8
Verifying a flatness-correction calibration
To verify the flatness-correction calibration, the
power sensor should be reconnected to the test
port to measure the test port power at individual
frequencies. Since the measurement system is
calibrated in 50W, inaccuracies will occur when
a device is not well matched. Since the flatness-
correction calibration is not a real-time power
leveling feature, it cannot correct for mismatches
that occur between the test port and the DUT.
Adjusting the frequency span after calibration
Once a flatness-correction calibration with the
maximum number of points (801 points) has been
completed, adjustments can be made to reduce the
number of trace points on the analyzer. The user
may also change the measurement frequency span
to a subset of the original calibration span without
invalidating the calibration. The source will output
corrected power at the appropriate measurement
points.
This capability is particularly useful for users who
are testing a number of devices with different
frequency spans at the same test station. In this
case, the user may choose to perform the flatness-
correction calibration with the maximum number
of calibration points across the full frequency
range. Subsets of the original calibration frequency
range can then be used to meet the specific testing
requirements of the individual devices.
When to recalibrate
The flatness-correction calibration does not need
to be repeated unless: (1) the user wants to cali-
brate over a wider frequency range, (2) the meas-
urement path between the source and the test
device changes, (3) the RF source power changes,
(4) the user wants to increase the number of meas-
urement points, or (5) the environmental condi-
tions under which the original calibration was per-
formed changes dramatically.
Keep in mind that the flatness-correction table
is automatically saved into register 1 of the RF
source. Any new calibration of the source will
overwrite the flatness-correction table. If multiple
flatness-correction calibrations are performed,
only the most recent calibration will be saved for
the DUT.
Using test set step attenuators with flatness correction
If lower test port power levels are desired, test
set step attenuators may be used. The frequency
response of the step attenuators can be eliminated
from the measurement by performing the flatness-
correction calibration with the appropriate atten-
uation enabled. High sensitivity power sensors
(8485D to 26.5 GHz and 8487D to 50 GHz) are
available for power measurements from –20 dBm
to –70 dBm. The limitation of making power meter
measurements at these low power levels is that the
actual calibration process takes considerably more
time since the power meter takes much longer to
settle at each correction frequency.
