In this paper, we present a low-cost and flexible high-speed data acquisition system based on software defined radio. An open-source software development toolkit, GNU Radio, is used to develop receiver's functionality. The hardware is used to perform characterization of electric and magnetic fields from electromagnetic fields measurements. This characterization is an important tool for understanding characteristics of lightning related phenomena such as preliminary breakdown pulse (PBP), K-processes, J-processes, Narrow Bipolar Pulses (NBP), etc. Currently scientists have proven that the use of high-speed data acquisition systems produce important information on lightning processes. However, some commercial high-speed data acquisition solutions can be quite expensive, so their uses in some contexts are not viable. The total hardware cost of the proposed instrument is low compared to commercially available high-speed data acquisition systems.