How to detect multiple changes in a cable

[jegjessing]

Hi,

I've seen on a TDR that the curve varies depending on the distance. This way you can determine multiple things, like 15ft out there some humidity, 40ft out there is a bend 100ft out there is a shortcut etc.

But all examples I've seen is a single pulse and a single result. How are these different results combined into one graph?
I suspect I would sent the pulse repeatedly and then, continuously reading the result varying the delay from I send to I read.

So "sweeping" the delay from I send the pulse to I read the analog value will give the curve that I see on the TDR?

Am I completely wrong or?

Attached is an image of what I mean.

Thanks in advance

[madires]

The diagram's x axis is titled "Time". The TDR sends one pulse and measures the reflections over time. With the speed of electrons in mind and the time delay of each reflection you can easily calculate the distance. As longer it takes for a reflection to reach the TDR as farther away is the cause of the reflection. Another thing to consider is that the speed of electrons depends on the medium, e.g. for coax cable it's about 66% of c.

[tggzzz]

Welcome to the world of sampling and signal processing.

If your device can collect samples and process them sufficiently quickly, then you can transmit a single pulse and measure the reflections. Usually that isn't the case.

In all cases the time/distance resolution is limited by the "aperture width" of the sampling circuit, dt.

Usually what happens is that you transmit a pulse and then sample the result a set time, t, later and then convert the sample to a digital signal using an ADC. After the ADC has completed, you can transmit another pulse and take another sample.

If you progressively change t to t+dt and convert the signals each change, then you will see reflections from progressively further away. Note that dt depends on the width of the sampling pulse, not the ADC conversion time. That is the mechanism by which old analogue TDRs such as the Tek 1502 can have 4GHz bandwidth yet only sample at 50kS/s, even though that looks like it violates the Nyquist theorem.

Alternatively if you take N samples at t (e.g. by adding each sample to an analogue voltage on a capacitor) before converting them, then the averaging can reduce the noise (by sqrt(N)) and increase the sensitivity/range. Of course it takes N times as long to do a complete sweep.

Less intuitively, you can also sample and convert to a digital signal N times, and then digitally adding the signals also averages and reduces the noise by sqrt(N). That can "improve" the ADCs resolution e.g. an 8-bit ADC can have 12 bits resolution - but only if the noise is random.

[dmills]

Another thing to consider is that the speed of electrons depends on the medium, e.g. for coax cable it's about 66% of c.

You might want to actually calculate the mean drift velocity of an electron in a copper conductor at reasonable current density, it is nothing at all like 0.6 light, in fact I (An unfit fat man) can trivially easily out run it!
 
What propagates at a significant fraction of light speed is the fields between and around the conductors, not the charged particles themselves.