PURPOSE: To demonstrate the reflection of pulses in terminated and unterminated transmission lines and to determine the speed of an electromagnetic wave in the transmission line.

DESCRIPTION: An oscillator produces pulses at a rate of about 100 khz, which are input into the transmission line. The scope is triggered on the initial pulse, and the pulse is displayed on the top trace of the dual trace scope as it enters the transmission line and as it returns. The pulse at the end of the transmission line is displayed on the bottom trace. For a short section of transmission line the input and output pulses almost line up. Using about 100 feet of transmission line the delay is approximately 150 ns; for this cable v/c is approximately 0.67 so the wave time is about 1.5 nanosecond per foot.

To see reflections, use only the top trace. The far end can be left open, shorted, or terminated with a 50 ohm terminator to prevent reflection, as shown in the center and right photographs above. The photographs below show the oscilloscope showing the 10 MHz pulse train (left), reflections with the far end of the cable connected to the lower trace of the oscilloscope, with the horizontal scale 250 nanoseconds per division (left center) and 100 nanoseconed per division (center), the far end terminated by 50 ohms (right center), and the far end shorted but not connected to the oscilloscope (right).

SUGGESTIONS: The actual measured length of the cable is 29.7 meters (approximately 97.5 feet). A detailed list of the instrument settings for this demo can be found here

See Question of the Week #227 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 5N10.30)

EQUIPMENT: Oscillator, dual trace scope, 100 feet of 50 ohm cable, shorting and 50 ohm terminators.

SETUP TIME: 5 min.