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NOTE
Loop Amplifier
Level Calibration
NOTE
System Maintenance
Theory of Operation
ONCE
If this option is selected, the source is polled for errors only
during the first sweep.
NEVER
If this option is selected, the source is not polled for errors
during any sweep.
If any of the basic conditions of the sweep (frequency range, number of
points, or power level) are changed, the entire sweep process (including
polling) begins over again.
This portion of the level control circuit is located within the 83651B RF
source. The loop amplifier compares the input from the detector feedback
path with a reference voltage. (The reference voltage is fixed; this leveling
system specifies an RF level not by setting the reference voltage, but by
setting the amount of gain in the feedback loop).
The output of the amplifier is applied to an integrator, which responds
rapidly to any change on its input, and drives the modulator in the RF output
path, so as to adjust the output power from the 83651B.
There are two level calibration processes, designed to counteract two
different sources of error:
•
Imprecision in the step attenuators
•
Frequency-related variations in mixer performance
The correction factors taken during these calibration processes are valid only
for a particular millimeter-wave controller, left test head, and right test head
calibrated together as a set. The cables connecting the testheads to the
controller also become part of the calibration. If a test head or the controller
is replaced, both calibration processes must be repeated.
Detector gain calibration
The step-attenuators in the programmable gain circuit are not precise enough
to be used without correction. Attenuator settings of (for example) 5 dB and
6 dB are nominally 1 dB apart, but the actual difference between the two
settings might be 0.9 dB or 1.1 dB. In order to regularize the increments of
attenuation (so that the actual effect of each attenuator setting is predictable),
a specific correction factor must be applied for each of the possible
attenuator settings.
5-18 8510XF Network Analyzer Systems
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