Once the optimum values were obtained, the design effort shifted to replicate the identified impedance value on a circuit board designing appropriate input and output matching networks. The manufactured input and output boards were populated and mounted into a test fixture to determine their s-parameters. When necessary, the matching networks were tuned to ensure a good agreement with the measured optimum impedances. After the board tuning, the PA was assembled and tested.

Both cases resulted in a first-pass success replicating gain, output power, and efficiencies that have been obtained from the Mesuro system to the first decimal point on the realized power amplifier, accounting for the losses of the input and output matching networks. Also, both realized PA designs achieved efficiencies above 80% over a relative bandwidth of 10% to 15%, resulting in absolute bandwidth of 100 to 200 MHz [1]. The design activities demonstrate the Mesuro system’s ability to significantly cut down the required number of design iterations even for complex PA modes while achieving performances that are close to theory.

What are the advantages of the load-pull process?

Devices are characterized at the impedances they will be used at in the end applications. The true performance can be measured at relevant impedances, and subtle effects can be measured. Also, devices are characterized at the high power levels that are present at the end applications. Predicted performance does not have to be “extrapolated” when it can be measured directly.

Also, power efficiency and its constituent waveform components are given to optimize the device in a directed manner and measure the resulting device performance in a state that will be used in the final application. Thus, we are not limited to limitations of behavioral models with untested conditions (independent harmonic impedance control, high reflection coefficients) and extrapolation of capabilities beyond measurements.

Furthermore, measurements of device states provide valuable information on the device reliability. All the parameters, from the effects of memory at dc/baseband to higher-order harmonic uWave frequencies, are characterized with sampling scope technology. And, the solution is not tied into a particular EDA system. Measurements can be ported to all design environments.

Why hasn’t this technology been viable until now?

Research work on active load-pull systems has been ongoing for last the last 10 years. There has been a recent migration from the development of systems to application of measurements. Case studies involving PA design, device characterizing, and modeling have all been studied and reported.

A recent product introduction, the Tektronix AWG7122B, has greatly simplified the commercial realization for active load-pull systems for the commercial wireless amplifier device industry. Also, Cardiff University has set up a commercial entity, Mesuro Limited, to deliver on the commercial realization of an Active Harmonic Load-Pull solution in a partnership with Tektronix. These Mesuro Active Harmonic Load-Pull products were commercially announced and demonstrated in June at IMS2009 (MTT-S) in Boston.

Where can I get more information on active harmonic load pull?

Active harmonic load-pull technology and its applications have been researched for more than 10 years at Cardiff University. Papers have been presented at international conferences discussing in detail the measurement system and its applications to device characterization and analysis, device modeling, and PA design.

References

1. P. Wright, A. Sheikh, Ch. Roff, P.J. Tasker and J. Benedikt, “Highly Efficient Operation Modes in GaN Power Transistors Delivering Upwards of 81% Efficiency and 12W Output Power,” 2008 IEEE MTT-S International Microwave Symposium Digest, Atlanta, Georgia, June 15-20, 2008

2. A. Alghanim , J. Lees, T. Williams, J. Benedikt, and P.J. Tasker, “Reduction of Electrical Baseband Memory Effect in High-Power LDMOS Devices using Optimum Termination for IMD3 and IMD5 using Active Load-Pull,” 2008 IEEE MTT-S International Microwave Symposium Digest, Atlanta, Georgia, June 15-20, 2008

3. J. Lees, T. Williams, S. Woodington, P. McGovern, S. Cripps, J. Benedikt, and P. Tasker, “Demystifying Device related Memory Effects using Waveform Engineering and Envelope Domain Analysis,” 38th European Microwave Conference, Amsterdam, Netherlands, Oct. 27-31, 2008

4. S. Woodington, T. Williams, H. Qi, D. Williams, L. Pattison, A. Patterson, J. Lees, J. Benedikt, P.J. Tasker, “A Novel Measurement Based Method Enabling Rapid Extraction of a RF Waveform Look-up Table Based Behavioral Model,” 2008 IEEE MTT-S International Microwave Symposium Digest, Atlanta, Georgia, June 15-20, 2008

5. A. Sheikh, J. Lees, J. Benedikt, P.J. Tasker, “Utilization of a Measurement Based CAD Tool for Enhanced PA Design Investigations,” 38th European Microwave Conference, Amsterdam, Netherlands, Oct. 27-31, 2008

6. C. Roff, Johannes Benedikt, Paul J. Tasker, and Mike Uren, et al, “Analysis of DC-RF Dispersion in AlGaN/GaN HFETs using RF Waveform Engineering,” IEEE Transactions on Electron Devices, volume 56, issue 1, Jan. 2009