Power-amplifier (PA) designers are often hampered in their use of electronic-design-automation (EDA) simulation tools because of deficiencies in large-signal models. Typically, an engineer must augment EDA tools with measured data before performing a simulation. This measurement can take the form of a few data points with a manual impedance tuner or it can be a full-fledged load-pull measurement. By carefully integrating load-pull measurements with EDA tools, it is possible to overcome the limitations of current large-signal device models and improve the overall process of designing PAs.

The load-pull measurement process is, of course, a method of applying known load states (impedances) to a RF power device and measuring pertinent performance attributes (i.e., power-out, efficiency, linearity, and input impedance). As parameters, such as frequency, drive power, supply voltage, and quiescent current, are swept during the measurements, the complexity and time required for the measurements grow geometrically. Such swept load-pull measurements extend the design-cycle time because physical prototypes and measurements must be made to fill in the missing pieces for the EDA tools.

It is also not trivial to apply the load-pull data to an EDA tool. Typically, designers must translate data from the load-pull measurement domain to the simulation framework. Connecting the two environments generally involves selecting impedance "points" from either contours or tabular data from the measurement side and manually entering them into the simulation tool. This manual approach is suitable for a design with narrow performance criteria, but is unrealistic when multiple attributes (power out, efficiency, and linearity over bandwidth, drive-power, and bias settings) are considered. Poor integration of load-pull data with EDA circuit-simulation tools is a major deficiency in the RF power design process. Designers require comprehensive load-pull information (both dependent and independent variables) in their EDA tools. They want an effective RF power model that can be implemented into their simulation just like any other component model.

Major EDA tool and measurement system vendors are beginning to recognize the value of integrating load-pull data with modeling. Applied Wave Research (El Segundo, CA) introduced a load-pull wizard in their circuit simulator, Microwave Office 2002.¹ This capability facilitates viewing load-pull data and establishing load-pull simulations with device models. It also allows data files from commercial measurement systems to be integrated to the simulator versus load only. However, frequency, drive-power, and bias are fixed values making it difficult to assess performance over bandwidth or other operating parameters. The Advanced Design System (ADS) from Agilent Technologies (Santa Rosa, CA) provides the ability to generate behavioral models for simulation speed enhancements using the Load-pullSetup and AmpLoad-pull elements,² but these capabilities do not address the deficiencies of the high-power device models. Recently, Agilent has announced the ability to import load-pull measurements in ADS2003.

On the hardware side, Maury Microwave (Ontario, CA) markets a translator converting load-pull files to Agilent ADS.³ Little information is available, however, on the dimensionality of the data or on linking these data to the simulator. Focus Microwaves (Dollard-des-Oreamux, Quebec, Canada) offers the µW-PADS⁴ and WinPADS⁵ software that use contours to drive circuit-matching networks using proprietary software. These programs employ a limited set of lumped- and distributed-circuit elements that may be optimized based on load-pull measurements; unfortunately, the platform is a non-mainstream EDA environment specializing only in PA design.

Engineers within the Global Telecom Solutions Sector (GTSS) infrastructure equipment group of Motorola (SBCL—Schaumburg, IL) have long recognized the chasm between load-pull measurements and simulation tools. In the mid-1990s, a program was initiated to bridge the load-pull/simulation gap. A primary tenet of this program was to merge and analyze load-pull measurements within mainstream EDA tools. A load-pull measurement system was used to measure device data, not extensively analyze it. Data reduction was reserved for the EDA simulation domain through the use of network analysis and optimization. This vision was realized by:

1. Extracting accurate load-pull data over a wide operating space including frequency, drive-power, bias, and load.
2. Integrating this entire measurement space into a database accessible to EDA design tools.
3. Developing simulation utilities to design RF PAs exploiting load-pull measurements.

The first two tasks are intertwined. Unfortunately, commercial load-pull systems have been unable to provide the multidimensional sweep in a format compliant with the EDA tools. In the past, curve-fitting was applied to the data to achieve EDA tool compliance. However, with the addition of independent variables, curve-fitting becomes untenable. Motorola's solution, therefore, was to acquire commercial load-pull hardware and internally develop custom control software. Focus Microwaves supplied tuner hardware and provided the software-calls into the tuner hardware. Load-pull control software was written in Labview™ software from National Instruments (Austin, TX). These tools allowed the development of a semicustom system using professional-grade tuner hardware. Control of the sweep algorithms also assured that the resultant data structure was EDA compliant without the intermediate data conditioning.