Hi everyone,
We have two parallel copper cable (a very standard, un-shielded, with 1.5
mm diameter) lines with lengths of 2500 meters (about 1500 miles). Normally
these lines are not connected to each other and suppose that there is no load
at each end of these lines (open circuit conditions). What we want to measure
is the distance where a short-circuit condition occurs or in other words an
approximate localization of point where these lines touch each other.
In the case of short-circuit, these 2 parallel cables form 2 different loops.
Suppose we have a measurement system at one end. We may send any signal from
one cable and recieve it (which travels all the way to the short-circuit point
on the first cable and enters at that point to the second cable) from the other
cable. Based on the information that I have read from the Modular Instruments
Course Manual, there will be an un-avoidable delay with the transmission of the
electricity signals :
"...Electricity travels through a wire at approximately two-thirds the speed
of light (200000 km/s - 0.2 m/nanoseconds), which means that it travels at
approximately two-thirds a foot per nanosecond... "
Here comes my question : Is it possible to measure the delay which is
introduced by the copper line to the signal transmission ? If it is so, we
may approximate the length of the cable on which the transmission occurs
(based on the measured delay) and simply find the point where the short-circuit
has occured (by diving the distance by 2).
I know that HS Digitizers (such as PXI-5122) are capable of timestamping to
nanosecond time precisions. Definition of TDC (time-to-digital converter) also
implies that :
"...A digitizer with a TDC uses the TDC to measure the time between the
trigger and the first sample. This allows for extremely accurate trigger
measurement. For example, without a TDC, when using a 100-MHz sample clock, the
trigger time would have a maximum error of one sample clock cycle or 10
nanoseconds. Some NI digitizers with a TDC can reduce
this error to 100 picoseconds or better..."
And we are also planning to use an Arbitrary Waveform generator (AWG) for tight
synchronization of the signal generation and recieving. In that regard, TClk
technique of the Synchronization and Memory Core (SMC) architecture is a
suitable candidate.
However I haven't used those techniques before and I am not sure about the
synchronization of these devices. Nevertheless, tenths of nanoseconds of
synchronization error is acceptable which would yield 2 - 20 meters of
localization error (less than %1 localization error for the overall line).
I need an approval of the usage for an HS-Digitizer and an AWG side-by-side in
this application. This kind of application seems very rare and the physics of the system should be
considered thoroughly.
All the valuable ideas are highly appreciated.
P.S. We are also thinking about to use a high precision DMM such like PXI-4071
for impedance or resistance measurement however the copper lines won't be in an
isothermal enviroment, there may exist temperature differences up to 20oC and
field conditions may also affect the impedance of the lines.
Best Regards,
Candan Caner
candancaner@yahoo.com
