
Build Your Own Time-Domain Reflectometer - elijahparker
http://www.allaboutcircuits.com/projects/build-your-own-time-domain-reflectometer/?utm_source=AAC+Members+Weekly+Newsletter&utm_campaign=e89113b233-AAC_Newsletter_10_16_15&utm_medium=email&utm_term=0_2565529c4b-e89113b233-266104569/
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fancyketchup
Aside from the things camperbob identified, this is pretty neat. The tricky
parts are getting the pulse generator right (you want a sharp leading edge and
flat top, with no ringing) and having an oscilloscope with a flat response
well into the GHz range.

One solution to the second challenge is to home-brew a sampling adapter, like
this one from 2000[1]. This is a good fit for TDR because of two factors
inherent in the TDR process: a) it is repetitive, so the usual drawbacks to
"sub-nyquist" sampling don't apply; and b) you have ready access to a pre-
trigger, so it is easy to see the leading edge of the signal of interest.

The first challenge requires a lot of cleverness or access to expensive test
equipment.

[1] [http://electronicdesign.com/boards/1-ghz-sampling-
oscillosco...](http://electronicdesign.com/boards/1-ghz-sampling-oscilloscope-
front-end-easily-modified)

~~~
CamperBob2
That's pretty neat. If you built a similar circuit with off-the-shelf parts
available today, it could be good for 8 GHz or so.

------
CamperBob2

       For all coaxial dielectrics, the relative permeability 
       is so close to 1 that we can just assume it is for our 
       purposes ( accurate to within 0.00000001%). Substituting 
       these values into our first equation we get
       
       Vp = 273,671,819.7 m/s or 10.7745 in/nS
    
       which means our signal will propagate at about 91.29% of 
       the speed of light, this percentage is known as the   
       velocity factor or VF.
    

Huh? That's nowhere near a typical coaxial velocity factor (most often around
0.66 for 50-ohm cables.)

~~~
rebootthesystem
Yup. Spent a good deal of time hand-timing coaxial cables for matched time of
flight. It's 66%.

[http://www.belden.com/techdatas/english/8281.pdf](http://www.belden.com/techdatas/english/8281.pdf)

[http://www.belden.com/techdatas/metric/9273.pdf](http://www.belden.com/techdatas/metric/9273.pdf)

