5. Math Functions
Figure 5.3. FFT trace (blue). The FFT plot was created from the section of the black live
trace that is currently red.
FFT traces can be stored, reloaded, and transferred to CT Viewer 2 just like normal scanned
traces.
5.4. Return Loss (S
11
) Traces
This feature uses precision open, short, and 50 ohm load (OSL) calibration standard measurements
to correct for systematic errors in the pulsing and sampling electronics, producing a more accurate
S
11
result on the Device Under Test (DUT). The procedure for creating an S
11
return loss plot is as
follows:
1. Prepare the CT100B by setting the trace resolution as desired by going to Resolution →
Fixed and using the M-FUNCTION knob. In general, a resolution of 1-2 ps is adequate to
take advantage of the full bandwidth and frequency resolution of the instrument. Larger
sample intervals can result in lower bandwidth resolution. Smaller sample intervals have
higher frequency resolution but increase sampling time.
2. Use the M-FUNCTION knob to set the smoothing factor to an acceptable level, usually in
the range of 64 to 256 depending on the application. Use higher smoothing factors for higher
bandwidth, lower noise measurements. Higher levels of smoothing improve the signal-to-noise
ratio (SNR).
3. With the DUT detached, place the leftmost cursor just to the left of the test plane
(the location where the calibration standards will be applied). This is usually just to the left
of the open at the test port or alternatively at the end of the phase stable cable.
4. Attach the DUT to determine the region of interest and place the rightmost cursor
just to the right of the region of interest. This is usually where steady state TDR trace
amplitude has been restored. Detach the DUT to continue calibration.
5. Select the SCAN → Math menu option to open the Math menu.
6. Select SParam & Normalize Tools. This will open the SParam & Normalize Tools menu.
80 CT100B TDR Cable Analyzers Operator’s Manual
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