The paper presents the analysis of the operation of a quadrature demodulator of a signals modulated with the use of Quadrature Phase Shift Keying (QPSK) taking into account the influence of parameters of a low-pass filter. Such quadrature demodulators are used in modern telecommunication systems because of the problem of separating out two Phase Shift Keying (PSK) modulated signals (in-phase and quadrature) from the QPSK-modulated signal arising at the receiving end, which is followed by detecting and determining the binary signal as “one” or “zero”. This task is performed by a two-channel demodulator of the receiver separately for in-phase and quadrature components of the QPSK-modulated signal. A wrong decision can be made due to the influence of the parasitic signal components, which occur at the inputs of decision devices. In the environment of the system UDF MAOPCs (User-Defined Functions Multivariate Analysis and Optimization of the Parametric Circuits) the influence of parameters of input signals and parameters of the synchronous detector on the output signals of the in-phase channel was analyzed. On the example of the synchronous detector, it is shown that application of the frequency symbolic method makes it possible to determine the effect of selected parameters of the circuit elements on the characteristics of functional units of telecommunications equipment in explicit form.
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