Mamadou Misbaou Bah
TCET 2220
Fall 2013
Project 2
Prof. Viviana Vladutescu
Applications of Load-Pull Systems in Transmission Lines
During this semester in the Transmission System course, we started by learning the theory behind electromagnetic waves, their characteristics and how they propagate in a medium, and as we moved on, we learn the transmission lines and some techniques used for impedance matching (such as quarter waves transformer, the matching by admittance of shunt stubs). During the class session we did some example related to impedance matching by the using calculation and by the use of the smith chart, witch a tool that uses a normalized scale to plot on the complex reflection coefficient plane in two dimension scales. The load pull technique, technique invented four decades ago has been used since in the characterization, measurement, design and optimization of transistor devices and Radio frequency power amplifiers (RFPAs).
The Load-Pull (LP) System on a transmission line (TL) is one of the most frequently techniques used for optimizing device performance in the non-linear domain. This system help assess the maximum the performance in a network by varying the impedance presented to the Device Under Test (DUT) most often transistors. Its help for the impedance matching in order to have the maximum power transfer to the load.
An active load pull system are related to the signal injection at the load-port of the DUT, they can used to synthesize very low input impedance, and they required high gain and high linearity on the feedback loop whereas the passive load pull cannot be used to synthesize the reflection coefficient near and on the boundary of the Smith Chart and is limited by the maximum achievement of the maximum of the refection coefficient.
Load pull include active and passive impedance tuner. The active load pull are divided into two main categories: the open-loop passive load-pull system and the active load-pull systems. Each one of them has some draw back. The actives load pull drawback is related to the fact that the output of the DUT depends on the device operation condition which makes it very slow for high measurement throughput application. In order to reduce these issues, one approach was the introduction of two probes called pre-matching and tuning. These two probes are placed side by side on the central conductor.
The presence of a loop structure makes the open the closed loop risky of oscillation.
The closed loop has some risk of oscillation that can explained by the fact that the synthetized reflection coefficient depend on the loop parameter, such as amplifier gain, attenuator and phase shifter. To solved this problem, some advance technique have been develop lately like the enhance loop passive load pull. A hybrid is a passive impedance tuner associated with an active load-pull system. The quarter wave is used to solve some problems related to the limitation of the pre-matching techniques, and consist of inserting a λ/4 impedance transformer between the device under Test (DUT) and the tuner. The λ/4 moves the matched impedance environment form 50 Ω to smaller impedance.
This paper enhances my knowledge not only in the application of the Smith, but also in reflection coefficient and impedance matching. I learn that one of the disadvantage of the passive load-pull system cannot be used for low output impedance. I found this paper to be a good example of impedance matching.
