Information about cathode ray oscilloscope

3. Cathode Ray Oscilloscope (CRO)
The cathode ray oscilloscope (CRO) is a type of electrical instrument which is
used for showing the measurement and analysis of waveforms and others
electronic and electrical phenomenon. It is a very fast X-Y plotter shows the input
signal versus another signal or versus time. The CROs are used to analyse the
waveforms, transient, phenomena, and other time-varying quantities from a very
low-frequency range to the radio frequencies.
Construction of Cathode Ray Oscilloscope
The main parts of the cathode ray oscilloscope
are as follows.
1.Cathode Ray Tube
2.Electronic Gun Assembly
3.Deflecting Plate
4.Fluorescent Screen For CRT
5.Glass Envelop

4. THERMIONIC EMISSION
If a tungsten filament is heated to about 2000◦C, some of the
electrons in the white hot metal gain enough energy to escape
from its surface. This effect is called thermionic emission and it
occurs in other metals and metal oxides as well.
The diagram below shows an experiment to demonstrate the
effect.
In the vacuum tube below there are two electrodes, called
cathode (−) and anode (+). The cathode in this case is the
tungsten filament. When the filament is switched on, it heated
up and emits electrons. These electrons accelerates towards
the anode, because it has a high positive potential. The glass
tube is vacuum, so the electrons will not collide with air
molecules and moves as a beam of electron. If the tube is not
vacuum the filament may also burn.

5. THERMIONIC EMISSION

6. Cathode ray oscilloscope (CRO)
A C.R.O consists of three main parts. They are electron gun,
deflection system and fluorescent screen.
When the filament heated up it emits electrons and moves
towards the anode in the electron gun and travels towards the
plates as a form of electron beam.
The Y-plates move the election beam vertically. This happens
when an external source of voltage – for example, an AC supply
– is connected across the Y-input terminals. The amount of
vertical movement can be amplified (magnified) by turning up
the gain control.
The X-plates moves the beam horizontally. Normally, the
movement is produced by a circuit called the time base in side
the oscilloscope. The time base is automatically applies a
changing voltage across the plates so that the spot moves from
left to right across the screen at a steady speed, flicks back to
the start, moves across gain… and so on, over again and again.

7. The screen of C.R.O is made up of fluorescent material, so when
the electrons strikes the screen a bright spot is produced on
the screen.
cathode
fluorescent
screen

11. Measuring voltages and time intervals.
A C.R.O shows a trace of a.c voltage as in the following diagram
the time base setting is 25ms/cm and the Y-gain setting is
5V/cm.
What is the maximum voltage of the supply and the time taken
to one complete oscillation?
Voltage of the supply = 5V
Time taken for one complete oscillation = 100ms

12. Measuring voltages and time intervals.
The diagram shows C.R.O screen connected to voltage supply.
The Y-gain control is set at 4V for each division on the screen.
State the value of the p.d in the voltage supply.
12V

13. Measuring voltages and time intervals.
The Y-plates of a cathode-ray oscilloscope (CRO) are connected to
an alternating voltage of amplitude 4.0 V and frequency 25 Hz.
The Y-gain of the CRO is set at 2.0 V / division and the time-base
is set at 0.01 s / division.
Calculate the time taken for one complete oscillation.
F =
1
𝑇
, T=
1
𝐹
= 1/25 = 0.04s
On the grid below, draw the trace on the screen of the CRO.

18. Logic gates are the basic building blocks of any digital system. It is an electronic circuit
having one or more than one input and only one output. The relationship between the
input and the output is based on a certain logic. Based on this, logic gates are named as
AND gate, OR gate, NOT gate etc.

