5. Math Functions
5.5.1. Return Loss (S
11
) Pre-Filter
The Pre-Filter can reduce aberrations in the S
11
trace. This can be combined with a Common
Mode Subtraction (CMS) transform on the input traces. To enable or disable these settings, do the
following:
1. Create an S
11
return loss plot using the procedure as described in Section 5.4.
2. Select the S
11
Options menu, then Pre-Filter Options.
3. Toggle Use this filter to apply filter to the S
11
trace.
4. Toggle Common Mode Subtraction (CMS) to apply this option to the S
11
trace.
5. Filter/CMS Offset Ratio is the start of the Aberration Filter and/or Common Mode
Subtraction relative to the S
11
trace test plane. The default value is 1.66, and is ideal in most
cases.
6. Frequency (MHz) is the aberration filter frequency. The default value is 200, and is ideal in
most cases.
7. Press OK to change the settings to those set above. The S
11
trace will update using the new
settings.
5.5.2. Return Loss (S
11
) Post-Filter
The Post-Filter can reduce excess noise in the S
11
trace. To enable or disable the setting, do the
following:
1. Create an S
11
return loss plot using the procedure as described in Section 5.4.
2. Select the S
11
Options menu, then Post-Filter Options.
3. Toggle Use this filter to apply filter to the S
11
trace.
4. Frequency (MHz) is the noise filter frequency. The default value is 20,000 (20 GHz), and is
ideal in most cases.
5. Toggle Apply Hanning to apply a Hanning Window to the filter.
6. Press OK to change the settings to those set above. The S
11
trace will update, although it
might not look visually different, depending on the settings.
5.5.3. Return Loss (S
11
) Phase Correction
Phase Correction reduces phase error oscillation in the S
11
trace. To enable or disable the setting,
do the following:
1. Create an S
11
return loss plot using the procedure as described in Section 5.4.
2. Select the S
11
Options menu, then toggle Phase Corr. to On. The S
11
trace will update,
although it might not look visually different, depending on the settings.
Phase Correction typically works best when started at the S
11
trace test plane. To choose a
different starting point:
3. Use the SELECT button to select a TDR trace, position the active cursor where desired,
then press Phase Corr. Start. The Phase Correction starting point will go to the active
cursor position, and the S
11
trace will update, although it might not look visually different,
depending on the settings.
82 CT100B TDR Cable Analyzers Operator’s Manual
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