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Chapter 16 465
Troubleshooting
Basic Signal Generator Operations
Figure 16-2 Reverse Power Solution
Compared to the original configuration, the ALC level is 10 dB higher while the attenuator
reduces the LO feedthrough (and the RF output of the signal generator) by 10 dB. Using the
attenuated configuration, the detector is exposed to a +2 dBm desired signal versus the
-15 dBm undesired LO feedthrough. This 17 dB difference between desired and undesired
energy results in a maximum 0.1 dB shift in the signal generator’s RF output level.
Signal Loss While Working with Spectrum Analyzers
The effects of reverse power can cause problems with the signal generator’s RF output when
the signal generator is used with a spectrum analyzer that does not have preselection
capability.
Some spectrum analyzers have as much as +5 dBm LO feedthrough at their RF input port at
some frequencies. If the frequency difference between the LO feedthrough and the RF carrier
is less than the ALC bandwidth, the LO’s reverse power can cause amplitude modulation of
the signal generator’s RF output. The rate of the undesired AM equals the difference in
frequency between the spectrum analyzer’s LO feedthrough and the RF carrier of the signal
generator.
Reverse power problems can be solved by using one of two unleveled operating modes: ALC off
or power search.
ALC Off Mode
ALC off mode deactivates the automatic leveling circuitry prior to the signal generator’s RF
RF LEVEL
CONTROL
SIGNAL GENERATOR
OUTPUT CONTROL
MIXER
LO
IF
ALC LEVEL/
DETECTOR
MEASURES
+2 dBm
ALC LEVEL
DETECTOR
MEASURES
- 15 dBm
REVERSE
POWER
LO FEEDTHRU
= - 5 dBm
RF INPUT
= - 8 dBm
LO LEVEL
= +10 dBm
10 dB
ATTEN
RF OUTPUT
= +2 dBm
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