19. Resistors
Resistors keep current and voltages at the levels needed for other
components to work properly. The resistance in ohms is marked on the
side using either the colour code or resistance code. The values is only
approximate. When the resistors are made the resistance can change
slightly from one to next.
The colour codes
Resistance value = 27000Ω
= 27 k Ω
The fourth ring gives the tolerance.
This tell us by how much resistance
may differ from the marked value.
Gold 5% , silver 10% and
no colour 20%

20. Potential divider (potentiometer)
Often a power supply or a battery provides a fixed p.d. to obtain
a smaller p.d., or a variable p.d., this fixed p.d. must be split
using circuit called a potential divider (potentiometer) as shown
below.
When the sliding contact is at N the p.d across the bulb is zero and not light
up. When the sliding contact moves from N to M the p.d. across the bulb
increases and it becomes brighter. The sliding contact at M gives the full

21. It also possible to make a potential divider connecting two
resistor in series as shown circuit below.
If the two resistors have same value of resistance, the
battery voltage is divided into same rate to each of the
resistor. In the circuit above the battery voltage of 6V is
divided into 3V to each resistor.

22. But when the two resistors have different value of resistance, the voltage
share by the battery to each resistor is different as shown below.
• Total resistance of the circuit = R = R1 + R2
• = 10 + 5 = 15 Ω
• Current through the circuit = V =IR
• I = V/R = 6/15 = 0.4A
• Voltage across 10 Ω resistor = V=IR
• V = 0.4 × 10 = 4V
• Voltage across 5 Ω resistor = V=IR
• V = 0.4 × 5 = 2V
In this case the 10Ω resistor gets 4V of p.d. from the battery and
5Ω resistor gets 2V of p.d from the battery. So the greater the
resistance, the greater share of the battery voltage.

23. The circuit below contains light dependent resistor (L.D.R), a
special type of resistor whose resistance falls when light shines on
it and in dark its resistance increases. The principle is used in
lamps which come on automatically at night
The LDR is a part of a potential divider, in day light the LDR has low
resistance and a low share of battery voltage-too low to switch the
bulb. In dark the resistance of the LDR rises considerably, and so does
it share of the battery voltage. Now, the voltage across the LDR is
high enough to switch bulb.
Light dependent resistor (a light
sensitive switch)

24. The circuit below contains thermistor a special type of resistor
whose resistance falls considerably when its temperature rises.
The principle is used automatic fire alarms.
The thermistor is part of a potential divider. At room temperature, the
thermistor has a high resistance and the major share of battery
voltage. As a result, the voltage across the resistor is not enough to
switch on the buzzer. When the thermistor is heated, its resistance
falls, and the resistor gets a much larger share of the battery voltage.
So the buzzer is on and starts to ring.
Thermistor (a temperature sensitive
switch)

25. The capacitor is designed to store electric charge (and hence
electrical energy). The energy is stored as an electric field
between two plates.
When you close the switch in the circuit below, there is a time
delay before the bulb lights up. This is because when the switch
closed it takes some time to charge the capacitor. The delay can
be increased by increasing the capacitance of the capacitor.
Capacitor (a time-delay swicth)

26. A diode is a component that allows electric current to in one
direction only. Its circuit symbol showing below represents this
by showing an arrow to indicate direction in which current can
flow. The bar shows that current is stopped if it tries to flow in
opposite direction.
Diodes are useful to for converting alternating current (which
varies back and forth) into direct current (which flows in one
direction only). This process is known as rectification and the
diode act as a rectifier. For example the d.c. motor in a drill
cannot work using a.c. current, so the diode is used to convert
a.c. current into d.c. current as shown the circuit below.
Diode (a rectifier)

27. Reed relay
A magnetic relay is a switch operated by an electromagnet. With
a relay a small switch with thin wires can be used to turn on the
current in a much more powerful circuit – for example, one with
large electric motor in it.
When the switch S in the input circuit is closed, a current flows
through electromagnet and it will magnetised. This pulls the iron
armature towards it, which closes the contact C. As a result
current flows through the output circuit and motor turn on.
Relays

28. Reed switch
The diagram below shows reed switch used to switch on warning
lamp.
When the temperature of the air rises the resistance of the
thermistor decreases, as a result current flows through the coil
and the ends of both wires inside the coil magnetised with
opposite pole. So both the wire attract each other and the circuit
of the warning lamp completes and lights up.