CATHODE RAY OSCILLOSCOPE
Introduction:
• The device which allows the amplitude of displayed primarily as a function of time is called
Cathode Ray Oscilloscope.
• CRO gives visual representation of time varying signals.
• The oscilloscope has become a universal instrument and is probably most versatile tool for
the development of electronic circuits and systems.
• The oscilloscope is the basic instrument for the study of all types of waveforms. It can be
employed to measure such quantities as peak voltage, frequency, phase difference, pulse
width, delay time, rise time and fall time.
BLOCK DIAGRAM OF CRO:
CATHODE RAY TUBE:
This is the heart of CRO.
• CRT consists of electron gun, which emits electrons to strike the phosphor screen through
deflecting plates. The deflecting plates are two types, horizontal deflecting plates and vertical
deflecting plates.
• The vertical deflecting plates move the beam up and down vertically.
• The horizontal deflecting plate’s moves the beam from left to right or right to left
horizontally.
TIME BASE GENERATOR:
• The time base generator is used to generate the saw tooth voltage required to deflect the
beam in the horizontal section.
HORIZONTAL AMPLIFIER:
• This is used to amplify the saw tooth voltage before it is applied to horizontal deflection
plates.
POWER SUPPLY:
There are two sections of a power supply block.
• High voltage supply(HVS): it is applied to CRT of the order of 1000V to 1500V.
• Low voltage supply (LVS): it is required for heater of electron gun and remaining circuits
except CRT only.
VERTICAL AMPLIFIER:
• This is a wide band amplifier used to amplify signals in the vertical section.
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DELAY LINE:
• It used to delay the signal time in the vertical sections.
TRIGGER CIRCUIT:
• The trigger circuit controls the instants at which the sweep voltage is to be applied to the
horizontal deflecting plates.
• This is used to convert the incoming signal into trigger pulses so that the input signal and the
sweep frequency can be synchronized.
FLUORESCENT SCREEN:
• The screen is inside face of CRT and coated with a fluorescent material called phosphor.
• Different types of phosphor of oscilloscope are designated as follows.
P1- green medium
P2-Blue-green medium
P5- blue very short
P11-blue short
GLASS TUBE (or) ENVELOPE:
• All the components of CRT are enclosed in an evacuated glass tube called envelope.
• This allows the emitted electrons to move about freely from one end of the tube to the other
hand.
BASE:
• The base is provided to CRT through which connections are made to the various parts.
CATHODE RAY TUBE:
The electron gun assembly produces a sharp beam of electrons which are accelerated to high
velocity. This focus beam of electrons strikes the flow of current screen.
The CRO consists of a CRT which is the heart of the CRO and some additional circuit to operate
the CRT.
The main parts of CRT are
1) Electron gun assembly
2) Deflection plate assembly
3) Fluorescent screen
4) Glass envelope
1) Electron gun assembly:
• The source of focused and accelerated electron beam is the electron gun which emits electron
and forms them into a beam consists of a heater, a cathode, a grid, pre accelerating anode.
• The electron gun section of the cathode ray tube provides a sharply focused electron beam
directed towards the fluorescent-coated screen.
• This section starts from thermally heated cathode, emitting the electrons.
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• The control grid is given negative potential with respect to cathode.
• This grid controls the number of electrons in the beam, going to the screen.
2) Deflection plate assembly:
• Electro static deflection is used in CRO for moving the electron beam horizontally and
vertically
• After leaving the electron gun assembly the electron beam passes through the deflection
plate assembly.
• The electron beam will be deflected vertically and horizontally under the influence of the
electric fields provided by the deflection plate assembly.
3) Fluorescent Screen:
• The front of the CRT is called the face plate.
• The surface inside of the face plate is coated with phosphor.
•
Light is emitted when an electron beam strikes phosphor material with high velocity.
4) Glass envelope:
• It is highly evacuated glass housing and maintains vacuum inside and supports the various
electrodes.
• The inner walls of CRT between neck and screen are usually coated with a conducting
material called “AQUADAG”.
NECESSITY OF TIME BASE VOLTAGE:
• If the two deflection voltages were held constant, the electron beam would strike a fixed
point on the phosphor screen and a stationary point would be visible on the screen.
• When an ac voltage is applied to a CRO, without applying the time-base signal , a vertical
line is only displayed indicating peak to peak voltage of input signal.
• In the above case, the input signal time period or frequency details will not be displayed on
the CRO screen.
Vertical line
•
To obtain such a display the signal voltage is applied to the vertical plates directly and it
moves through horizontal plates by a sweep voltage applied to the horizontal plates.
• The horizontal sweep voltage produces the time base by moving the spot horizontally with
the time, while the signal moves the spot vertically in proportional to the voltage at a
particular instant of time.
DEFLECTION SENSITIVITY:
• The deflection sensitivity of a CRT is defined as the deflection of the screen per unit
deflection voltage.
• It is expressed in cm / volt.
Deflection sensitivity S = D/ Ed
= L ld / 2 d Ea m/V or cm/V
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CONDITIONS FOR STATIONARY WAVEFORMS IN CRO:
• In order to obtain the stationary waveform on CRO screen, the horizontal sweep frequency
and the vertical input frequency signals must be applied to the deflecting plates at the same
time.
• Synchronization has to be done to obtain a stationary waveform.
