Service manual

Agilent 382A Operating and Service Manual 9
Operation
Set calibrating attenuator to zero and replace short with terminated
attenuator to be tested. Trigger final sweep, which will be the SWR versus
frequency characteristics of the attenuator under test.
Since we wish to determine if the SWR is below 1.15, we can calibrate the
reflectometer for this value and check that the attenuator under test stays
below that value for all frequencies within its rated band. From a
reflectometer calculator we obtain for SWR = 1.15 a return loss of 23.1 dB
With the use of the reflectometer calculator (obtainable from your local
Agilent field office or from the factory) the errors of such a set-up can be
determined and taken into account. The return loss of a SWR of 1.15 is
23.1 dB, but taking into account the ambiguity due to the directional coupler
directivity, the adjusted return loss setting will move to the lower end of the
ambiguity limit: 24.5 dB. One more possible error that must be taken into
account is the calibration error of the attenuator setting the value of the
return loss (specifications call for ±2% or 0.1 dB whichever is greater). This
figure will increase the ambiguity limit by 2% to 25 dB. There could be one
more error involved in this measurement, namely the multiple-mismatch
error due to the change of attenuator setting of the 382A attenuator with
respect to the detector and the auxiliary arm of the directional coupler. This
error can be neglected if at least 10 dB of attenuation is left always in the
attenuator since the phase and magnitude of the reflection coefficient of the
rotary-vane attenuator will not change appreciably if it is held above 10 dB
at all times.
For the above reasons we will have to set a return loss of 25 dB for a SWR of
1.15. The procedure is as follows:
1. With the instruments connected as in Figure2 on page8, the calibrating
attenuator set to 35 dB, and the short connected, run a slow trace. If a
moveable short is used, rapidly phase the short while sweeping. The
average of the fine grain variation of the trace is the calibration curve.
2. Disconnect the short used for calibrating.
3. Connect one arm of the 382A under test to the setup where the short was
originally connected.
4. Set the calibrating variable attenuator to 10 dB.
5. Connect a moveable load to the other port of the attenuator.
6. Set the Model 382A under test for 0 dB Attenuation.
7. Trigger the sweep again. This is the test trace. If this trace approaches
the calibration trace, phase (spin) the load rapidly during this trace to
record all possible phases of the source and load mismatch. The actual
test trace is the average of the fine grain variation of the recording. This
procedure will balance out the source and load mismatch errors.