User`s guide

464 Chapter 16
Troubleshooting
Basic Signal Generator Operations
Figure 16-1 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 16-2 on page 465 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