Emi unit ii ppt

By
Dr. G.N. Kodanda Ramaiah /
P. Siva Nagendra Reddy

INTRODUCTION:
 The cathode-ray oscilloscope (CRO) is a multipurpose display instrument used for the
observation, measurement , and analysis of waveforms by plotting amplitude along y-axis
and time along x-axis.
 CRO is generally an x-y plotter; on a single screen it can display different signals applied
to different channels. It can measure amplitude, frequencies and phase shift of various
signals. Many physical quantities like temperature, pressure and strain can be converted
into electrical signals by the use of transducers, and the signals can be displayed on the
CRO.
 A moving luminous spot over the screen displays the signal. CROs are used to study
waveforms, and other time-varying phenomena from very low to very high frequencies.
 The central unit of the oscilloscope is the cathode-ray tube (CRT), and the remaining part
of the CRO consists of the circuitry required to operate the cathode-ray tube.

The CRO consists of the following:
(i) CRT
(ii) Vertical amplifier
(iii) Delay line
(iv) Horizontal amplifier
(v) Time-base generator
(vi) Triggering circuit
(vii) Power supply

1.CRT:
It is the heart of the oscilloscope. When the electrons emitted by the electron gun
strikes the phosphor screen of the CRT, a visual signal is displayed on the CRT.
2. Vertical Amplifier
The input signals are amplified by the vertical amplifier. Usually, the vertical
amplifier is a wide band amplifier which passes the entire band of frequencies.
3. Delay Line
As the name suggests that, this circuit is used to, delay the signal for a period of time
in the vertical section of CRT. The input signal is not applied directly to the
vertical plates because the part of the signal gets lost, when the delay Time not
used. Therefore, the input signal is delayed by a period of time.
4. Time Base Circuit
Time base circuit uses a uni junction transistor, which is used to produce the sweep.
The saw tooth voltage produced by the time base circuit is required to deflect the
beam in the horizontal section. The spot is deflected by the saw tooth voltage at a
constant time dependent rate.
5. Horizontal Amplifier
The saw tooth voltage produce by the time base circuit is amplified by the horizontal
amplifier before it is applied to horizontal deflection plates

6. Trigger Circuit
The signals which are used to activate the trigger circuit are
converted to trigger pulses for the precision sweep
operation whose amplitude is uniform. Hence input signal
and the sweep frequency can be synchronized.
7. Power supply:
The voltages require by CRT, horizontal amplifier and vertical
amplifier are provided by the power supply block. Power
supply block of oscilloscope is classified in to two types
(1) Negative high voltage supply
(2) Positive low voltage supply
The voltages of negative high voltage supply is from -1000V
to -1500V.The range of positive voltage supply is from 300V
to 400V

The main parts of Cathode
Ray Tube are:
•Electron Gun Assembly
•Focusing Anodes
•Deflection Plates Assembly
•Screen for CRT

Basic Features of a CRT
1. Size: Size refers to the screen diameter. CRTs for oscilloscopes are available in sizes of
1, 2,
3, 5, and 7 inches. 3 inches is most common for portable instruments
For example a CRT having a number 5GPI . The first number 5 indicates that it is a 5 inch
tube.
Both round and rectangular CRTs are found in scopes today. The vertical viewing size is
8 cm and horizontal is l0 cm.
2. Phosphor: The screen is coated with a fluorescent material called phosphor. This
material determines the color and persistence of the trace, both of which are indicated
by the phosphor.
The trace colors in electrostatic CRTs for oscilloscopes ale blue, green and But green.
White is used in TVs. and blue-white, orange, and yellow are used for radar
Persistence is expressed as short, medium and long. This refers to the length of time
the trace remains on the screen after the signal has ended.

The phosphor of the oscilloscope is designated as follows. Pl --Green medium
P2--Blue green medium
P5--Blue very short
P11--Blue short
These designations are combined in the tube type number. Hence 5GPl is a 5
inch tube with a medium persistence green trace.
Medium persistence traces are mostly used for general purpose applications
Long persistence traces are used for transients, since they keep the fast
transient on the screen for observation after the transient has disappeared.
Short persistence is needed for extremely high speed phenomena, to prevent
smearing and interference caused when one image persists and overlaps
with the next one.
P11 phosphor is considered the best for photographing from the CRT screen.

3. Operating Voltages: the CRT requires a heater voltage of 6'3 volts ac or dc at
600mA.
Several dc voltages are listed below. The voltages vary with the type of tube used.
(i) Negative grid (control) voltage 14 V to - 200 V.
(ii) Positive anode no. 1 (focusing anode) -100 V to - ll00 V
(iii) Positive anode no. 2 (accelerating anode) 600 V to 6000 V
(iv) Positive anode no. 3 (accelerating anode) 200 v to 20000 V in some cases
4. Deflection Voltages: Either ac or dc voltages will deflect the beam.
 The distance through which the spot moves on the screen is proportional to
the dc, or peak ac amplitude.
 The deflection sensitivity of the tube is usually stated as the dc voltage (or peak
ac voltage) required for each cm of deflection of the spot on the screen
5. Viewing Screen:
 The viewing screen is the glass face plate, the inside wall of which is coated with
phosphor.
 The viewing screen is a rectangular screen having graticules marked on it.
 The standard size used nowadays is 8 cm x l0 cm (8 cm on the vertical and 10 cm
on horizontal).
 Each centimeter on the graticule corresponds to one division (div). The standard
phosphor color use d nowadays is blue

