Coherent addition of intermodulation distortion in spectrum analyzers

Third-order intermodulation distortion testing using CW signals has long been known to be subject to errors due to the coherence of the spectral components generated in the device under test and the analysis device. The same effect occurs when the test signal is noise-like, such as a W-CDMA signal, even though such a signal appears to be noncoherent. This paper demonstrates the effect, so that the reader can be better prepared to optimize the accuracy of measurements of adjacent channel leakage in W-CDMA components.