power compression as a function of second harmonic (8 GHz) loads on a 4 GHz GaAs FET PA. Blue circles: LP measurements,

black solid: efficiency contour levels.

? m a s t e r s e r i e s

LP measurements have become a de facto high-frequency characterization standard for several microwave and RF applications.

? m a s t e r s e r i e s

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June 2013

Originally, LP systems were based on power meters

and mechanical tuners, as shown in Figure 3, where

a pair of power meters is used to monitor the input

and output power levels. All test bench components

(e.g., cables, transition, and couplers) are precharacter-ized, while tuners are measured at several frequen-cies under many possible states with a lengthy and

bulky precalibration procedure by a vector network

analyzer (VNA). When performing the DUT measure-ments, the controlling software de-embeds all the lin-ear parts and uses the tuner lookup table to compute the impedance shown at the DUT ports. This system precalibration can be considered the Achilles’ heel of traditional LPs, since the accuracy strongly depends on how close the effective measurement conditions are to the ones experienced during the calibration; furthermore, only the acquired tuner states can be actually used. Another issue for the overall accuracy is the output power meter position which, as shown in Figure 3, is cascaded to the mechanical tuner. This arrangement is particularly critical for highly reflec-tive loads, i.e., close to open or short circuits, since the rather poor tuner transmission toward the power meter results in a system-reduced dynamic range [12].

Recently, these limitations have been surpassed by the commercial introduction of real-time LP and,

today, all the modern test benches

are based on this strategy [13]–[15].

Although known since the 1980s

[16], [17], modern real-time or vector

receiver LP systems have ultralow-loss couplers between the DUT refer-ence plane and the tuners, as shown in

Figure 4. Now the VNA is used for the measurement of the impedance pre-sented at the DUT and for the power

levels [18]. I n this way, real-time LPs have vector measurement capability,

and the setup calibration is fast and

broadband since the same methodol-ogy established for VNAs is adopted. One of the most relevant aspects of

this approach is that tuner and system

block precalibrations are no longer required, since the loads at the DUT ports are measured in real time dur-ing the DUT characterization.The VNA is also used to acquire the absolute power levels, while a power meter is only required for the calibra-tion. This feature is extremely impor-tant since VNAs have a much higher Figure 3. Passive LP.Microwave Source Input Variable

Attenuator

Input Amplifier Power Meter Power Meter Power

Meter

Input Tuner

Output Tuner

Port 1Port 2

DUT VNA

P1P2

Input Load

Output Load

Figure 4. Real-time LP.

D i f -D i f f e r e n t i a l C u r r e n t (m A -2210-11

24.7 dBm 27.3 dBm 2)

1m Load at Second

Harmonic

Figure 2. An example of time domain differential harmonic LP. Fundamental

frequency 1 GHz. Voltage and current waveform for two values of second

harmonic load.

LP data are usable only at the

operative conditions where they have been carried out.

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dynamic range than power meters, and it has been demonstrated that the overall measurement accuracy is at least one order of magnitude higher than those of traditional solutions [12]. Accuracy is fundamental to successfully use an LP system for the optimization of the amplifier performances because the characteriza-tion of a modern transistor with large gate periphery, which typically has highly reflective reflection coef-ficients .,09>L C ^h emphasizes any small residual calibration uncertainty [12]. This problem is further stressed when high-power-added efficiencies (>80%) at very high C must be measured. In this case, real-time LPs are the only possible solution since power-added efficiency uncertainties of a few % are almost impossible to achieve with traditional power-meter-based LP systems [19]. Low-loss directional couplers are key components of these novel LP setups because conventional tuners can be adopted without compro-mising the possibility of synthesizing highly reflec-tive loads. Coupler behavior and performances play a decisive role not only in terms of insertion loss, but also concerning directivity, coupling, return loss, and stability behavior, which must be ensured across the whole frequency band. Although systematic uncer-tainties, hence coupler effects, can be accounted for, and thus removed with the bench calibration, good hardware is mandatory to reach the desired measure-ment stability and repeatability [19]. Today, ultralow loss couplers have insertion loss below 0.25 dB and flat coupling up to 35 GHz [20].

