E.5.13 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
- 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).
- You can measure the speed of light in the cable from the data
above, finding about 2X 108m/s.
- 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
- 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.
- 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.
Suggested by Brad Tippens
|