Metrological Requirements for an Impedance Meter for Computational Automatic Antenna Tuning Units

Abstract

One of the conditions to implement measuring and computing methods for discrete Antenna Tuning Units (ATU) configuring is to establish dependencies between the accuracy of measuring antenna parameters at the operating frequency and the matching quality in an extremely short period of time. The priority task is to regulate the nomenclature of measured quantities, ranges of their values, algorithms for processing measurement information performed by the computational component of the measuring system, and to develop requirements for measurement accuracy. The analysis results of the applied impedance measurement methods made it possible to choose the approach for measurement in the RF I-V range of decametric waves (ammeter-voltmeter). Such a technique provides the best accuracy in the wide range of impedances required for practical application. In particular, this method is used in advanced measuring devices produced by Agilent Company, ensuring better performance compared to vector network analyzers. The detailed measuring system based on the RF I-V method includes a high-precision coaxial port for accurate impedance measurement in a given frequency range. The article describes the procedure for meter single-port calibration to reduce systematic errors, and options for including a voltmeter and an ammeter for measuring low and high impedance values. The measurement accuracy effect on broadband transformers with low losses, limiting the operating frequency range from below, and showing a distorting effect on the circuit being measured, is taken into account. A method to synthesize the requirements for two circuits of an impedance meter for computational Automatic Tuning Units (ATU) is presented, and corresponding calculation expressions are obtained that can be used to determine the dynamic range and permissible error values. In particular, it is shown that the ATU meter should measure impedances corresponding to the values of the standing wave coefficient up to 1,000...2,000. When constructing an impedance parameter meter, it is advisable to transfer the measuring signals to an intermediate frequency of the sound range of 2...10 kHz. Audio signals can be digitized with a 24-bit analog-to-digital converter and filtered with a digital filter with a narrow band, about 100 Hz. The dynamic range of undistorted 20-bit quantization is 120 dB, which is sufficient to ensure the required accuracy.