Solving Problems
If You Encounter a Problem
Chapter 478
Signal Loss Occurs While Working with Mixers
If you experience signal loss at the signal generator’s RF output during low-amplitude,
coupled operation with a mixer, you can solve the problem by adding attenuation and
increasing the RF output amplitude of the signal generator.
Figure 4-1 shows a hypothetical configuration in which the signal generator provides a low
amplitude signal to a mixer.
Figure 4-1 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 4-2 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
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