140. Transmission Line

Transmission Line

A signal generator capable of 10 MHz is hooked to a T-connector at channel 2 of an oscilloscope through an 8 meter 50 ohm cable as shown above. Channel 1 of the scope is hooked to the same source through a short cable.

Input a pulse at 1 MHz; sweep at 0.5 ms/cm

1. With no termination at channel 2, channel 1 shows the pulse and its reflection at the open end of the cable. Channel 2 shows the arriving pulse between the two (shown on scope screen above).
2. You can measure the speed of light in the cable from the data above, finding about 2X 108m/s.
3. You can show the effect of an open, completely absorbing, or closed termination on the reflected pulse. Use a completely absorbing terminator on the T at channel 2, and the reflected pulse disappears. Short the T at channel 2, and the reflected pulse inverts.

Input a sine wave

1. At low frequencies (100 KHz) the cable acts like a wire; the signals at channel 1 and channel 2 are in phase and of equal amplitude. But at higher frequencies, the 8 m cable begins to act like a transmission line. At about 6 MHz the channel 1 signal goes to zero as the reflected signal from the open end cancels the transmitted signal. With the 8 m cable terminated with a complete absorber, the signals become equal in amplitude again, but the signal through the 8 m cable is phase shifted 90 degrees at 6 MHz.
2. Get the wavelength from l/4 = L by nulling the Channel 1 signal with open termination. From the frequency of the generator, you can now verify c = ln, comparing with the above-measured value of c.