Signal Loss While Working With A Spectrum Analyzer - Keysight E8267D User Manual

Troubleshooting
RF Output Power Problems
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 13-2
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.
Figure 13-2 Reverse Power Solution
DETECTOR
MEASURES
+2 dBm
ALC LEVEL
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
316
shows a similar configuration with the addition of a 10 dB
SIGNAL GENERATOR
OUTPUT CONTROL
ALC LEVEL/
RF OUTPUT
= +2 dBm
RF LEVEL
CONTROL
DETECTOR
MEASURES
– 15 dBm
REVERSE
POWER
RF INPUT
= – 8 dBm
10 dB
ATTEN
LO FEEDTHRU
= – 5 dBm
Keysight E8357D/67D & E8663D PSG User's Guide
MIXER
LO
LO LEVEL
= +10 dBm
IF