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246 Chapter 12
Troubleshooting
RF Output Power Problems
Figure 12-2 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 a Spectrum Analyzer
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.
Setting ALC Off Mode
ALC off mode deactivates the automatic leveling circuitry prior to the signal generator’s RF output.
In this mode, a power meter is required to measure the output of the signal generator and assist in
achieving the required output power at the point of detection.
Use the following steps to set the signal generator to the ALC off mode:
1. Preset the signal generator: press
Preset.
2. Set the desired frequency: press Frequency and enter the desired frequency.
3. Set the desired amplitude: press
Amplitude and enter the desired amplitude.
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