4. Operating Instructions
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.
4.13.2. 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 the 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 3.13.1]; 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. Toggle Ω/ft Correct. to On 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 per unit length setting will modify 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 4.38 and Figure 4.39.
Use this correction to make more accurate impedance measurements on a length of cable. Since
uniform cables tend to have a characteristic resistive loss per unit length or “dribble up”, one
technique is to adjust the correction until the cable is as flat as possible. Another technique is to
attach a known impedance reference, such as a resistive terminator, to the cable and adjust the
ohms per unit length correction until the measured impedance reads the correct value.
72 CT100B TDR Cable Analyzers Operator’s Manual
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