Design of a very high resolution network analyser

A major characteristic of a network analyzer is its frequency resolution; i.e. the minimal distance between two adjacent frequency lines the analyzer can handle. The authors show how this frequency resolution can be enhanced. The frequency resolution can be improved either by decreasing the sampling frequency or increasing the number of time samples. The solution adopted is real-time interpolation in the generator and real-time subsampling in the acquisition unit. With this technique the generated output signal is computed in real-time starting from a small set of samples. This small set cannot contain enough information to cover the entire frequency band. However it is possible to put that narrow-band signal in any desired portion of the frequency band so that the instrument bandwidth is not reduced. Likewise, on the acquisition side the large amount of incoming samples is immediately decimated to a much smaller set and only this set has to be stored in memory. This also implies that during one measurement cycle only a fraction of the total bandwidth can be measured. The process of interpolation and subsampling is described, with the emphasis on the practical implementation choices that have to be made