Service manual

8 Agilent 382A Operating and Service Manual
Operation
Standing Wave Ratio This test will obtain the SWR by measuring the reflection coefficient and
calculating the SWR. Connect the instruments as shown in Figure 2.
Figure 2 RF Pre-Insertion – Y-Z Recorder Setup for ? Tests
Procedure for using this set-up consists of shorting the output (100%
reflection) and then drawing a calibration grid of traces on the recorder
around the expected value by varying the calibrating attenuator. For
instance, if a reflection coefficient of 0.10 is expected, a ca1ibration trace for
this value at the frequencies swept by the sweep oscillator can be obtained
by setting the calibrating attenuator to 20 dB. Attenuation values for setting
the return loss by means of the calibrating attenuator for other reflection
coefficients may be obtained either by mean of a reflectometer calculator
(obtainable from your local Agilent field office) or from the expression:
Return Loss (dB) – 20 log
10
?
The negative sign in this expression merely indicated that this attenuation is
a loss.
The effect of reflections from the load may be minimized, by moving the
load through at least a half-wavelength rapidly to present the maximum
possible signal to the indicator at any frequency. However, the speed of
moving the load must not be faster than the X-Y recorder can respond. When
using this technique, the fact that the signal goes beyond the limits does not
necessarily mean the attenuator is out of specifications (the reflections from
the load and the attenuator may have added). The true value is the average of
the small signal (fine grain) variations in the trace.