Power supply
It provides the voltages required by the cathode ray tube
to generate and accelerate the electron beam.
 Cathode ray tube (CRT) requires high voltage for pre-
accelerating and accelerating anode, low voltage required
for heater, control grid, focusing anode and the other
circuits of CRO.
Vertical Amplifier:
The signal under the analysis is to be applied to
vertical deflection plates through the vertical
amplifier.

 This FET input stage is followed by a BJT emitter follower,
to match the medium impedance of
 FET output with the low impedance input of the phase
inverter.
 This phase inverter provides two anti phase output signals
which are required operate the push pull output amplifier.
 The push-pull output stage delivers equal signal
voltages of opposite polarity to the vertical plates of the
CRT.
 The advantages of push-pull operation in CRO are similar
to those obtained from push-pull operation in other
applications; better voltage cancellation ran the source or
power supply (i.e. dc), even harmonic suppression,
especially large 2nd harmonic is cancelled out, and greater
power output per tube as a suit of even harmonic
cancellation.
 In addition, a number of defocusing and non linear effects
are reduced, because neither plate is at ground potential.

The vertical amplifier consists of several stages, with
fixed overall sensitivity gain expressed in V/div.
The advantage of fixed gain is that the amplifier can be
more easily designed to meet the requirements of
stability and B.W.
The vertical amplifier is kept within its signal
handling capability by proper selection the input
attenuator switch.
The first element of the pre-amplifier is the input
stage, often consisting of a FET source follower whose
high input impedance isolates the amplifier from the
attenuator.

Delay line
 If both vertical and horizontal signals arrives at the
same time to the corresponding deflection plates,
then only we will get exact waveform.
 But vertical signal arrives much early compared to
the horizontal signal.
 For this reason, the vertical signal at the output of
the vertical amplifier should be delayed with the
help of delay line.
 The delay time is almost equal to 200nsec.

Types of delay lines:
 Lumped Parameter delay Line
 Distributed Parameter delay line

Trigger circuit
 This is triggered by the portion of the vertical amplifier
output.
 This circuit initiates then time base generator. It is the
link between the vertical input and horizontal time
base.
 Trigger circuit is used to synchronize horizontal
deflection with vertical deflection.

TIME-BASE GENERATORS:
The CRO is used to display a waveform that varies as a function of time. If the
wave form is to be accurately reproduced, the beam should have a constant horizontal
velocity.
As the beam velocity is a function of the deflecting voltage, the deflecting voltage
must increase linearly with time.
A voltage with such characteristics is called a ramp voltage. If the voltage
decreases rapidly to zero—with the waveform repeatedly produced, as shown in Fig. 14-
6—we observe a pattern which is generally called a saw-tooth waveform.
The time taken to return to its initial value is known as flyback or return time.
Time base generator is used to generate saw tooth voltage, required to deflect the
beam in horizontal section.
In saw tooth wave form, the deflecting voltage increases slowly and linearly with
respect to time and reduces to zero quickly (fast) i.e. raise time is high and fall time
is less.

Time-base generator using UJT:
The continuous sweep CRO uses the UJT as a time-base generator. When power is
first applied to the UJT, it is in the OFF state and CT changes exponentially through RT
.
The UJT emitter voltage VE rises towards VBB and VE reaches the plate voltage
VP.
The emitter-to-base diode becomes forward biased and the UJT triggers ON.
This provides a low resistance discharge path and the capacitor discharges rapidly.
When the emitter voltage VE reaches the minimum value rapidly, the UJT goes
OFF. The capacitor recharges and the cycles repeat.
To improve the sweep linearity, two separate voltage
supplies are used; a low voltage supply for the UJT and
a high voltage supply for the RTCT circuit. This circuit
is as shown in Fig. 14-7(c).
RT is used for continuous control of frequency within a
range and CT is varied or changed in steps. They are
sometimes known as
timing resistor and timing capacitor.

Horizontal Amplifier
 The saw tooth voltage produced by the time base
generator may not be of sufficient strength.
 Hence before giving it to the horizontal deflection
plates, it is amplified using the horizontal amplifier.

CATHODE-RAY TUBE:
 The electron gun or electron emitter, the deflecting
system and the fluorescent screen are the three major components of
a general purpose CRT. A detailed diagram of the cathode-ray oscilloscope is
given in Fig. 14-2.

