Application of a Dual Quadrature Demodulator to Improve the Accuracy of Spectrum and Frequency Measurements

Quadrature demodulation occupies an important place among the methods of measuring the frequency of non-sinusoidal signals. There are known applications of a quadrature demodulator to perform accurate measurements of the amplitude and phase values of individual harmonics, as well as electrical power parameters. The main problem of using a quadrature demodulator to perform high-precision measurements of signal parameters is related to the inverse ratio of static and dynamic error components when performing measurements. To ensure a low static error, the quadrature demodulator requires high-order output filters. This leads to a large dynamic error due to the considerable time required to completion of the filter transition process. The paper attempts to solve this problem by introducing an additional quadrature demodulator that performs similar measurements, but with a phase-shifted version of the input signal by π/2 relative to the original one. The proposed approach reduces the order of the output filters of the demodulator, and, therefore, the resulting dynamic error. The main features of the implementation of the proposed approach for a phase shifter on a time delay block, a digital integrator and a pair (a lowpass filter for one demodulator and a bandpass filter for the second) of second-order digital IIR filters are considered. The variants of application of the proposed approach for the case of measuring the parameters of the main spectral component and harmonics are proposed. The sources of the static component of the error are considered. The effectiveness of the proposed modification in terms of final accuracy, setting time and the number of arithmetic operations compared with the traditional approach to the construction of demodulators is evaluated. Analytical relations have been obtained that make it possible to estimate the final static component of the measurement error.