1406 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 59, NO. 5, MAY 2010 A Multiport Measurement System for Complex Distortion Measurements of Nonlinear Microwave Systems Walid S. El-Deeb, Student Member, IEEE, Noureddine Boulejfen, Senior Member, IEEE, and Fadhel M. Ghannouchi, Fellow, IEEE Abstract—An effective multiport measurement system for the characterization of N -port nonlinear microwave devices is pro- posed. The proposed measurement system has the capability of measuring the S-parameters of linear microwave circuits and the amplitude-modulation-to-amplitude-modulation (AM/AM) and amplitude-modulation-to-phase-modulation (AM/PM) con- versions for fundamental and harmonic frequencies for nonlinear microwave devices on single-step measurement without any need for reconnection or change in the calibration technique or mea- surement setup. It is found suitable that the proposed system can accurately quantify the impact of crosstalk on the performance of multibranch ampliﬁers and multiport nonlinear microwave systems in terms of power efﬁciency and signal distortion. Index Terms—Active load-pull, calibration, microwave transi- tion analyzer (MTA), multiport measurements, nonlinear charac- terization, power ampliﬁer (PA), radio frequency (RF). I. I NTRODUCTION C URRENTLY, the most important challenge in the design of mobile communication base stations is the realization of high-efﬁciency and high-power ampliﬁers (PAs). The design of these PAs requires consideration of the effect of output load impedance variations on critical PA parameters, such as output power, drain efﬁciency, and intermodulation distortion [1]–[3]. The effect of output load impedance variation on the performance of the PA is called the load-pull effect. Load- pull measurements are widely used in radio frequency (RF) PA design and characterization, because they allow for direct measurement of the device under test (DUT) under actual oper- ating conditions. By varying the source and load impedance for the given biasing conditions, the performance of the transistor can be optimized to meet the desired performances in terms of output power, linearity, and/or power-added efﬁciency of the transistor [4]. Manuscript received June 30, 2009; revised January 15, 2010. Current version published April 7, 2010. The Associate Editor coordinating the review process for this paper was Dr. Gerd Vandersteen. W. S. El-Deeb and F. M. Ghannouchi are with the Department of Electrical and Computer Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: wseldeeb@ucalgary.ca; fghannou@ieee.org). N. Boulejfen is with the Department of Electrical Engineering, University of Hail, 2440 Hail, Saudi Arabia, and also with iRadio Lab, Department of Electrical and Computer Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: nourane@uoh.edu.sa). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TIM.2010.2045033 Several measurement systems have been developed for the load-pull large-signal characterization of microwave devices using either a vector network analyzer (VNA) [4], [5] or a six- port reﬂectometer [6], [7]. These measurement systems have been designed to characterize two-port DUTs using load-pull measurements at the fundamental frequency and/or at a number of its harmonics. The multiport VNA is very expensive and not available to many research laboratories. Characterization of an N -port DUT using a two-port VNA is time consuming since it requires C 2 N successive two-port measurement series to obtain the full scattering parameters (S-parameters) of the DUT. During these measurements, (N − 2) matched loads have to be used to terminate the ports of the device that are not connected to the network analyzer. As the number of ports of the DUT increases, the process becomes more complicated and inaccurate since there are many measurement steps that have to be performed. Furthermore, many useful measurements for N -port DUT char- acterization cannot be performed using a two-port measurement series. For example, the effect of crosstalk between the ports of an N -port DUT on the whole performance of the device cannot be measured using a two-port VNA, as it will be discussed later. In this paper, a multiport measurement system that is suit- able for the characterization of linear and nonlinear multiport microwave devices is presented as an extended version of the work reported in [8]. The system has the ability of making full large-signal char- acterization of a multiport DUT at the fundamental frequency and its harmonics in a one-step measurement connection, which saves time and increases the accuracy and credibility of the measurement results. The system gives accurate results with input power variation using only one calibration procedure at any power level within the range of variation [8]. The proposed measurement system is based on the use of the microwave tran- sition analyzer (MTA) as a receiver since the system requires access to a complex wideband harmonic receiver or high-speed time-domain sampler [9]. Although the MTA is an old and obsolete instrument, it is still available and useful; hence, it can be integrated in a measurement system to perform complex multiport nonlinear measurements, provided that a suitable calibration procedure and an appropriate test set are used. In this paper, the system is used for the characterization of a commercial two-port PA (ZFL-2500) by measuring its amplitude-modulation-to-amplitude-modulation (AM/AM) and 0018-9456/$26.00 © 2010 IEEE