Working with
Spectrum
Analyzers/Reverse
Power Effects
Reverse p ower is a problem with sp ectrum analyzers that do not
have preselection capability. Some analyzers haveasmuchas
+5 dBm LO feedthrough coming out of their RF input, at some
frequencies. The eects of reverse power are less in the hetero dyne
band (0.01 to 2.0 GHz) where the power amplier provides some
broadband matching. Similarly, at frequencies ab ove 2.0 GHz,
reverse p ower that is within 10 MHz of the swept signal generator's
frequency may b e partially absorb ed by the YIG lter. If the
frequency dierence is small enough to be within the leveling
system bandwidth (typically 10 kHz CW, 200 kHz sweep or AM),
the eect of reverse power is amplitude mo dulation of the swept
signal generator's output. The AM rate equals the dierence in RF
frequencies. Reverse p ower problems may b e treated by using the
unleveled mo de. There are two unleveled mo des, ALC o and search.
To set the swept signal generator to the ALC o mo de:
1. Refer to menu map 1.
2. Press
4
ALC
5
.
3.
Select
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
Leveling Mode ALCoff
.
In this mo de, the swept signal generator provides RF power with
no ALC correction and therefore requires a po
wer meter to set a
particular p ower.
To set the swept signal generator to the search mo de:
1. Press
4
ALC
5
.
2.
Select
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
Leveling Mode Search
.
In this mo de, the swept signal generator is in the normal ALC
mo de until the desired power level is reached, then the ALC is
disconnected.
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