The Keysight (Agilent) N4421A S-parameter test set is a key component of an Keysight (Agilent) physical layer and balanced-measurement system. When combined with an Keysight (Agilent) PNA Series E8364B Option 014 vector network analyzer and an external PC equipped with an IEEE-488 GPIB card, this test set makes characterizing high-speed digital interconnects as well as, fully balanced or balanced-to-single-ended RF components much easier.The N4421A S-parameter test set requires the N4425A Balanced Measurement System software or the N1930A Physical Layer Test System software for control of the 4-port VNA system, calibration, and data display and analysis.