Introd to load pull syst and applic
TCET 2220 Project 2 Assessment
Prof. Viviana Vladutescu
Name: Arie Meiman
Date: May 6, 2013
Course: TCET 2220 TRANSMISSION SYSTEMS
Applications of Load-Pull Systems in Transmission Lines
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The research paper is “Introduction to Load-Pull systems and their Applications”, and its written by Mohammad S. Hashmi and Fadhel M. Ghannounchi. This paper presented the most common concepts and techniques of load-pull design and testing in modern wireless power amplifier. As well as the differences in benefits and limitation of both, active and passive load-pull matching circuits techniques. In addition, two of the most recent development in load-pull techniques were discussed as well.
Load-Pull (LP) System is commonly used on non-linear devices, such as transistors and power amplifiers (PAs), with large signals passing through them. Under large signal operating conditions, load-pull is used for RF and microwave power amplifier (PA) design, to achieve the most optimal results.
Since our course is about transmission lines, and its characteristics, this paper directly related to the covered material in the class. Topics, such as reflection coefficient, and smith chart are very familiar to me since we studied them in depth during the semester. Therefore I’m able to use the context of this paper and reference it to our course. This paper gave me some additional insights on the overall understanding of the smith charts parameters, and how they are implemented in different applications. The first reference, “Automatic Load Contour Mapping for Microwave Power Transistors” was the most helpful to understand in more depth the terminology and concepts mentioned in this paper.
There are a few differences between the passive and active load pull systems. Passive load pulls systems are easy to use, low implementation and maintenance costs, and they don’t have any oscillation. Standard passive load pull is usually unable to synthesize reflection coefficients near the boundary of the Smith chart. Where, active load pull techniques synthesize reflection coefficients near and on the boundary of the Smith chart, and therefore can synthesize extremely small impedances for matching DUTs.
The main disadvantage of passive load pull is the limitation of synthesized impedances due to limitation of the maximum achievable magnitude of reflection coefficient. With closed loop systems, there is a risk of oscillations that can happen since a closed loop structure is used. The DUT(device under test) presents an insertion loss causing the oscillations in the closed loop structure. A passive impedance tuner and active load pull called a hybrid load pull setup, is used to achieve the desired load pull functionality.
The pre matching technique is limited and requiring impedances less than one Ohm. When a high power DUTs requiring a load pull setup with high reflection coefficients, the approximate optimal impedance is known. Quarter wave transformer technique is a special case of pre matched systems, where pre matching is fixed. The quarter wave transformer moves the matched impedance environment from 50Ohm to some other smaller value, resulting in enhanced tuning range, and reduced Smith chart coverage.
The quarter wave transformer virtually acts the same as Klopfenstein transformer, where the main difference relies in their frequency range. Klopfenstein transformer has a much wider bandwidth (100Hz-12GHz), and its replaces the quarter wave transformer, to allow full harmonic load pull applications.
Incorporation of an impedance transformer in a passive load pull enhances the reflection coefficient tuning range, whereas the incorporation of such a transformer in an active load pull reduces the stringent requirement on the loop amplifier for achieving a high reflection coefficient.
There are two latest developments in load pull configurations presented in this paper. The first is called enhanced loop passive load pull technique, and it consists of an impedance tuner and passive loop cascaded together. In this technique, the passive loop first moves the matched point farther from 50(Ohm) by Đ“ loop, and therefore synthesizes the high reflection coefficient at the load reference plane.
Reading suggestions:
In Smith charts: Lines and Fields in Electronic Technology, By Stenley & Harrington
In load pull systems: F. Blache, J.-M. Nebus, P. Bouysse, L. Jallet, A novel computerized multi-harmonic active
load-pull system for the optimization of high-efficiency operating classes in power transistor,
in IEEE International Microwave Symposium Digest, Orlando, USA (June 1995), pp. 1037–1040
