TDR Cable Fault Locator Calculator: Reflection Coefficient, Echo Polarity and Distance to the Fault
Turn a TDR trace into the distance to a cable fault and what kind of fault it is.
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How this calculator works
Enter the characteristic impedance of the cable, the impedance at the change (or a very large number for an open circuit and 0 for a short), the echo time from the TDR trace, and the velocity factor of the cable.
The calculator finds the reflection coefficient, the size of the reflected step and the level of the trace after the echo, and the speed of the signal in the cable.
It halves the round-trip time to give the distance, and gives the VSWR, the return loss and what the echo means: open, short, a higher impedance or a lower one.
Formulas used
- \Gamma = \frac{Z_2 - Z_0}{Z_2 + Z_0}
- v_p = VF\cdot c
- d = \frac{v_p\Delta t}{2}
- \mathrm{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}
Worked example
With the default numbers in the calculator above:
- Reflection coefficient: 0.20 (positive: the impedance rises)
- Size of the reflected step and the level the trace shows after the echo: Echo 0.20 V; the trace steps from 1.00 to 1.20 (in units of the launched step)
- Speed of the signal in the cable: 2.01 x 10^8 m/s
- Distance to the fault (the echo travels out and back, so halve the time): 10.04 m
- Round trip distance: 20.09 m
- Voltage standing wave ratio and return loss: VSWR 1.50, return loss 13.98 dB
- What the echo means: A rise in impedance: a thinner section, a high resistance fault, or a partial break
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Impedance at the fault or the far end (0 = short circuit, a very large number = open circuit) 0 ohm.
- Reflection coefficient: -1.00 (negative: the impedance falls)
- Size of the reflected step and the level the trace shows after the echo: Echo -1.00 V; the trace steps from 1.00 to 0.00 (in units of the launched step)
- Speed of the signal in the cable: 2.01 x 10^8 m/s
- Distance to the fault (the echo travels out and back, so halve the time): 10.04 m
- Round trip distance: 20.09 m
- What the echo means: About -1: a short circuit
Common questions
What does a positive echo mean?
The impedance has gone up, which is a thin section, a high-resistance joint, or a break in the cable. A full +1 echo is an open circuit.
What does a negative echo mean?
The impedance has gone down: wet or crushed cable, a tap or a short circuit. A full -1 echo is a short.
Why halve the time?
The echo travels to the fault and back, so the measured time covers twice the distance.
How accurate is the distance?
As good as the velocity factor. Calibrate it on a known length of the same cable.
Related calculators
Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Pozar, Microwave Engineering (transmission lines and reflection); Keysight, Time Domain Reflectometry Theory.
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