PicoScope 7 Software
Available on Windows, Mac and Linux
Flick a switch in an electric circuit and the resulting effect (for example a light coming on) seems instantaneous. Electrons drift slowly in the circuit yet the electromagnetic wave travels through the circuit at unimagininably high speed.
This simple demonstration using a PC-based oscilloscope gives a clear picture of the time taken for a pulse to travel 200 m in a coaxial cable. It also provides graphic evidence of signal attenuation.
In view of the limited number of applications for a pulse generator in the ‘A’ level syllabus and the relatively large requirement for signal generators, suitable pulses were produced by a signal generator giving a 100 kHz square wave through an R-C pulse-shaping network. (A note is included on pulse shaping- see below)
An ADC-200/50 PC–based oscilloscope (and PicoScope software), with its timebase set to 2 µs per div, was employed.
The coaxial cable was purchased from RS components (a considerable time ago). It is much easier to use if kept on its drum!
The 68 ohm resistors equal the characteristic impedance of the cable and prevent reflections.
The capacitor combines with the resistor and shapes the square wave into sharp pulses (see the note on pulse shaping).
Both ends of the outer sheath of the coax are connected to earth.
The signal generator is set at an amplitude of about 1 volt.
Channel A on the scope then shows the pulses before they enter the coax, channel B shows the corresponding pulses as they arrive after travelling 200 m in the cable a satisfying 1 µs later, equivalent to a speed of 2 x 10^8 m·s-1.