( A similar demonstration is listed as Activity 70D Advancing Physics, AS2000 )

Introduction

Educational data logger

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.

Equipment required

Experiment Setup

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 2µs 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!

Carrying out the Experiment

The circuit diagram is given below.

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 200m in the cable a satisfying 1us later, equivalent to a speed of 2 x 10^8 ms-1.

Results

Waveform as seen on PicoScope

  • 100 kHz square wave "differentiated" by RC network (see further note)
  • Positive peaks represent the leading edge of the square wave
  • Period (read from scope) is 10 µs (ie. 100kHz frequency)
  • BLUE trace is pulse at start of coax cable
  • RED trace is pulse detected after 200m of travel down cable
  • Time delay between red and blue is 1us
  • 200 m in 1ms = 2 x 10^8 ms-1.
  • The Y-gain for both channels is 500mV so that attenuation of the red pulse is nicely shown another good discussion point.

Converting the 100kHz Squarewave to Pulses PicoScope view.

  • The 100 kHz square-wave (blue trace/channel A) is produced at the signal generator.
  • The pulses are produced by the 68 ohm resistance and capacitance, C.
  • A suitable values for C is 4700pF a substitution box was used.
  • The R-C network effectively differentiates the square-wave, a good discussion point for mathematically-inclined students (and teachers).