3. Operating Instructions
5. A set of terminators, both open and short, are included with the CT100B for calibration
purposes. Attach these terminators to the CT100B when prompted to do so (pn:
CT100-AC-ISS, CT100-AC-ISMM, CT100-AC-IBS, CT100-AC-IBMM).
When the reference measurement is finished, cursor measurements will now appear in blue at
the right hand of the screen, and the notification
using vert. ref.
will appear underneath.
A green line will appear at the position of the active cursor (see step 3 above). Vertical reference
is applied to all points to the right of the green line. Standard vertical measurements are used
for all points to the left of the green line. The position of this line can be changed with the
Vert.
Ref Start
menu option.
By default, the impedance value at 0 m
ρ
(millirho) is taken to be 50 Ω. This value can be
changed with the
Vert. Ref. Center Imp.
menu option. If you are using an impedance matcher
or balun adapter, the center impedance should be changed to the value of impedance of the
adapter. For instance, if you are using a 100 ohm differential pulse splitter balun to measure
differential impedance of twisted pair cable, then the center impedance should be changed to
100 Ω.
The CT100B is now configured for enhanced accuracy impedance measurements across a wide
range of impedances.
The
Vert. Ref. On/Off
menu option can be used to toggle the vertical reference on and off as
needed for a given cable length setting.
3.11.16. Cable Resistive Loss Correction
The CT100B has a system for correcting for resistive loss (“dribble up”) in cables. This allows
the operator to measure impedance more accurately through long and/or lossy cables.
Under the
Measurement
menu, there is an item to set ohms per unit length and another item to
toggle the correction on and off. The calculations of this feature assume a linear serial resistance
to an attached cable and no other loss. The procedure for performing resistive cable loss
correction is as follows:
1. Select the
MENU
→
Measurement
menu.
2. Select
Vertical Correction
submenu.
3. Adjust the trace of interest so that it fits on the screen and position the active cursor to
the right of the trace you wish to correct.
4. Select
Adjust
Ω
/ft.
(
Note:
This menu option will show the horizontal units set in
MENU
→
Settings
→
Meas. Settings
→
Horiz. Units
[see Section 3.11.28.1]; i.e., if set to meters,
the menu option will read
Adjust
Ω
/m
instead. This is true for all steps in this section.)
5.
Select
Ω
/ft Correct. Off
to turn on the correction then press
Set
Ω
/ft Pos.
to set the
correction endpoint. The endpoint will appear as a dotted orange vertical line.
6. Rotate the
M-FUNCTION
knob to change the ohms per unit distance correction setting.
Use the Ω
/ft Correct On
toggle menu option to turn the correction on or off.
Changing the
Ohms/m
setting will change the slope of the cable’s trace as the resistive loss is
subtracted, and consequently the impedance readings for all points on the cable’s trace and
beyond. The effect and accuracy of the ohms per unit length value is dependent on the value for
pulse velocity. The effect of resistive cable loss correction on a relatively lossy 72 ohm RG-59
cable is shown in Figure 3.31 and Figure 3.32.
48
CT100B TDR Cable Analyzers Operator’s Manual
