The paper deals with problems of very high impedance (|Z| < 10 GΩ) measurements in a wide frequency range (10^-5 ... 10⁶ Hz). The measurements are aimed to identify electrical parameters of different anti-corrosion coatings. The construction of the measuring system designed for monitoring the anti-corrosion coatings effectiveness on various types of technical objects is presented. The circuit solutions analysed refer, in particular, to the input adapter, the measuring signal generator, and the phase-sensitive detector. The test results of the whole measuring path are shown and some parameters determining low and high band of the system frequency range are pointed out.

The transfer of information theory from data communications to digital measurement in the form of the coding theorems, descriptive languages and data patterns, has provided new digital signals. In this paper, shift keyed symbols are used to design Symbolic Addition (SA), Phase Shift Keyed (PSK) and Frequency Shift Keyed (FSK) signals. These allow different aspects of a frequency response to be investigated with one signal and one experiment. Symmetrical versions of these signals allow a set of Multifrequency Identification Patterns (MIPs) which can capture and monitor different multifrequency characteristics. SA signals are useful for obtaining information at the phase crossover allowing the determination of controller settings in auto-tuning or adaptive control. PSK signals ‘zoom in’ and give accurate details of, say, a peak in the closed loop frequency response while FSK signals highlight two or more portions of the frequency response. This allows the system parameters to be continuously monitored and analysed. These shift keyed signals are applied to different systems where an MIP or frequency information may be obtained.

Experimental results concerning the behaviour of some high value resistors versus applied dc voltage are given. The measurements have been carried out at IEN on typical standard resistors, normally available in National and secondary laboratories, in the field 10 MΩ ÷ 1 TΩ by means of the measurement systems described in [1, 2]. The measurement results, with which we tried to determine the voltage coefficients, showed for most of the measured resistors a behaviour versus voltage that is the addition of linear and quadratic components.

A compact, low cost harmonics measurement equipment, harmonics monitorTM, has been developed using a DSP based mixed signal ASIC. Each effective current harmonics from the basic frequency up to the 19^th order harmonics and the total harmonics distortion ratio can be obtained in real time. In stead of using a conventional FFT method a 6^th order, digital IIR filtering method was used for the accurate measurement. It allowed the computation to be in real time and the configuration of the main circuitry to be simplified into a single chip, thereby reducing the system cost. To compensate gain drop of ADC outputs over a continuous wide range of frequency the second order equalizing digital IIR filter was applied as well. The measurement accuracy has been realized within ± 0.5% of full scale from the basic frequency up to 19th order harmonics frequency.

For the calibration of power meters, it is preferred to use virtual sources that supply voltage and current at their outputs with high accuracy and in a manner that the phase angle between the two electric quantities can be set to the desired value. Due to the direct calibration procedure, these sources are required to be stable and accurate as well. As a transformer is included in the voltage- and current amplification circuits, the inherent nonlinearity of transformers and the handling of problems due to the effect of diffuse parameters (stability, accuracy and distortion) become of great importance. The high accuracy requirements reawakened the mathematical- and physical models that describe the operation of transformers as a function of frequency, with various diffuse parameters also taken into account. In the implementation the dependence of various diffuse quantities (inductance, capacitance) and the iron losses on the signal level raises difficulties; in fact, these result in the alteration of the corner points in the transfer function as well as undesirable accentuation.