Besides the couplers, there are several different solutions on the receiver side: from PCI extensions for instrumentation (PXI )-based VNAs to time domain receivers with high-speed sampling scopes [6], [21]. The simpler and more economic approach is adopting a conventional VNA with a direct sampler/mixer access option [13]. If an external switch unit is added to increase the effective number of receiver channels, the measurement of other quantities such as the source reflection coefficient S C can be also carried out [15]. Figure 5 shows an example of a real-time-based system. All real-time LPs typically are able to provide harmonic measurements of reflection coeffi-cients at the input and output and, if a proper phase reference unit is available, time domain waveforms also can be acquired [4]. Table 1 presents a comparison of the main measurement features of traditional and real-time systems.

Automatic Load Technologies

Today, load technology spans from modern tuners with multiple slugs to innovative active loads. The common specifications are ?frequency range

?harmonic tuning capabilities ?maximum power handling

?maximum voltage standing wave ratio (VSWR).

Switch Unit

Loop Amp Loop Unit

Couplers Source

DUT

VNA

Figure 5. A real-time LP with an active loop. (Photo

courtesy of HFE.)

When performing the DUT

measurements, the controlling software de-embeds all the linear parts and uses the tuner lookup table to compute the impedance shown at the DUT ports.

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There are three main load technologies available on the market today:

?mechanical tuner ?open-loop active load ?closed-loop active load.

Mechanical tuners have been the first solution adopted to automatically change impedance in an LP test bench. They rely on one or multiple slugs inside an airline. A set of stepper motors accurately move the slugs along the airline as well as push the slugs close to the center conductor, as shown in Figure 6. Horizontal and vertical slug motions change the reflection coef-ficient phase and amplitude, respectively. Tuner tech-nology has greatly improved in the last 20 years, from single-slug systems to the sophisticated multislug harmonic tuners of today. In most recent tuners, sev-eral slugs are present with different frequency behav-ior. Different impedances at fundamental frequency and harmonics can thus be independently synthe-sized [22]. An example of a modern harmonic tuner is shown in Figure 7. Since the different slugs interact, to independently control each harmonic impedance, a sophisticated software is required, which accounts for the multidimentional tuner map, and synthesizes the desired impedances.

A lt houg h moder n t u ners ca n sy nt he si ze almost purely reactive reflection coefficients (Max

VSWR > 200), the presence of cables, fixtures, probes,

and connector losses reduces the maximum achiev-able reflection coefficient at the DUT reference planes. Since losses increase with frequency, this issue is more marked at millimeter-wave but it is, however, important at wireless bands where the characteriza-tion of high-power transistors with sub-ohm match-ing impedance is normal. Furthermore, when on-wafer LP measurements are needed, since vibrations due to mechanical tuners are unacceptable, a coaxial cable must be placed between the probe and the tuner, which again negatively affects the maximum attain-able reflections.

For these reasons, the active load techniques have been introduced. From the original Takayama and Bava works [23], [24], the active load technology has reached the multiharmonic solutions available today. The working principle of an open-loop active solution is the injection at the load side of a signal generated from an independent source that can be controlled in magnitude and phase. The original idea used the main signal generator followed by a splitter and a controlling network, as shown in Figure 8, while in

Slug

Movement Phase Control

Electrical Equivalent

L 1

L 2

L 3L 3

Z

The working principle of a mechanical tuner.

Mechanical tuners have been the first solutions adopted to automatically change impedance in an LP test bench.

First Harm Slug

Second Harm Slug

Third Harm Slug Fourth

Harm Slug I/O Coax

Figure 7. A modern multiharmonic tuner. (Photo courtesy of Focus Microwaves.)

DUT

Figure 8. An active open-loop concept. The backward wave is obtained by attenuating and shifting the source signal.

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today’s active loads, several independent

phase-locked synthesizers are typically used [25] (Figure 9). Each source has an independent control of phase and mag-nitude of its output signal, thus a set of

backward waves at harmonic frequencies is obtained and different harmonic loads

are synthesized (Figure 9). The backward

wave, set by the generators, is indepen-dent from the incident one, therefore, if for any reason the DUT output power changes, the load moves. In other words,

this approach does not synthesize an actual load but instead ensures a constant and well-defined ratio between the back-ward and forward wave. This is a major

drawback. For example, if a load close to

the edge of the Smith chart is set with the device already in compression and the

input power is reduced, the load seen by the DUT can

exit the Smith chart causing DUT damages if no cor-rective control actions are undertaken. Even if today’s

active open loops are controlled by sophisticated

algorithms that measure the effective DUT loads

and automatically operate to keep it inside the Smith

chart, the control latency time is questionable when

very highly reflective loads are required.

