Pulse Reflections in a Coax Cable

Presentation Date: 

Thursday, February 25, 2016

What it shows:  A voltage pulse, injected into a long coaxial cable, will travel down the length of the cable and undergo a reflection at the other end. The nature of that reflection depends on how the cable is terminated at the other end. Shorting the cable at the far end produces an inverted reflection. With no termination (an "open" end), the reflected pulse is not inverted. When the impedance of the termination matches that of the cable, there is no reflection.

Knowing the length of the cable and noting the amount of time it takes the pulse to come back allows one to calculate the propagation speed; this turns out to be 85% of the speed of light in a vacuum.

How it works: Any pulse generator capable of producing very short duration pulses (say, 100 ns or less in width ... if the pulse is any wider, the reflection will overlap the initial pulse) will do the job. The Wavetek model 801 50 MHz Pulse Generator works well for this demo. The cable (30.55 m / 100 ft 12 AWG Ethernet 50-ohm coaxial cable) is wound up into a manageable-size coil. Both ends have BNC connectors. The pulse generator and oscilloscope for observing the pulses are both connected to the near end through a T-connector. The far end is terminated by a 250-ohm potentiometer so that the impedance can be adjusted between zero and 250 ohms. It's attached with clip leads, so you can easily disconnect it if you wish to have an open end.

Setting it up:The Tektronix TDS 3014B or the 2221A can be used. Vertical scale, 10 or 20 volts/div. Horizontal scale 100 ns/div. DC coupling. 50-ohm input impedance. Expect a 240 ns delay between initial and reflected pulse (don't forget that the distance is a round trip).

The Wavetek 801 can be found upstairs in the 191 lab.