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.