The closed-loop technology is the other commer-cial solution available for active load synthesization. In

this case, the backward wave is obtained from the inci-dent one through a directional coupler placed at the DUT output, as shown in Figure 10. With the closed loop approach, feedback is created and the active load behaves at any effect as a frequency selective passive tuner, where the backward wave is linearly linked to the incident one. The main issue of this technology is

related to oscillations, which can affect the system. This problem was solved in the late 1990s with the inclusion

of losses inside the loop [26] and inserting a narrow-band filter (Figure 10). Thanks to these improvements, today’s systems are as reliable and oscillation free as the open-loop setups [15]. An example of commercial active load is shown in Figure 11. If more active loops are combined, a harmonic load is realized, as shown in Figure 12, where each loop is centered at a differ-ent frequency to independently control the harmonic

terminations.DUT

3f 0

2f 0

f 0

f 0

Figure 9. A multiharmonic open-loop active load. The backward signal is obtained by combining several independent sources controllable in magnitude and phase.

Variable Atten Coupler

Losses Amplifier Phase Shifter A VNA

C L = C · A · G · L · e j U

U a m f 0b m YIG Filter C G L Figure 10. The active-load-loop concept: the backward wave is obtained by attenuating and shifting the incident

one taken through the loop coupler.

Figure 11. A commercial active loop. (Photo courtesy of

HFE.)

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The main drawback of both active technologies is the high power required by the load amplifiers, which, to avoid undesired harmonic contributions, must work

under strictly linear conditions. Therefore, they must operate in strong back-off conditions and the maxi-mum power requirements may result in unsustain-able costs. For this reason, a mixed approach combin-ing a tuner with one or multiple active loads has been proposed (Figure 13). The tuner acts as a prematcher, reducing, under certain conditions, the loop ampli-fier power requirements [27]. This solution, however, requires a rather sophisticated software to indepen-dently control the modulus and phase of the harmonic terminations [28].

The tuning speed is another important param-eter to consider. Active, electronically controlled solutions clearly overcome traditional mechanical tuners. However, the cost effectiveness of modern multislug tuners offers a good compromise in term of costs and performances, especially for in-fixture high-power measurements. I nstead, concerning on-wafer characterization, vibration free, compact, and fast-active loads, which replace the bulky mechani-cal tuners on the prober, represent a clean and very effective solution.

The frequency range of all solutions span from the wireless range to millimeter wave in different band, while passive tuners are also available at lower fre-quencies (100 MHz).

Latest Developments

The latest development of harmonic LP techniques span from broadband LP on modulated signals to four-port time-domain differential LPs [6], [29]. Test-ing devices under complex modulated signals may offer a new view of their performances but the com-plexity of the system raises the cost because of the highly specialized hardware required [21]. The dif-ferential time domain LP represents the latest evolu-tion of real-time active loop systems. Here, a four-port

Figure 14. A four-port differential active LP system.

The closed-loop technology is the other commercial solution available for active load synthesization.

Variable Att

Coupler C L

Amplifier

Phase Shifter

YIG Filter

2f 0

f 0

Figure 12. A harmonic active closed-loop scheme.

C L

f 0

2f 0

C

Prematch Tuner Figure 13. An active loop with prematching tuner.

measurement system is built up with frequency and time-domain capability, along with the use of two or more active loads to independently control the com-mon and differential mode load at fundamental and harmonic frequencies, as shown in Figure 14. Again, the complexity of this system raises the costs, but the measurement capabilities of a four-port time/fre-quency domain differential LP is unsurpassed, and complex devices like mixers, differential driven cir-cuits, dual gate, or push-pull transistors can be char-acterized more easily and deeply.

Conclusions

An overview of modern harmonic LP technologies has been presented. Today’s LP systems offer pow-erful measurement capabilities for nonlinear charac-terization, and they represent a fundamental tool for supporting the design of high-power PAs or, more generally, nonlinear circuits at high frequencies. A final remark concerns the evolution of measurement technology that will bring LP characterization more and more embedded inside the modern fabrication and design flow.

Acknowledgments

The authors wish to thank Valeria Teppati of Politecnico di Torino, Cristos Tsironis of Focus Microwaves, and Marco Garelli of High Frequency Engineering (HFE) for the precious discussions and material. References

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