Architecture_Preselected_Spectrum_Analyzer

Figure 7-7. Preselection; dashed lines represent bandwidth
of tracking preselector

The word eliminate may be a little strong. Preselectors do not have infinite
rejection. Something in the 70 to 80 dB range is more likely. So if we are
looking for every low-level signals in the presence of very high-level signals,
we might see low-level images or multiples of the high-level signals. What
about the low band? Most tracking preselectors use YIG technology, and
YIG filters do not operate well at low frequencies. Fortunately, there is a
simple solution. Figure 7-3 shows that no other mixing mode overlaps the
1 mixing mode in the low frequency, high IF case. So a simple low-pass filter
attenuates both image and multiple responses. Figure 7-8 shows the input
architecture of a typical microwave spectrum analyzer.

Figure 7-8. Front-end architecture of a typical preselected spectrum analyzer