The security of software defined radio (SDR) software is essential to the trustworthiness of the overall radio system. When designing and developing security solutions for SDR software, its performance requirements, such as stringent real-time constraint, need to be considered. In this paper, we describe a tamper resistance scheme that was designed to thwart the unauthorized tampering of SDR software. This scheme utilizes code encryption and branch functions to obfuscate the target program while enabling the program to satisfy its performance requirements. The scheme employs a technique called the Random Branch Function Call (RBFC), which enables a user to control the tradeoff between integrity checking frequency and the overhead. We have rigorously evaluated the scheme using various performance metrics and quantified the relationship between the end-to-end delay overhead (caused by the tamper resistance scheme) and voice quality in the context of a voice communication network.