Download Cathode ray oscilloscope

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.
cathode (C)
Focussing anode (A1)
Y plates
Electron beam
X plates
vacuum
grid (G)
+5 kV
Accelerating anode (A2)
Fluorescent screen
Figure 1
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.
1
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 lefthand 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.
right
right
right
left
left
left
Figure 2
The rise time is usually between 1 s 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
d.c. input
a.c. input
2
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.
d.c. input with the time base off
a.c. input with a slow time base
d.c. input with the time base on
a.c. input with a fast time base
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.
3
The cathode ray tube screen showing various inputs
(a) time base off (the small circles have been added to help you see the spot)
no input – spot adjusted left
d.c input lower plate positive
d.c input upper plate more positive
a.c input
no input
d.c input – upper plate positive
d.c input – lower plate positive
low frequency a.c input
high frequency a.c. input
a.c input with a diode
no input
d.c input upper plate positive
(b) time base on
4
large amplitude a.c input
small amplitude a.c input
a.c. input with a slow time base
a.c input large Y gain
voice or music
a.c. input with a fast time base
a.c input small Y gain
full wave rectification
5
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.
y
Let  be the phase difference between the two signals
yo
and let the signal applied to the x plates be x = x0sin(t) y1 = yosin
and that applied to the y plates be y = yo sin (t+)).
But when x = 0, sin(t) = 0, giving t = 0.
At this point y = y1 = yo sin , and hence  may be
found.
Examples of the traces for two particular phase
differences are shown in Figure 5.
y
x
Figure 4
y
y1 = yosin
yo
yo
y1 = yosin
x
x
=0
Figure 5
 = 90o = /2 c
6