Technical data

85108L System Manual
Operation
High Power Measurements
3-35
Changing the Signal Path After Calibration
If any attenuator or other external equipment is changed after calibration, the measurement
results cannot be specified except by your own estimation of the error contribution of the change.
For example, when the port attenuation is changed with correction On, the message CAUTION:
CORRECTION MAY BE INVALID is displayed. The operator must judge whether the error is tolerable
in the particular application and how to compensate for the change.
The only reason to change the internal attenuators or external equipment between calibration and
measurement is to maximize the levels under both conditions, thus minimizing uncertainty due to
noise. Many factors will enter into the decision of whether it is more accurate to calibrate at a low
signal level without changing the setup, or change the setup to optimize levels for both calibration
and measurement. Changing the Port 1 or the Port 2 attenuator does not seriously change the test
set mismatch, directivity, or isolation characteristics, but will change the frequency response
magnitude and phase. This difference in the frequency response between calibration and
measurement can be normalized using the 8510 trace memories.
The following three procedures can help minimize the errors due to changing the signal levels
between calibration and measurements.
Set the Attenuators
If the Port 1 or Port 2 attenuators are changed, the following procedure can minimize the errors.
1. Connect a short or a thru and set the Port 1 and Port 2 attenuators for best IF signal levels
during calibration.
2. Perform appropriate measurement calibration.
3. With an appropriate standard connected, set the Port 1 and Port 2 attenuators to the setting
required for operating the test device.
If you view the response of a short circuit, notice that changing the Port 1 attenuators has negligible
effect on the S11 marker reading, and that changing Port 2 attenuators has negligible effect on the
S22 marker reading. This is due to the way that the attenuators are paired. Both the reference and
the test signal are changed an approximately equal amount.
However, when viewing S21 or S12, changing the Port 1 or Port 2 attenuators offsets the marker
reading by the difference in the Port 1 and Port 2 values.
Store Trace Memories
The main frequency response effects of changing the attenuators can be compensated for by using
the 8510 trace memories and trace mathematics as follows:
1. Connect the thru used for calibration. Set the port attenuators to the value used for
measurement of the device. Recheck the user parameter levels to be sure they are below
10 dBm.
2. Press
DISPLAY, then DATA -->MEMORY to store the S21 trace in default trace memory 2. This trace
is the frequency response difference of the S21 signal path between calibration and
measurement.
3. Press
S12 DISPLAY, DATA --> MEMORY to store the S12 trace in trace memory 3. This trace is the
frequency response difference of the S12 signal path between calibration and measurement.
Now that these traces are stored, they can be used to normalize the corrected data.