• This requires that the frequency of vertical input signal must be equal to or an exact multiple
of the sweep generator signal frequency, as shown in below.
CONDITIONS FOR FLICKER FREE WAVEFORMS IN CRO:
The waveform observed in a CRO must be free from flicker. In order to get flicker free waveform,
the sweep frequency generated by the horizontal sweep generator must be greater than the
persistence of vision frequency of 16Hz.
Flickering is due to measurement of low frequency signals (below 30Hz).
The conditions for flicker free waveforms are given below.
1. Use low frequency sweep generators
2. Use CRO in auto or external triggering mode.
3. Connect the signal to vertical input and external trigger to external trigger input.
TRIGGERED SWEEP:
• To display voice or music signals, the pattern falls in and out of sync as the frequency and
amplitude of the music varies resulting in an unstable display.
• Sweep generator produces output only at the command of trigger pulses.
• Trigger pulses can be derived from waveform under observation or from an external source
• R3 & R4 form Voltage divider
• When the Power is ON, UJT is in non-conducting state and Ct charges through Rt towards
VBB.
•
•
•
•
•
Capacitor charges until diode becomes forward biased and conducts to clamp the capacitor to
VD
If negative trigger pulse of sufficient amplitude is applied to base-2 of UJT, it fires Ct
discharges rapidly through UJT.
The trigger pulse initiates the retrace before the sweep can be generated
Now the UJT switches off the capacitor again charges toward VBB until it is clamped again at
VD. This process repeats as long as the supply voltage and trigger pulse present.
The sufficient condition for the above operation is VBB > VD > VP where VP is peak
voltage of UJT.
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FUNCTION AND USE OF VARIOUS CONTROLS AND INPUT TERMINALS OF
CRO:
The various input terminals provided on CRO are:
1. Horizontal amplifier input: ( X-input)
This is used for connecting the external horizontal signal as a input for horizontal amplifier.
2. Vertical amplifier input: ( Y-input)
This signal under testing is connected to the vertical amplifier input.
3. Basic controls:
a) Intensity: This control is for correct brightness of the trace on the screen.
b) Focus: This control is for sharp trace on the screen.
c) Astigmatism: This is another focus control. With the help of focus control and
astigmatism control, a very sharp spot can be obtained both in the centre and also at the
edges of the screen.
4. X-shift or horizontal position control: This control is for moving the pattern left to right and
right to left on the screen.
5. Y- Shift or vertical position control: This control is used for moving the pattern up and down
on the screen.
6. Time/Div. control: This control adjusts the horizontal gain that is the width of the pattern.
7. Volts/Div. control: This control adjusts the vertical gain that is the height of the pattern.
8. Variable speed: This control adjusts the internal sweep frequency into sub multiples of
vertical input signal.
9. EXT: In EXT position, the horizontal deflection system is fed from an external input.
10. Coupling (AC-DC-GND): It decides the mode of input coupling i.e.., whether the input to
the vertical amplifier is to be a.c coupled, d.c, coupled or GND.
VOLTAGE MEASUREMENT:
To measure the voltage from the CRT display, one must observe the setting of the vertical
attenuator expressed in V/Div. and the peak to peak deflection of the beam, i.e.., the number of
divisions. The peak to peak value of voltage is then computed as follows
𝑽𝒐𝒍𝒕𝒔
𝑵𝒐.𝒐𝒇 𝒗𝒆𝒓𝒕𝒊𝒄𝒂𝒍 𝒅𝒊𝒗𝒊𝒔𝒊𝒐𝒏𝒔
Vp-p = (
)𝑿
𝒅𝒊𝒗.
𝟏
TIME PERIOD AND FREQUENCY MEASUREMENT:
The period and frequency of periodic signals are easily measure with an oscilloscope. The
waveform must be displayed such that a complete cycle is displayed on the CRT screen. Accuracy
is generally improved if a single cycle displayed fills as much of the horizontal distance across the
screen as possible.
𝒕𝒊𝒎𝒆
𝑵𝒐.𝒐𝒇 𝒅𝒊𝒗.
)𝑿
The period is calculated as:
𝑻=(
𝒅𝒊𝒗.
The frequency is then calculated as F =
𝒄𝒚𝒄𝒍𝒆
1
𝑇
SAMPLING OSCILLOSCOPE:
• Sampling oscilloscope is used to improve the high frequency performances.
• Generally higher input frequencies cause the electron beam to move so fast across the screen
that only a very faint trace is produced.
• Sampling oscilloscope overcomes this difficulty by producing a low frequency DoT
representation of the signal.
• Each DOT represents an amplitude sample of the input signal.
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Block diagram:
OPERATION:
• The input waveform which is to be measured is applied to sampling gate.
• When a trigger input is applied to oscillator, it starts, the ramp generator and linear ramp
voltage is produced. The linear ramp voltage is applied to voltage comparator it compares
the ramp voltage with stair case voltage.
• When two voltages are equal the stair generator advance one step and a sampling pulse is
generated and applied to sampling gate then the sample of input voltage is taken amplified
and applied to vertical deflecting plates for every cycle.
APPLICATIONS OF CRO:
1. Waveform analysis
2. Amplitude (voltage) measurement
3. Time period measurement
4. Frequency measurement
5. Phase angle and time delay measurement
6. Current measurement
Radio applications: Visual display of signal, composite, video signal, to display response of tuned
circuits.
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