The BK Precision 391A is a high-performance, programmable AC power source that provides a stable and reliable power supply for testing and evaluating electronic devices. It features a wide range of voltage and frequency settings, making it suitable for a variety of applications. The 391A also includes a built-in waveform generator, which allows users to create custom waveforms for testing purposes. With its advanced features and capabilities, the BK Precision 391A is an ideal solution for engineers and technicians who need a versatile and reliable power source for their testing needs.