3. Operating Instructions
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.
7. Select
S
11
Calibration
. After going through an informational message box, the
S
11
Calibration
menu will appear and the CT100B will ask for the open terminator. Attach
the open terminator and press
Scan Open
, or alternatively press
Use Selected
to use the
currently selected trace as the open calibration standard. For the best results when using
type SMA terminators, use a calibrated 3.5 in-lbs (0.4 Nm) torque wrench (pn:
CT100-TL-TORX7) to tighten the test standards as well as the DUT.
8. Follow the same process as instructed to scan or use the selected trace for the short and 50
ohm load calibration standards.
9. The instrument will now ask for the Measured “Device Under Test” (DUT). Press
Use
Live
to create a live S
11
trace,
Use Selected
to create a static S
11
trace from the currently
selected trace, or
Scan Measured
to scan the current live TDR trace.
10. Select the S
11
return loss trace. Use the cursors to measure return loss at cursor and
average return loss between cursors.
Figure 3.33 shows an S
11
return loss plot and TDR trace for a 2.4 GHz WiFi antenna. The
return loss at 2.4 GHz (2400 MHz) is 20.9 dB.
3.11.18. Return Loss (S
11
)Options
There are several mathematical transforms to apply to the S
11
trace and its input traces that
can, in some cases, help reduce irregularities in the final Return Loss plot. These options can be
found under
SCAN
→
Math
→
SParam & Normalize Tools
→
S
11
Options
. If the options are
changed when no S
11
traces exist, then they will be applied to the next S
11
trace created.
Otherwise, they are applied to the last S
11
trace that was selected using the
SELECT
button.
The options can be combined as necessary, and will change the last selected S
11
trace in real
time.
3.11.18.1. 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:
50
CT100B TDR Cable Analyzers Operator’s Manual
