Time domain reflectometry exercise.

Group members:

Bogdan Dumitru

Krisztian Kovacs

Jakub Slupecki

Odense 07-12-2011

This exercise was about discovering if there is something wrong with the cable. It may help to diagnose the cable.

Measurements can show if the cable is squeezed somewhere and if everything is OK with the cable.

For that experiment we were guided by an article from electronics magazine.

The experiment involved building an oscillator circuit using 555 timer and some hex inverters to invert the signal.

The 20 meter (75 ohm) coax cable is connected to the resistor. The experiment was to see oscilloscope picture and

count the cable length due to signal reflection speed. Also putting different things on the end of the cable to count

reflection coefficient factor.

Instead of 100nF we are using smaller capacitor 220 pF, so we have about 1.8 MHz frequency.

Calculations:

Calculating the speed of the signal in the cable.

The velocity for RG6 cable is 0.66 or 0.83. We will stick with 0.83 factor.

Speed of light = ⁄

Therefore: ⁄ ⁄

So the signal passes roughly 25 meters of cable in time of .

The total way to travel is 40 meters. So the time needed is .

From the picture we can see that between two points there are roughly 6.5 squares and one square

is long, so:

Since there is a measure error involved and cables on the vero board are rather long the result is

not exactly the same as counted earlier number.

The conclusion from that is that the cable is OK; there are no rough corners or squeezes on its way.

In practice it is a way to check if the cable is not damaged.

Reflection coefficient calculations:

1. Shorted

Signal is reflected; reversed polarity.

2. Open

Signal is reflected; the polarity is the same.

3. 75 ohm resistor

No reflection; cable is terminated.

Conclusion This exercise is a good method of checking if a cable is in good condition or if it is damaged. This method is

used in diagnostics to check physical condition of a cable. The reflection coefficient tells if there is a signal

reflection. That activity was used in older days to fix the problem of having “shadows” on TV screen.