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System Maintenance
Theory of Operation
Level Control
In some applications of network analysis (particularly in testing of passive
devices), absolute power levels are less important than relative power levels.
However, in other applications, absolute power levels are very significant,
and must be controlled.
For example, when active devices are tested, level control is needed as a
safeguard against applying excessive input power. Control of absolute power
levels is also necessary for purposes of performance verification (for
example, to test the dynamic range of the network analyzer).
RF Leveling vs.
In a typical level control circuit, a coupler is placed in the RF path, and the
IF Leveling
coupled RF signal is applied to a detector. The detector voltage is fed back to
an amplifier which controls the RF source. This loop amplifier drives the RF
source so as to achieve a particular detected RF level.
Figure 5-4
Comparing RF and IF Leveling Systems
The level control circuit in the 8510XF includes a refinement of this basic
leveling loop design: the coupled RF signal is not directly detected. Instead,
the RF signal is applied to a mixer, and the IF output of the mixer is detected
(the difference between these two approaches is illustrated in
Figure
5-4).
Because the IF level varies with the RF level, the IF (suitably amplified) can
be substituted for the RF in the leveling loop. The loop amplifier drives the
RF source so as to achieve a particular detected IF level (and therefore a
particular RF level).
5-14 8510XF Network Analyzer Systems
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