TDR Mode

Time Domain Reflectometry (TDR) transforms frequency-domain S11 data into a time-domain view, letting you see reflections along a cable or transmission line as a function of distance. Use it to measure cable length, locate faults, or identify impedance discontinuities.

Note

TDR works with S11 (reflection) data only. Make sure your trace is set to the S11 channel before enabling TDR.

Enable TDR mode

1. Connect the cable or device under test to Port 1 (CH0).
2. Ensure the active trace is on the S11 channel.
3. Tap DISPLAY → TDR.
4. Select a processing mode (see below).

Processing modes

Low Pass Impulse

Shows reflections as sharp impulse spikes. Each discontinuity along the cable produces a distinct peak. Best for:

• Locating connectors, splices, or damage
• Measuring distance to a specific fault

Low Pass Step

Shows the impedance profile as a step function along the cable. The vertical axis represents impedance. Best for:

• Identifying impedance changes along a transmission line
• Seeing the characteristic impedance of different cable sections

Bandpass

Uses only the measured frequency data without extrapolation. Produces a symmetric response. Best for:

• Measurements where the low-frequency assumption is not valid
• Situations where you need the raw bandpass response

Set the window function

The window function controls the trade-off between resolution and sidelobe suppression:

1. From the TDR menu, tap WINDOW.
2. Select one of three options:
   • MINIMUM — sharpest resolution, highest sidelobes
   • NORMAL — balanced (recommended for most use)
   • MAXIMUM — lowest sidelobes, reduced resolution

Set the velocity factor

The velocity factor accounts for the signal propagation speed in your cable, which is slower than the speed of light. Without the correct velocity factor, distance readings will be inaccurate.

1. From the TDR menu, tap VELOCITY FACTOR.
2. Enter the value as a percentage (e.g., enter 70 for a velocity factor of 0.70).
3. Tap Ok to confirm.

Common velocity factors

Cable type	Velocity factor	Enter as
RG-405 / Semi-rigid	0.70	70
RG-58	0.66	66
RG-213	0.66	66
RG-174	0.66	66
LMR-400	0.85	85
Air dielectric	0.95—1.00	95—100

Tip

If you do not know the velocity factor, measure a cable of known length and adjust the velocity factor until the TDR distance reading matches the actual length.

Caution

For accurate TDR measurements, calibrate the VNA before enabling TDR mode. An uncalibrated measurement will show artifacts that can be mistaken for real cable faults.