The growth of interest in the development of reduced-scale electrocardiogram (ECG) system based on field-programmable gated-array (FPGA) design platform is increasing. This study provides initial result of mapping digital signal processing to hardware design for specific purpose. In this paper, a part of digital signal processing for atrial fibrillation classification was implemented to register-transfer level (RTL) design. The specific part was feature extraction of ECG signal. The algorithm of ECG signal feature extraction was natural frequency from second-order system for detecting atrial fibrillation. By applying high-level synthesis method, three designs were implemented for natural frequency behavior. The designs were two Single-Cycle (Design 1 and Design 2) and Multi-Cycle fully-constraint (Design 3), of which logic utilization consist of 2530, 36 and 1, respectively. Performance evaluation among all designs were compared.