Time domain vernier method
Time Domain Vernier Method or TDV, is a reflectometry method used to locate intermittent faults in wires (such as aircraft wires) by measuring time delay between incident and reflected signals. A test system sends a high frequency signal down the wire which is then reflected back by any impedance discontinuity. This reflected signal returns to the test system after a delay which can be converted into a measurement of the distance to the fault by the following formula:
-
- 2d = VOPT
where T is the delay of the reflected signal, VOP is the velocity of propagation on the cable and d is the distance to the fault.
Difficulties
The VOP depends on the type of wire (which May Be unknown) and its orientation relative to other metallic structures (which is usually unknown). For instance, in aircraft wiring VOP is normally around 67% of the speed of light, but can vary by +/- 10%. Any error in VOP causes a direct error in the measurement of distance, regardless of which type of reflectometry is used.
On top of the first reflected signal from the impedance and the second reflection from the end of the wire, multiple reflections can occur from branches, junctions and other impedances which may overlap and confuse the data. However, methods to identify these sources and account for them more accurately have been developed.
See also
- Time-domain reflectometry
- Spread-spectrum time-domain reflectometry