Why does the spectrum analyzer block diagram on page 216 use a bandpass filter?

Why not use a lowpass filter as you did in the example on page 217?

 

Using surface acoustic wave (SAW) filters it's possible to design very good

bandpass filters. However, using SAW filters at low frequencies results in

very large structures with poor filtering characteristics. The physcial

dimensions of the filters just become impractical.

 

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