Time Domain Reflectometry by pengxuebo

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									                  SEE 3 Field Theory Laboratory

                   Time Domain Reflectometry.
1. Introduction.
       The domain reflectometry is a widely used technique for testing
transmission lines, optical fibres and printed circuit boards. It may also
be used to determine frequency response.
 (1) Using the pulse generator and oscilloscope, set up a short pulse of
duration 150 ns and of amplitude 2.5 V and having a pulse repetition
frequency of 100 kHz. The signal generator output resistance should be
50.
(2) Connect the delayed trigger output of the signal generator to the
external trigger input of the oscilloscope. Switch the trigger source of
the oscilloscope to external. You may now adjust the delay control of the
signal generator to centre the pulse waveform in the screen of the
oscilloscope.
(3) Carefully sketch in the results form the incident and reflected
pulses. You will have noticed that the reflected pulse is not an exact
replica of the incident pulse but is somewhat distorted. This is due to two
imperfections of the transmission line. These are:
       (i) attenuation on the line due to losses in the copper and
dielectric.
       (ii) dispersion due to the different frequencies (of which the pulse
is composed in terms of Fourier analysis) travelling at slightly different
speeds, so that frequencies undergo differing group delay.
(4) Time domain reflectometry is also used to locate faults on cables or
optical fibres, especially in cases where they are buried or otherwise
inaccessible. To determine the length of the transmission line to the open
circuit, measure on the oscilloscope the time taken by the pulse to
traverse the line. Remember the reflected pulse will have traversed the
line twice. The velocity factor, which is the ratio of the velocity of
propagation to the velocity of light, is 0.6. Complete this section of the
results form.
(5) The board provided with the experiment contains each of the
following terminations:
       (a) an open circuit;
       (b) a matched load ( 50 );
       (c) a 25  load;
       (d) a 75  load.


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For each of these terminations calculate and measure the reflection
coefficient, . The reflection coefficient is the ratio of the reflected
wave to the incident pulse
                               Vreflected
                          
                               Vincident
This may be measured directly on the oscilloscope, neglecting the effects
of attenuation. The reflection coefficient may be calculated by

                               Z L  ZO
                          
                               Z L  ZO
Complete the results form for each termination, including the open
circuit.
(6) When using fast digital circuits it is necessary to consider the
connections between gates as transmission lines. Reflections may then
arise when the input and output impedances of the gates are incorrectly
matched.
       To investigate these concerns, terminate the transmission line in
the diode on the board. The diode represents the base to emitter
junction of an input transistor in a gate.
       Increase the amplitude of the pulse incident on the diode to 5 V.
This turns the diode on. Verify that the diode now acts as short circuit.
       Decrease the amplitude of the incident pulse until it is
substantially less than the diode cut-in voltage. You should now see the
diode acting as an open circuit.
       Vary the amplitude of the incident pulse until the amplitude of the
pulse reflected from it is minimised, neglecting the effects of
propagation delays. Its dynamic resistance now approximates 50 .
Complete a sketch of the waveform on the results form.
(7) With an open circuit load, increase the duration of the incident pulse
to 2 ms, so that the reflected pulse interferes with the next incident
pulse. Sketch the resultant waveform on the results form and explain the
effect.
(8) Note that the reflected pulse also represents the impulse response
of the line. By calculating the inverse Fourier transform of the reflected
pulse, the frequency response of the line may be obtained.




                                      2
                                  Results Form

                      Time Domain Reflectometry.




Open Circuit
 Sketch the waveform observed from an open circuit in the grid shown
above. Indicate the scales of the vertical and horizontal axes.
      Vertical............V/div               Horizontal.............sec/div

Length of Line

              L = 0.6 x c x t /2 = ..................m.
                    where c = 3 x108 m/s .




                    Open      Short         Matched       25   75



Calculated
Reflection
Coefficient
Measured
Reflection
Coefficient




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                                 (a)                                                                        (b)

(a) Matched diode
 Sketch the waveform observed from the diode where the incident pulse
is such as to minimise the reflected pulse. Indicate the scales of the
vertical and horizontal axes.
      Vertical............V/div           Horizontal.............sec/div

(b) Multiple reflections
 Sketch the waveform observed for the case of multiple reflections.
Indicate the scales of the vertical and horizontal axes.
      Vertical............V/div               Horizontal.............sec/div

Comment, observation or conclusion

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                            you leave the laboratory!
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