Changing Signal Path States After System Calibration
Changing the Test Port Attenuators
Changing an internal attenuator or any external equipment after completing the calibration means the measurement results cannot be specified. You must use your own estimation of the potential error contribution due to the change.
For example, when the port attenuation is changed with correction ON, the message CAUTION: CORRECTION MAY BE INVALID displays. You must judge whether the error is tolerable in the particular application and how to compensate for the change.
The application question is: “Does increasing the signal level during calibration improve the calibration enough to risk a possible increase in error contributions when you change the setup?”
The only reason for changing an internal attenuator or external equipment between calibrations and measurements is to maximize the signal level under both conditions, thus minimizing uncertainty due to noise.
Many factors enter into a
•is it more accurate to calibrate at a low signal level without changing the setup, or
•is it more accurate to change the setup to optimize levels for both the calibration and measurement
Changing attenuators at Port 1 or Port 2 does not change the test set mismatch, directivity, or isolation characteristics severely. It does change the frequency response magnitude and phase, however. The difference between frequency response calibration and measurement can be normalized by using HP 8510C trace memories.
If only the attenuators at Port 1 or Port 2 are changed, use the following procedure to minimize errors:
1.Connect a short, or a thru, and set Port 1 and Port 2 attenuators for the best IF signal levels during calibration.
2.Perform the appropriate measurement calibration.
3.Connect the correct calibration standard, set Port 1 and Port 2 attenuators to the value required for operating the test device.
HP 8517B