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310 Agilent N5161A/62A/81A/82A/83A MXG Signal Generators User’s Guide
Troubleshooting
RF Output
The figure at right shows a
configuration in which the
signal generator provides a
low amplitude signal to a
mixer.
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 −5dBm LO
feedthrough 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. Because the detector responds to its total input power
regardless of frequency, this excess energy causes the ALC to reduce the RF output. In this example,
the reverse power across the detector is actually greater than the ALC level, which can result in loss
of signal at the RF output.
The solution at right 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
increases to +2 dBm and
transmits through a 10 dB
attenuator to achieve the
required −8 dBm amplitude
at the mixer input.
Compared to the original
configuration, the ALC level
is 10 dB higher while the
attenuator reduces the LO
feedthrough (and the signal
generator’s RF output) 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.
Mixer
LO
ALC Level
= −8 dBm
RF Level
Control
Signal Generator
Output Control
Detector
measures
−8dBm
ALC level
Detector
measures
−5dBm
reverse
power
LO Feedthru
= −5 dBm
RF Output
= −8 dBm
LO Level
= +10 dBm
IF
Effects of Reverse Power on ALC
Mixer
LO
ALC Level/
RF Output
= +2 dBm
RF Level
Control
Signal Generator
Output Control
Detector
measures
+2 dBm
ALC level
Detector
measures
−15 dBm
reverse
power
LO Feedthru
= −5 dBm
RF Output
= −8 dBm
LO Level
= +10 dBm
IF
Reverse Power Solution
10 dB
ATTEN