Pei, Xiaoshuai, et al. “Measurement of Soil Electrical Conductivity Based on Direct Digital Synthesizer (DDS) and Digital Oscilloscope”. International Journal of Agricultural and Biological Engineering, vol. 12, no. 6, Dec. 2019, pp. 162-8, https://ijabe.migration.pkpps03.publicknowledgeproject.org/index.php/ijabe/article/view/4840.