3 ms·

Build Your Own Time-Domain Reflectometer

• CamperBob2 11y ago
  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 11y ago
    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
• fancyketchup 11y ago
  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-oscilloscope-front-end-easily-modified http://electronicdesign.com/boards/1-ghz-sampling-oscillosco...
  • CamperBob2 11y ago
    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.