Specifications

90
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.
3
2
4
3
5.3
4.7
6
4 4.4
1+
5 5.6
1
–
6
LO frequency (GHz)
Preselector
bandwidth
Signal frequency (GHz)
Figure 7-7. Preselection; dashed lines represent bandwidth
of tracking preselector
3 GHz
3 - 7 GHz
Sweep generator
Display
3.9214 GHz
Analog or
Digital IF
321.4 MHz 21.4 MHz
3.6 GHz
To
external
mixer
321.4 MHz
Preselector
300 MHz
Input
signal
Atten
Low band
path
High
band path
Figure 7-8. Front-end architecture of a typical preselected spectrum analyzer