Design Of A Reflectometer System Based On Six-port Technique

The load-pull measurement system can measure the large-signal characterization ofnonlinear transistors, such as impedance, power flow and reflection coefficients.Generally, an active load-pull measurement system consists of two six-portreflectometers and three passive two-port devices. In China, most of the wirelesscommunication frequency bands range from0.8GHz to2.5GHz. In order to make anactive load-pull system with high precision and low cost, we designed a six-portreflectometer in this thesis.The design scheme of the hardware circuit of a six-port reflectometer is explainedin detail. In this scheme, the wideband six-port junction makes up of four widebandhybrids which form from two8.34dB coupler. Simulation and test results show that thissix-port junction satisfies the requirement of the system, with reasonable powerallocation and good q-point distribution. Then we designed four detection circuits usingRMS power detectors and four calibration standard parts. Normal FR4printedcircuit-boards are used for the processing of hardware circuits, leading to reduced costs.Another key point of this thesis is calibration algorithm. Three methods were usedto calibrate the six-port reflectometer. Differences between their calibration accuracywere also compared. We solve the reflection coefficient by different methods and theiradvantages and disadvantages are discussed according to the experiment results.Measurements show that Hoer method is best for calibration while the linear method isthe optimum method for the calculation of the reflection coefficient in our system. Byanalyzing the calabrated systerm constants, we found that actual characterization of therealized six-port reflectometer gave good agreement with simulation results.Finally, a six-port reflectometer is builded by combining the six port network,detectors and other equipments. As we calibrate the six-port system by Hoer methodand calculate the reflection coefficient by linear method with universal computers, thedigital data must be collected by DSP at first. A group of measurement results fordifferent loads are listed to compare with that obtained using the vector network analyzer. Experimental results verify that the reflectometer can measure the accuratereflection coefficient in the designed frequency band and is suitable for constructing theactive load-pull system.