The cathode ray oscilloscope

This versatile instrument was developed by Brown in 1897 from the cathode ray tube. It has many uses, including voltage measurement, observation of wave forms, frequency comparison and time measurement. Figure 1 is a simplified diagram of a cathode ray oscilloscope.

At the centre of the instrument is a highly evacuated cathode ray tube with the following features:
(a) a heated cathode C to produce a beam of electrons - a typical beam current is of the order of 0.1 mA;
(b) a grid G to control the brightness of the beam;
(c) an accelerating anode A₂ - a typical potential difference between A₂ and the cathode would be about +1000 V;
(d) a pair of plates Y₁ and Y₂ to deflect the beam in the vertical direction;
(e) a pair of plates X₁ and X₂ to deflect the beam in the horizontal direction;
(f) a fluorescent screen F on which the beam of electrons falls - in many modern oscilloscopes this is coated with zinc sulphide, which emits a blue glow when electrons collide with it, while there are other coatings that glow for some seconds after the beam has passed so enabling transient events to be seen more clearly;
(g) a graphite coating to shield the beam from external electric fields and to provide a return path for the electrons (see below);
(h) a mumetal screen which surrounds the tube and shields it from stray magnetic fields.

The focusing and accelerating systems are connected at different points along a resistor chain. Focusing is achieved by varying the voltage applied between the two anodes A₁ and A₂.

Since secondary electrons are emitted from the screen when the electron beam hits it, the phosphor coating of the screen and the inner graphite layer of the tube are both earthed to prevent a large build-up of static charge on the tube.

In the double beam oscilloscope there are two Y plates with an earthed plate between them to split the beam into two. Two traces are then observed on the screen. This can be most useful when comparing phase differences or making lapsed time measurements.

The deflection system

The beam may be moved 'manually' in the X- and Y-directions by applying a d.c. or a.c. voltage to the X- and Y-plates. Alternatively it can be moved using the time base system.

The time base circuit applies a saw-tooth waveform to the X-plates, as shown in Figure 2. The beam is moved from the left-hand side of the screen to the right during the time that the voltage rises to a maximum, and then is returned rapidly to the left as the voltage returns to zero. This fly-back time should be as short as possible.

The rise time is usually between 1 ms and 1 s for most oscilloscopes used in schools, but time base speeds of many seconds or of fractions of a microsecond can be obtained on more elaborate instruments.

If a voltage is now applied to the Y-inputs, the variation of this voltage with time may be displayed on the screen. Some such variations are shown in Figure 3, together with the effect of various alterations of time base speed or input frequency.

Cathode ray oscilloscope traces

Figure 3 shows the appearance of the oscilloscope screen when a variety of different signals are applied to the Y-plates.

The following diagram shows you various patterns that can be made on the screen with two different inputs


The speed of the time base will change what we see on the screen even if the input signal is kept the same. The following four diagrams show this.

Because the deflection of the spot depends on the voltage connected to the Y plates the CR0 can be used as an accurate voltmeter. The oscilloscope is also used in hospitals to look at heartbeat or brain waves, as computer monitors, radar screens and is also the basis of the television receiver.

The cathode ray tube screen showing various inputs

(a) time base off (the small circles have been added to help you see the spot)



(b) time base on

Measurements with the cathode ray oscilloscope

The primary uses of the cathode ray oscilloscope (CR0) are to measure voltage, to measure frequency and to measure phase.

(i) Measuring voltage
Because of its effectively infinite resistance, the CR0 makes an excellent voltmeter. It has a relatively low sensitivity, but this can be improved by the use of an internal voltage amplifier.

The oscilloscope must first be calibrated by connecting a d.c. source of known e.m.f. to the Y-plates and measuring the deflection of the spot on the screen. This should be repeated for a range of values, so that the linearity of the deflection may be checked. The known e.m.f. is then connected and its value found from the deflection produced.
Most oscilloscopes have a previously calibrated screen giving the deflection sensitivity in volts per cm or volts per scale division. In this case a calibration by a d.c. source may be considered unnecessary.

(ii) Measuring frequency
Using the calibrated time base the input signal of unknown frequency may be 'frozen', and its frequency found directly by comparison with the scale divisions.
Alternatively the internal time base may be switched off and a signal of known frequency applied to the X-input. If the signal of unknown frequency is applied to the Y-input, Lissajous figures are formed on the screen. Analysis of the peaks on the two axes enables the unknown frequency to be found.

(iii) Measuring phase
The internal time base is switched off as above and two signals are applied as before. The frequency of the known signal is adjusted until it is the same as that of the unknown signal. An ellipse will then be formed on the screen and the angle of the ellipse will denote the phase difference between the two signals

We can see this in Figure 4.
Let f be the phase difference between the two signals and let the signal applied to the x plates be x = x_osin(wt) and that applied to the y plates be y = y_o sin (wt+f)).

But when x = 0, sin(wt) = 0, giving wt = 0.
At this point y = y₁ = y_osin f, and hence f may be found.
Examples of the traces for two particular phase differences are shown in Figure 5.