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Normally, the analyzer attempts to phase-lock
near the start frequency of the sweep. When the
analyzer cannot phase-lock at the start frequency
though, the analyzer performs a "pre-tune cali-
bration" routine. During pre-tune calibration the
analyzer attempts to phase-lock at a set frequency
(100 MHz for the Agilent 8753D family), which
could be below the start of the analyzer's sweep.
The pre-tune calibration routine would be per-
formed, for example, if the analyzer is switched to
external reference mode without a signal attached
to the R channel. When the external R channel
signal is connected the analyzer is attempting to
phase-lock at the pretune frequency (100 MHz).
With a band-limited device in the R channel
path (e.g., a booster amplifier) there may not be
sufficient signal at this pretune frequency and
phase-locking errors can occur on some analyzers.
On the 8753E network analyzer, turning "PLL
Auto" off (located in the Service Modes menu) will
stop the analyzer from performing the "pre-tune
calibration" routine. Turning "PLL Auto" off rarely
affects the accuracy of instrument measurements.
Another way to prevent this phase-locking error
is to use frequency-offset mode in the 8753E. In
frequency-offset mode the analyzer assumes that
a band-limited device is being measured, and as a
result the analyzer never goes outside the sweep
range to phase-lock. Setting LO = 0 Hz when in
frequency-offset mode allows the analyzer to be
used normally.
10
Band-limited devices in the R channel path may
cause phase-locking problems in the Agilent 8720E
family of network analyzers unless the analyzer
has the high-power option (Option 085) or frequency-
offset option (Option 089). With either of these
options, phase-locking is not a problem. This is
because the internal reference switch switches to
the internal signal when phase-locking. After the
analyzer is phase-locked, the reference switch
returns to the external signal and the analyzer
is ready to make measurements.
Calibration
Calibrating this configuration is similar to the
calibration performed in Configuration 2. Only a
response calibration is possible since only forward
measurements can be made. Perform the response
calibration by connecting the output of the coupler's
through arm to the attenuator on test-port 2 of
the analyzer.
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