Purpose: To become familiar with the operation of an oscilloscope and to use it for making measurements of varying voltages and short time intervals.
Equipment: The oscilloscope is very useful for observing phenomena which change quickly with time. The ordinary voltmeter cannot be used to measure potential differences which fluctuate even as slowly as 60 times per second because the inertia of the movement prevents it from following the fluctuations. The oscilloscope uses a beam of electrons which have very small masses ( me = 9.1 x 10-31 kg ) and can respond very quickly to rapid changes even as great as rate of 106 Hz. The indicating mechanism of the oscilloscope is called a cathode ray tube (Fig. 1). This tube consists of an evacuated glass envelope in which the electrons are produced by heating up an oxide coated piece of metal (the cathode).
The electrons are then focused and accelerated along the axis of the tube by applying potential differences to various discs (anodes) containing holes that allow a well-defined beam to pass after attaining kinetic energies of several kev. The beam of electrons strikes the end of the tube (screen) which is coated with a fluorescent phosphor that emits a spot of light wherever the electrons are stopped. Between the anodes and the screen are two sets of parallel plates oriented at right angles to each other so that a potential difference applied to either set will produce a deflection of the beam as it passes between the plates. Making the upper plate [+] and the lower one [-] will cause the beam to be deflected upwards to point P in Fig. 1. The set of plates producing a vertical deflection is called the y-plates, the set producing a horizontal deflection, the x-plates. Looking at the front of the tube with zero voltage on both sets of the deflecting plates, a spot is observed at the center of the screen (Fig. 2a). Applying a potential to the y-plates as shown in (Fig. 2b) produces a vertical displacement of the spot. A potential applied to the x-plates produces a horizontal displacement (Fig. 2c). The displacement produced is proportional to the potential difference on the deflecting plates (See question 2). Since there is no current passing between the deflecting plates, the oscilloscope is effectively a voltmeter with infinite resistance.
Fig. 2: Position of electron beam as seen on oscilloscope screen with (a) zero applied voltage, (b) steady voltage on y-plates, (c) steady voltage on x-plates.
An AC voltage applied to the y-plates (Fig. 3) will produce a vertical line which appears to be continuous. If the frequency of the applied voltage were only a few cycles per second, we would see the spot moving, but due to our persistence of vision and the time interval required for the phosphor to stop emitting light after the electrons are removed, the effect at even moderate frequencies is to produce a continuous line. The length of the line is proportional to the Amplitude of the AC voltage. Circuits build into the oscilloscope provide for amplifying very weak signals so they can be measured.
Fig. 3: An Alternating Voltage Applied to Y-plates Produces a Vertical Line
Suppose a voltage which varies linearly with time is applied to the x-plates. If this voltage is made to repeat the cycle by suddenly returning to the initial value after a given time interval t , a horizontal line would appear on the screen. Fig. 4 represents this voltage (saw-tooth voltage). Under these conditions the x deflection varies linearly with time.
Simultaneously applying the signal in Fig. 3 to the y-plates and that in Fig. 4 to the x-plates and adjusting the period of the signal in Fig. 3 so that it is the same as t in Fig. 4 will produce a sine curve as shown in Fig. 5. Remember in this figure the vertical axis is proportional to the potential difference across the y-plates and the horizontal axis is proportional to the time. If the period of the saw-tooth voltage is an integer multiple of the period of the sine wave, the pattern will be stable with one or more oscillations on the screen. To achieve this stability an internal circuit allows the period of the saw-tooth wave to be varied and part of the signal applied to the y-plates is used to start (trigger) the saw-tooth oscillations.
