3. Operating Instructions
Figure 3.36.
S
11
between cursors with loosened connector showing average return loss of
-26.7 dB from 756 MHz to 3 GHz (7.7 dB excess return loss compared with the tightened
connector in Figure 3.35).
3.11.19. Improving S-Parameter Measurements
Note that testing short microwave cables directly from the test port may cause unacceptable
noise due to the presence of secondary reflections in the TDR trace. In general, this shows up as
excessive spectral noise. In this case, time windowing can be used to restore measurement
accuracy:
1. Attach a 2 ft. phase stable cable to the test port of the instrument.
2. Place the leftmost cursor just to the left of the end of the phase stable cable.
3. Identify the start of the multiple reflections on the cable under test and exclude this region
with the cursors (see Figure 3.37).
4. Perform SParam calibration as described in Section 3.11.17.
The time-windowed trace will have substantially reduced noise due to exclusion of the secondary
reflections.
3.11.20. Smith Charts
The Smith chart is a useful graphical tool for plotting reflection coefficients and complex
impedance values and can be used to simplify impedance matching. For a very brief
introduction to Smith charts see Section 5.12. The CT100B allows the operator to display
S-parameter return loss and cable loss traces on a Smith chart. To create a Smith chart, use the
following procedure:
1. Create an S
11
return loss plot using the procedure as described on page 49.
2. With the S
11
trace selected, adjust the cursors to minimum and maximum frequency
desired on the Smith chart.
3. Go to
SParam Tools
and select the
Smith Chart
option to toggle Smith chart display.
4. Use the
HORIZONTAL POSITION
knob to move the cursor along the complex
impedance curve.
54
CT100B TDR Cable Analyzers Operator’s Manual
