Theory: Oscilloscope

The oscilloscope is one of the most important and versatile electronic measuring instruments used in the lab today. This instrument is composed of a cathode ray tube with amplifiers, time bases and other electronics that enable voltages to be displayed pictorially. This allows one to observe the effects on an electric signal by a circuit and to study any physical quantities (eg sound) that have been converted to an electrical signal.

Figure 1

The actual working of the oscilloscope is based on the electron that, because of its charge, will experience a force in an electric field. Hence, if a stream of electrons is sent through an electric field, it is deflected by an amount that is dependent on the strength of the field. By using a separate electric field for both x and y directions, the electron beam can be moved and positioned to strike some point on a distant screen. These electric fields are caused by voltages applied to deflection plates. Since the displacement of the beam is proportional to these voltages, one has a simple system for plotting a graph. By moving the beam more swiftly than the eye can follow, lines can be drawn. If the beam is moved horizontally at a fixed rate, and a varying voltage is applied to deflect it in the y-direction, then a graph of voltage versus time is produced. This forms the basis of an oscilloscope's operation.

 In practice, the beam of electrons is created by boiling electrons off a hot wire (the cathode) and accelerating them through several cylindrical tubes of higher potential (anodes). The anodes are also used to shape (focus) the electron beam. The beam then travels through x and y deflection plates towards a phosphor screen which glows when the electrons strike it. The deflection of the beam is directly proportional to the voltage applied to the deflection plates. This is controlled by x and y amplifiers (see Fig. 1). In addition, the x deflection has the option of having a time base applied to it that simply sweeps the beam across the screen at a constant rate. With the screen marked off in xy coordinates, a graph of voltages or voltage versus time can be plotted.

Figure 1 also shows the inputs to the oscilloscope. Placing a voltage on the y-input will cause the observed spot on the screen to move to a position up or down depending on the voltage (a negative voltage on the input with respect to ground will cause the spot to move down). Similarly, the spot can be positioned horizontally through the x-input. Placing a voltage on both x and y inputs moves the spot to that xy position. If an alternating voltage is applied to the inputs, the spot will move displaying the instantaneous values of the voltage. The x-input can be switched to a time base that moves the spot across the screen at a constant speed. The spot will start to move from the edge of the screen each time a pulse is applied to the trigger input of the time base. This pulse can come from the y-input or some other source and is generally used to synchronise the events happening on the y-input to form a stationary picture on the screen of the y-input versus time.

All oscilloscopes have similar switches (but may be arranged differently). Therefore learning to operate any one particular oscilloscope will usually enable one to operate any oscilloscope. The manual appendix shows the oscilloscopes used in the first year lab with descriptions of all the switches. The best way of learning how to use an oscilloscope is to experiment ("play") with each one of the switches to learn what it does. Notice the switches are divided into sections based on their functions.