Agilent Spectrum Analysis Basics - Agilent Technologies

1+
Signal frequency (GHz)
6
5.3
4.7
4
3
2
3
Preselector
bandwidth
4 4.4 5 5.6
LO frequency (GHz)
1–
6
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 very 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.
Atten
Low band
path
3 GHz
3.9214 GHz
321.4 MHz 21.4 MHz
Analog or
Digital IF
Input
signal
High
band path
3 - 7 GHz
To
external
mixer
3.6 GHz
300 MHz
321.4 MHz
Preselector
Sweep generator
Display
Figure 7-8. Front-end architecture of a typical preselected spectrum analyzer
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