Time-Domain Reflectometer

 

 

Background

 

A time-domain reflectometer (TDR) is a device which allows us to see the reflections from a discontinuity or load on a transmission line. In this lab, you will construct your own TDR and use it to measure reflections from various terminations.

 

Procedure

 

I. Construct a TDR

 

1.      Place a BNC T-connector on the input to the oscilloscope.

2.      Connect one side of the T to the square wave function generator with a short very short line.

3.      Set the function generator to output a 0.1V, 100kHz square wave (or square pulse if it is available).

4.      For simplicity in viewing you may want to check your wave on the scope and set the voltage offset so that the wave goes from 0 V to some positive voltage (the default is for the wave to be symmetric around 0 V; if you can't raise the minimum voltage to zero using the offset adjustment, then adjust the amplitude vernier to make a smaller peak voltage).

5.      Use the symmetry or percent duty adjustment on the generator to set the pulse width to minimum, then set the time resolution on the scope so that you just see the entire pulse (for most setups this will be around 100nsec).

6.      Be sure the scope is set to high impedance input.  For most of the scopes in this lab you can press the coupling adjustment button for the scope channel which you are using.  A menu will appear at the bottom of the display.  Verify that the 50 Ohm setting is off, meaning that you are correctly set to high impedance.

 

II. Measurements

 

Connect a long (at least 5 meters) length of RG-58 coaxial cable to your TDR.

 

A.     For the following five termination types sketch the wave form at the generator end of the coax.  Label the times and amplitudes of all of the major events of each waveform.  Be sure to include the entire wave form corresponding to one square pulse input.  Why might this be easy to miss for the short circuit and the inductor? 

1.      Open circuit

2.      Short circuit

3.      50 ohm load

4.      Capacitor

5.      Inductor

For the capacitor and inductor you should also measure the time constant of the circuit. This can be easily approximated by measuring the time it takes for the signal to move two thirds (67%) of the way from an initial voltage (the points of the voltage spikes in this case) to a final voltage.

 

  1. Measure the characteristic impedance of the RG-58 coaxial cable. You can approximate this in two ways:

1.      Assume the output impedance of the generator is 50 ohms, and compare the unterminated voltage (connected directly to the scope with high impedance input and no transmission line connected to the other side of the T connector) to the voltage divider obtained when the coax is attached to the T connector.

2.      Attach the 50 ohm load. Measure very carefully the small difference between the initial voltage and the voltage after reflecting from the load (in many cases this small difference will be too small to distinguish from the noise in the system, making this method virtually impossible). Calculate the reflection coefficient and work backward to get the characteristic impedance.

Explain which method you chose and why.  Show or explain how you used your measured values to obtain the characteristic impedance.

 

III. Analytical Comparisons

  1. Determine analytically all voltage levels for each termination type measured in Part II.
  2. Explicitly explain the relationships between the timing of major events for each termination type assuming the same transmission line was used. (We will be determining propagation speed in the next part of the lab, so limit your responses to a qualitative explanation of your observations from part II regarding timing.)
  3. Calculate the expected time constants of the L and C responses.  Remember that the resistance seen by the loads in this case is limited to the characteristic impedance of the transmission line which they are connected to. (This should only require a very simple calculation)

 

IV. Propagation on the RG-58 Coax

  1. Measure the propagation speed on the RG-58 coax (the length should be written on a piece of tape on the coax).  If your wave appears to be traveling faster than the speed of light please reconsider your calculations before handing in your write-up.
  2. Using the TDR and the measured propagation speed found in part 1, measure the length of another length of RG-58 (different from the one used in part 1 and greater than 5 meters in length) and compare with its actual length.
  3. Using the TDR, measure the length of a short (no more than 3 meters) length of RG-58 coax to within 10 cm.  Explain any difficulties experienced while making this measurement on a shorter line.

 

Be sure to follow the description of expected lab submissions.

 


Copyright © 1998 by Michael A. Jensen. All rights reserved.

Last updated on Jan 12, 2005