Working
with
Reverse power is a problem with spectrum analyzers that do not
Spectrum
Analyzers/Reverse
Power
Effects
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 CW 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 CW 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 CW generator to the ALC off mode:
1.
Refer to menu map
1.
2. Press
IALC).
3.
Select Leveling Mode
ALCoff.
In this mode, the swept CW generator provides RF power with
no ALC correction and therefore requires a power meter to set a
particular power.
To set the swept CW generator to the search mode:
1. Press
m.
2.
Select Leveling Mode Search.
In this mode, the swept CW 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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