"Double adjustable pots are better than R's" Uh - oh. For 1 GHz/Gigabit Ethernet? Some of those types of 'R's *may* not present a purely resistive (like 50 + j0, 200 + j0) load but rather appear as "50 +j20" instead - - user beware and be aware ... Jim ----- Original Message ----- From: "Peter L. Peres" To: Sent: Saturday, September 08, 2001 2:13 PM Subject: Re: [OT]: Help! need 120 ohm cat5 cable > Quick recipe to figure out cable impedance: > > Fit a R = suspected Zo of cable at each end (without removing from roll), > short one of them to the shield, the other goes to a signal generator (can > be any square or pulse wave with sharp - < 20ns - edges), and connect > scope to cable inner and shield at the generator side. Adjust the scope to > view at most one half period on X. > > If the R's do not match the cable the waveform will be extremely badly > distorted on the scope (and you can use some math to determine the > reflection coeff. and thus Z0). I use(d) the same method to determine if > reclaimed cable was 'kinked' or damaged (after comparing to a new roll > using the same method). > > Double adjustable pots are better than R's and allow the cable to be > matched in real time (and measured if you attach a scale). These used to > be available as spare parts for test equipment ($$$). Ganging two 250 Ohm > pots (not wirewound - cermets more likely) may yield good results. > > Since you do not need access to the far end of the cable the measurement > can be done with already laid cables, if the far end is terminated. This > is the ancestor of cable TDR. > > Peter > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu