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85
Signal Generator
With optional step
attenuator
RF output
Frequency
mixer
Local oscillator
RF amplitude control
0dB attenuator
Detector
measures
Detector
measures
LO feedback
-8dBm ALC level
-5dBm
feedback
amplitude
LO level
MF
Depending on frequency, it is possible for most of this LO feedthrough energy to enter the
detector. Because the detector will respond to its input amplitude regardless of frequency, this energy
causes the ALC to reduce the output amplitude. In this example, the reverse power across the
detector is actually greater than the ALC level, therefore the amplitude output of the generator is
actually shut down.
In the example shown in Fig.3.12, a -8dBm amplitude output is manually generated with the
attenuator. In this example, the attenuation is 10dB, the ALC level is +2dBm and the LO frequency of
frequency mixer is 10dBm. With the 10dB attenuator, the reverse power (-5dBm) of LO is attenuated
to
-15dBm, thus the detector is exposed to a +2dBm ALC level and the undesired -15dBm reverse
power. This 17dB difference results in a 0.1dB shift in the signal generator‘s output level. Set the
generator manually with an attenuator:
Operation steps:
Step 1. Open the Ampl Config Dialog
Press the
【
Amplitude
】
key or tap [Amplitude] on the touchscreen to open Ampl Config Dialog.
Step 2. Set Attenuation Style in ALC system to Manual and set the attenuation value.
Select [Atten Config] in the Ampl Config Dialog, rotate RPG on the front panel clockwise (or
counterclockwise) to select Attenuation Style box, and press the knob to select Manual; rotate RPG
again to select Attenuation input box, and press the knob to make it editable; or tap Attenuation Style
on the touchscreen to select Manual, and then tap Attenuation input box to make it editable. Then,
set the Attenuation to 10 dB.
Step 3. Set ALC
Select [Atten Config] in the Ampl Config Dialog, rotate RPG on the front panel clockwise (or
counterclockwise) to select ALC input box, and press the knob to make it editable; or tap ALC input
box on the touchscreen to make it editable. Now, set ALC to 2dBm.
Fig.3.11 Effects of Reverse Power
（
-8dBm Coupling Output)
