A Real-Time Microwave Network Analyser Using Multiport Techniques

Abstract

The first ever pilot prototype of a real-time swept—frequency network analyser for general laboratory use is presented. Using multi-port reflectometer techniques it gives real-time animated plots of all S-parameters of a general two-port microwave device over an octave frequency band. The measurement accuracy is ±0.1dB at unit reflection coefficient and the dynamic range is 50dB. The seven-port reflectometer networks were designed to give the best signal to noise ratio in the results for reflection coefficient with limited microwave source power. Schottky Barrier Diode power detectors are used for speed, but their non-linearity and noise voltage are the major sources of measurement error in the complete instrument. A specialised high speed low level data acquisition system has been designed to convert their voltage outputs into digital form. Linearisation of the diodes’ detection characteristics is carried out by the real-time data processing program. This is a major piece of software written on a 16—bit (MC68010-based) microprocessor system. It carries out all the steps in the calculation of reflection coefficient and S-parameters from the raw detector output readings. A high resolution colour graphic display is the principal output device of the reflectometer system. A new calibration method for a multi-port reflectometer using the minimum number of standard terminations has been developed. It uses a new general approach to the multi-port reflectometer calibration problem. It can use the frequency independent short circuit, open circuit and matched load standards used by current superheterodyne automatic network analysers, requiring only one more termination to be connected to the reflectometer during calibration. Existing two-state two-port S-parameter measurement and calibration methods have been extended so that they can be used by a network analyser that does not incorporate bandwidth restricting ferrite isolators. No extra two-port devices are used in the full network analyser calibration, and only three extra measurements need be made.