Time Domain Transform

Time Domain Reflectometry (TDR) transforms frequency-domain measurements into time-domain data, allowing you to locate faults in cables, see impedance discontinuities, and analyze transmission line characteristics.

Enable Time Domain Transform

Tap the screen to open the menu
Navigate to DISPLAY > TRANSFORM
Tap TRANSFORM to toggle between ON and OFF

When enabled, the horizontal axis changes from frequency to time/distance.

Transform Modes

The NanoVNA-H offers three transform modes accessible via DISPLAY > TRANSFORM:

Bandpass Mode

Best for general TDR measurements
Shows both positive and negative reflections
Use when measuring cables or transmission lines
Select: DISPLAY > TRANSFORM > BANDPASS

Low Pass Impulse

Simulates a traditional TDR impulse response
Requires sweep to start near DC (low frequency)
Best for locating discontinuities
Select: DISPLAY > TRANSFORM > LOW PASS IMPULSE

Low Pass Step

Simulates traditional TDR step response
Shows impedance profile along the line
Better for seeing impedance levels
Select: DISPLAY > TRANSFORM > LOW PASS STEP

Window Functions

Window functions control the trade-off between resolution and dynamic range. Access via DISPLAY > TRANSFORM > WINDOW:

Window	Resolution	Sidelobes	Use Case
MINIMUM	Best	Highest	Maximum resolution, single discontinuity
NORMAL	Good	Medium	General purpose (beta=6)
MAXIMUM	Reduced	Lowest	Multiple discontinuities, high dynamic range

MINIMUM (Rectangular): No windowing applied. Highest resolution but also highest sidelobes.
NORMAL (Kaiser beta=6): Balanced resolution and sidelobe suppression. Good default choice.
MAXIMUM (Kaiser beta=13): Lowest sidelobes but reduced resolution.

Velocity Factor

The velocity factor converts time to physical distance. Set it based on your cable type:

Go to DISPLAY > TRANSFORM > VELOCITY F.
Enter the velocity factor as a percentage (1-100)
Tap a unit key or ENTER to confirm

Common velocity factors:

Cable Type	Velocity Factor
Air/vacuum	100%
RG-58 (foam)	79%
RG-58 (solid)	66%
RG-174	66%
RG-213	66%
LMR-400	85%
Hardline	88%

Shell Commands

# Enable/disable transform
transform on
transform off

# Set transform mode
transform impulse   # Low pass impulse
transform step      # Low pass step
transform bandpass  # Bandpass mode

# Set window function
transform minimum   # Rectangular window
transform normal    # Kaiser beta=6
transform maximum   # Kaiser beta=13

# Combine multiple settings
transform on impulse normal

Measurement Tips

For Cable Fault Location

Set frequency sweep from 50 kHz to 300 MHz (or higher)
Enable transform: DISPLAY > TRANSFORM > ON
Select LOW PASS IMPULSE mode
Set appropriate velocity factor for your cable
Look for peaks indicating discontinuities

For Impedance Profiling

Use a wide frequency sweep
Enable transform
Select LOW PASS STEP mode
The vertical axis shows impedance along the cable

Resolution Considerations

Time domain resolution depends on the frequency span:

Resolution = Velocity Factor / (2 x Frequency Span)

For example, with a 900 MHz span and 66% velocity factor:

Resolution = 0.66 x 3x10^8 / (2 x 9x10^8) = 0.11 meters (11 cm)

Understanding the Display

In time domain mode:

X-axis: Time (or distance if velocity factor is set)
Y-axis: Reflection coefficient or impedance (depends on trace format)
Peaks: Indicate impedance discontinuities (opens, shorts, connectors)

Interpreting Peaks

Peak Direction	Meaning	Typical Cause
Positive (upward)	Higher impedance than Z0	Open circuit, connector gap, broken conductor
Negative (downward)	Lower impedance than Z0	Short circuit, water ingress, crushed cable
No peak	Matched impedance	Good cable or termination

In Low Pass Step mode, the display shows a running impedance profile — the Y-axis directly represents impedance at each point along the cable. A flat line at 50 ohms means the cable is uniform. Deviations indicate impedance changes.

In Bandpass and Low Pass Impulse modes, you see impulses (spikes) at each discontinuity. The height of the spike corresponds to the severity of the mismatch.

Range and Aliasing

The maximum unambiguous distance is determined by the sweep point spacing:

Max Range = (Velocity Factor × c) / (2 × Frequency Step)

Where Frequency Step = (Stop - Start) / Points.

Sweep Range	Points	Max Range (VF=66%)
50 kHz – 300 MHz	101	~33 meters
50 kHz – 300 MHz	401	~33 meters
50 kHz – 900 MHz	101	~11 meters
50 kHz – 900 MHz	401	~11 meters

Choosing Between Modes

Mode	Best For	Requires Low Start Freq?
Bandpass	General fault finding, quick scans	No
Low Pass Impulse	Precise fault location, reflections	Yes (50 kHz recommended)
Low Pass Step	Impedance profiling along a cable	Yes (50 kHz recommended)

Bandpass mode works with any sweep range, making it the most flexible. Low pass modes give cleaner results but require the sweep to start near DC — if your start frequency is too high, the transform quality degrades and you may see ringing artifacts.

Troubleshooting

Unexpected Peaks at Time Zero

A large peak at the very beginning of the time domain display usually means the calibration plane is at the connector — everything before the cable appears as a discontinuity. This is normal. Calibrate at the end of the cable to move the reference plane.

Ringing Around Peaks

Ringing (oscillations around a peak) is caused by the abrupt frequency cutoff at the edges of the sweep. Use the NORMAL or MAXIMUM window function to suppress sidelobes at the cost of slightly reduced resolution.

Distance Readings Are Wrong

Check these in order:

Velocity factor — is it set correctly for your cable type?
Calibration — did you calibrate at the start of the cable?
Sweep range — low pass modes need the sweep to start near 50 kHz