CPFSK modulation and more in quadrature on STM32 microprocessors

The author, an engineer with a background in telecommunications, shares practical experience in implementing a radio modem based on the STM32 microcontroller (Cortex-M3 core). The focus is on developing an efficient modulator for the AX.25 protocol, which utilizes Continuous-Phase Frequency-Shift Keying (CPFSK). The article discusses the transition from professional test equipment to accessible microcontrollers and the optimization of signal synthesis algorithms. The author demonstrates the process of creating assembly language functions for the Cortex-M3 that allow for the fast and high-quality generation of smooth, continuous-phase signals. This article is useful for embedded systems developers interested in digital signal processing and low-level code optimization for STM32 microcontrollers, aiming to implement complex modulation methods without relying on specialized DSP processors.