It is now important to emphasize some of the problems related to the measurement of CSO, CTB, and crossmodulation. The common method of making composite distortion measurements uses a spectrum analyzer operating in the LOG display mode. In this mode, spectrum analyzers measure noise and noise-like signals in error. They measure noise as approximately 2.5 dB weaker than the actual power. This may result in discrepancies among the measuring methods.
Great caution is required when correlating or interpreting measurements by other methods.
Crossmodulation measurements can also be a problem. Using a spectrum analyzer to measure the distortion sidebands directly can result in large errors. The desired measurement is actually amplitude crossmodulation. Many active devices generate phase crossmodulation with magnitudes that are 30 dB above the amplitude crossmodulation. The spectrum analyzer can not differentiate between the amplitude and phase sidebands and as a result great errors can occur.