Introduction The Agilent N5531S measuring receiver system is the successor to the venerable (and now discontinued) 8902A measuring receiver. The center piece of the N5531S is an optional fi rmware personality module for the Agilent PSA Series spectrum analyzers (Option 233). The N5531S measuring receiver system matches or outperforms the 8902A in every operating mode, while extending the fundamental operating frequency range from 1.3 GHz (8902A) to as high as 50 GHz. Accurate power-level measurement was a key capability in the 8902A that has been carried over to the N5531S. This capability is essential for the calibration of signal-generator output level and step-attenuator accuracy. Among the more important performance parameters for any signal generator are the absolute and relative accuracies of its output power level. The highest-accuracy measurement of the output level of a signal generator requires a power meter and sensor. However, modern power sensors can measure only part of the output range of modern signal generators. Below −70 dBm, some other method must be used. The traditional approach has been to check the relative accuracy of the generator’s output level for power levels below the range of power sensors. The power meter is employed to establish a reference power level, and a sensitive, linear measuring receiver is used to verify level accuracy relative to that reference down to −140 dBm.