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Entry  Thu Dec 10 13:10:25 2009, Alberto, Configuration, LSC, 166 LO Disconnected 
    Reply  Thu Dec 10 18:31:44 2009, Alberto, Configuration, LSC, 166 LO Disconnected 
       Reply  Fri Dec 11 09:30:09 2009, Koji, Configuration, LSC, 166 LO Disconnected 
          Reply  Fri Dec 11 11:42:26 2009, Alberto, Configuration, LSC, 166 LO Disconnected 
             Reply  Fri Dec 11 14:12:32 2009, rana, Configuration, LSC, 166 LO Disconnected 
                Reply  Fri Dec 11 19:51:13 2009, Alberto, Configuration, LSC, 166 LO Disconnected 
                   Reply  Sat Dec 12 07:36:56 2009, rana, HowTo, Electronics, How to Measure the Length of a Cable: Interferometry 
Message ID: 2403     Entry time: Sat Dec 12 07:36:56 2009     In reply to: 2402
Author: rana 
Type: HowTo 
Category: Electronics 
Subject: How to Measure the Length of a Cable: Interferometry 

Need to measure the length of the cable, but too lazy to use a measuring tape?

Then you too can become an expert cable length measurer by just using an RF signal generator and a scope:

  1. Disconnect or short (not 50 Ohm term) the far side of the cable.
  2. Put a T on the near side of the cable.
  3. Drive the input of the T with your signal source.
  4. Look at the output of the T with the scope while sweeping the signal source's frequency knob.

The T is kind of acting like a beamsplitter in an asymmetric length Michelson in this case. Just as we can use the RF phase shift between the arms to measure the Schnupp asymmetry, we can also use a T to measure the cable length. The speed of light in the cable is documented in the cable catalog, but in most cases its just 66% of the speed of light in the vacuum.

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