www.anritsu.com | 11When measuring close in spurious components, the shape of the IF filter becomes important. The filter skirt inclination is determined by the ratio of the filter bandwidth at –60 dB to that at –3 dB (Figure 11).This skirt inclination is known as the ‘shape factor’ of the filter and provides a convenient guide to the filter quality. The most common type of IF filter is known as the Gaussian filter, since its shape can be de rived from the Gaussian function of distribution. Typical shape factor values for Gaussian filters are 12:1/ 60 dB:3 dB, while some spectrum analyzers utilize digital filters where the shape factor can be as low as 3:1. Digital filters appear to be better in terms of frequency resolution, but they do have the drawback of sharply increasing the scan time required to sweep a given frequency range. Figure 12 shows the effects of scanning too fast for a given IF bandwidth filter. As the scan time decreases, the displayed amplitude decreases and the apparent bandwidth increases. Consequently, frequency resolution and amplitude uncertainty get worse, and some analyzers will warn you that you are now in an ‘UNCAL’ mode.Figure 11Δf @ –3 dBΔf @ –60 dBFigure 12