W
‘orking
with
SpectrGm
Analyzers/Reverse
Power Effects
Reverse power is a problem with spectrum analyzers that do not
have preselection capability. Some analyzers have as much as
+5
dBm
LO feedthrough coming out of their RF input, at some
frequencies. The effects of reverse power are less in the heterodyne
band (0.01 to 2.0 GHz) where the power amplifier provides some
broadband matching. Similarly, at frequencies above 2.0 GHz,
reverse power that is within 10 MHz of the swept signal generator’s
frequency may be partially absorbed by the YIG filter. If the
frequency difference is small enough to be within the leveling
system bandwidth (typically 10 kHz CW, 200 kHz sweep or AM),
the effect of reverse power is amplitude modulation of the swept
signal generator’s output. The AM rate equals the difference in RF
frequencies. Reverse power problems may be treated by using the
unleveled mode. There are two unleveled modes, ALC off and search.
To set the swept signal generator to the ALC off mode:
1. Refer to menu map 1.
2. Press
IALC).
3.
Select Leveling Node ALCoff .
In this mode, the swept signal generator provides RF power with
no ALC correction and therefore requires a power meter to set a
particular power.
To set the swept signal generator to the search mode:
1. Press
a.
2.
Select Leveling Mode Search.
In this mode, the swept signal generator is in the normal ALC
mode until the desired power level is reached, then the ALC is
disconnected.
1-32 Getting Started Advanced
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