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Chapter 12 245
Troubleshooting
RF Output Power Problems
Figure 12-1 Effects of Reverse Power on ALC
The internally leveled signal generator RF output (and ALC level) is -8 dBm. The mixer is driven
with an LO of +10 dBm and has an LO-to-RF isolation of 15 dB. The resulting LO feedthrough of
-5 dBm enters the signal generator’s RF output connector and arrives at the internal detector.
Depending on frequency, it is possible for most of this LO feedthrough energy to enter the detector.
Since the detector responds to its total input power regardless of frequency, this excess energy causes
the ALC to reduce the RF output of the signal generator. In this example, the reverse power across
the detector is actually greater than the ALC level, which may result in loss of signal at the RF
output.
Figure 12-2 on page 246 shows a similar configuration with the addition of a 10 dB attenuator
connected between the RF output of the signal generator and the input of the mixer. The signal
generator’s ALC level is increased to +2 dBm and transmitted through a 10 dB attenuator to achieve
the required -8 dBm amplitude at the mixer input.
RF LEVEL
CONTROL
SIGNAL GENERATOR
OUTPUT CONTROL
MIXER
LO
IF
ALC LEVEL
= - 8 dBm
DETECTOR
MEASURES
- 8 dBm
ALC LEVEL
DETECTOR
MEASURES
- 5 dBm
REVERSE
POWER
LO FEEDTHRU
= - 5 dBm
RF OUTPUT
= - 8 dBm
LO LEVEL
= +10 dBm