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Time domain reflectometry (TDR)

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manchiraju

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Hi,
I wanted to implement the TDR principle in my laboratory as I was against the idea of buying one. I found out few circuits on the net, but I would like to implement a very basic TDR without the use of a Sampling head/ t-connector(as found in most cases on net and this forum). Can I implement it with just a function generator, CRO, cables and other devices (as available in my lab) ??
 

Yes you can. Locate a power buffer ( IC such as a DS0026 or better yet a newer low level mos fet gate drivers like the IR4426 ) capable of 50ns rise and fall times. If lower rise and fall times are used the returns will not be as accurate. A low inductance power supply with very short leads ( 1 inch total power supply loop ) Place the output of the buffer at the end of the coax with a low inductance 10 ohm resistor in series. Look across the resistor for the input pulse and the return. I hope this is of some use.
Yar.
 
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    ferdem

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You mean to say that i use the pulse generator to send a pulse across the power buffer whose output is connected to one end of coax and in series with a resistor?
Can I look for the incident and the return using a single probe to the oscilloscope?
 

That's the whole idea although you can of course measure the time delay with a counter rather than an oscilloscope.

The principle is to send a fast edged pulse along the cable from one end and see how long it takes to bounce back from the far end. The far end could of course be a break in the cable rather than it's physical end. An unterminated cable or a break will not absorb the pulse so much of it will simply reflect back. The time delay between outgoing and returned pulse is caused by twice the cable length, there and back again, but you must take into account the velocity factor to convert the delay into a real distance.

Brian.
 

Hey ppl,
Please answer these 2 Qs
1) I need to design a TDR circuit to measure the dielectric constant of soil. Can you please tell me what all components are required because I have to order them as soon as I can and make it work in the next 3 days (VERY URGENT). What I have in my lab is a pulse generator, CRO, their corresponding probes (should I order separate cables or these will suffice???) ,resistors,caps etc.
2) After designing the first circuit I have to communicate with a wireless system and send data across. So, keeping that in mind, can you answer the first question!
 

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