V.V. Kiselev - Ph.D.(Eng.), Associate Professor, Department of Design and Technologies in Electrical Engineering, Perm National Research Polytechnic University
V.D. Volodin - Senior Lecturer, Department of Information Technologies and Automated Systems, Perm National Research Polytechnic University
A.A. Sharonov - Senior Lecturer, Department of Information Technologies and Automated Systems, Perm National Research Polytechnic University
Modern microcontrollers include a wide range of periphery devices including analog to digital converters, timer counters with PWM functions, digital to analog converters and others. These periphery devices in combination with high-performance computing core, such as ARM Cortex-M3 or Cortex-M4 allow to make devices with the functions that used to be made only with the help of analog devices. An oscillator phase-locked loop system can be referred to such devices. Not long ago such systems were used for high frequency generators and frequency synthesizers. But modern microcontrollers with advanced periphery devices allow to implement self-tuning frequency circuits into a wide range of different devices, for instance power regulators. Such systems need a low-frequency system PLL to be made.
In case of microcontroller use, most elements of a phase-locked loop system will be made with special software. One of such elements is a voltage-controlled oscillator. Its software implementation is given in the article.
The paper presents the equation of voltage controlled oscillations. Since all the digital systems including a microcontroller of PLL system, function with sampled signals, the voltage controlled oscillator equation will be adapted for digital systems. The developed software takes into account not only the frequency of generated signal but also sampling frequency. The value for sampling frequency was 8000 Hz. Moreover, integration operation was substituted for the summarization operation of controlling voltage measured values. The software contains the functions to prevent overflow of variable, including the sum of controlling voltage instrumentation results. The article also includes firmware flow chart of voltage-controlled oscillator software.
One of the basic key elements of voltage controlled oscillations equation is a controlling voltage multiplication coefficient. There are graphs of oscillator output frequency dependence on different values of controlling voltage multiplication coefficient. On the basis if these graphs the physical sense of multiplication coefficient was defined, as well as the expression for its calculation depending on the required values of oscillator output frequency. The result of the research has proved that multiplication coefficient determines minimal and maximal frequency of voltage-controlled oscillator output signal.
The accuracy of the derived expression was experimentally tested on a real breadboard with a microcontroller. In the paper, there are graphs of generated signal frequency dependence on the value of controlling voltage on the basis of a ma-thematical model and experimental test. The test has proved the validity of the derived expression.
The introduced software implementation and the mathematical model of the voltage controlled oscillator may be further used to construct a phase-locked loop system with different low frequency systems.
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