 The diagram shows that when the delay line is not
used, the initial part of the signal is lost and only part
of the signal is displayed. To counteract this
disadvantage the signal is not applied directly to the
vertical plates but is passed through a delay line
circuit, as shown in Fig.7.13. This gives time for the
sweep to start at the horizontal plates before the signal
has reached the vertical plates. The trigger pulse is
picked off at a time t0 after the signal has passed
through the main amplifier. The sweep generator
delivers the sweep to the horizontal amplifier and the
sweep starts at the HDP at time t0 + 80 ns. Hence the
sweep starts well in time, since the signal arrives
 at the VDP at time t0 + 200 ns

Electron Gun:
 In the electron gun of the CRT, electrons are emitted,
converted into a sharp beam and focused upon the fluorescent
screen.
 The electron beam consists of an indirectly heated cathode, a
control grid, an accelerating electrode and a focusing anode.
 The electrodes are connected to the base pins. The cathode
emitting the electrons is surrounded by a control grid with a
fine hole at its centre.
 The accelerated electron beam passes through the fine hole.
 The negative voltage at the control grid controls the flow of
electrons in the electron beam, and consequently, the brightness
of the spot on the CRO screen is controlled.

HORIZONTAL DEFLETING SYSTEM
 The horizontal deflecting system consist of a time base Generator and an output
amplifier.
 The UJT is used to produce the sweep. When the power is first applied, the UJT is
off and the CT charges exponentially through RT.
 The UJT emitter voltage VE rises towards VBB and when VE reaches the peak
voltage VP, as shown in Fig. 4.3, the emitter to base '1' (B1) diode becomes
forward biased and the UJT triggers ON.
 This provides a low resistance discharge path and the capacitor discharges rapidly.
The emitter voltage VE reaches the minimum value rapidly and the UJT goes OFF.
The capacitor recharges and the cycle repeats.
 To improve sweep linearity, two separate voltage supplies are used, a low voltage
supply for UJT and a high voltage supply for the RT CT circuit.
 RT is used for continuous control of frequency within a range and CT is varied or
changed in steps for range changing. They are sometimes called as timing resistor
and timing capacitor respectively.
 The sync pulse enables the sweep frequency to be exactly equal to the input signal
frequency, so that the signal is locked on the screen and does not drift.

Deflection Systems:
 Electrostatic deflection of an electron beam is
used in a general purpose oscilloscope. The deflecting
system consists of a pair of horizontal and vertical
deflecting plates.
 Let us consider two parallel vertical deflecting
plates P1 and P2. The beam is focused at point O on the
screen in the absence of a deflecting plate voltage.
 If a positive voltage is applied to plate P1 with
respect to plate P2, the negatively charged electrons are
attracted towards the positive plate P1, and these electrons
will come to focus at point Y1 on the fluorescent screen.

Deflection Systems:
The deflection is proportional to the deflecting voltage between the plates. If
the polarity of the deflecting voltage is reversed, the spot appears at the point Y2, as
shown in Fig. 14-3(a).

Deflection Systems:
 To deflect the beam horizontally, an alternating voltage is applied to the horizontal
deflecting plates and the spot on the screen horizontally, as shown in Fig. 14-3(b).
 The electrons will focus at point X2. By changing the polarity of voltage, the beam will focus
at point X1. Thus, the horizontal movement is controlled along X1OX2 line.

Display waveform on the screen:
Figure 14-5(a) shows a sine wave applied to vertical deflecting plates and a
repetitive ramp or saw-tooth applied to the horizontal plates.
 The ramp waveform at the horizontal plates causes the electron beam to be
deflected horizontally across the screen.
 If the waveforms are perfectly synchronized then the exact sine wave applied
to the vertical display appears on the CRO display screen.

Fluorescent Screen:
 Phosphor is used as screen material on the inner
surface of a CRT. Phosphor absorbs the energy of the
incident electrons. The spot of light is produced on the
screen where the electron beam hits.
 The bombarding electrons striking the screen,
release secondary emission electrons. These electrons are
collected or trapped by an aqueous solution of graphite
called “Aquadag” which is connected to the second anode.
 Collection of the secondary electrons is
necessary to keep the screen in a state of electrical
equilibrium.
 The type of phosphor used, determines the
color of the light spot. The brightest available phosphor
isotope, P31, produces yellow–green light with relative
luminance of 99.99%.

TIME-BASE GENERATORS:
 The CRO is used to display a waveform that varies as a function of time. If the
wave form is to be accurately reproduced, the beam should have a constant horizontal
velocity.
 As the beam velocity is a function of the deflecting voltage, the deflecting
voltage must increase linearly with time.
 A voltage with such characteristics is called a ramp voltage. If the voltage
decreases rapidly to zero—with the waveform repeatedly produced, as shown in Fig. 14-
6—we observe a pattern which is generally called a saw-tooth waveform.
 The time taken to return to its initial value is known as flyback or return time.
Time base generator is used to generate saw tooth voltage, required to deflect the
beam in horizontal section.
In saw tooth wave form, the deflecting voltage increases slowly and linearly with
respect to time and reduces to zero quickly (fast) i.e. raise time is high and fall time
is less.

Probe Specifications:
 Attenuation
 Bandwidth
 Input Capacitance
 Input Resistance
 Accuracy
 Rise Time
 Distortion/Aberration
 CMRR
 Time Delay
 Max. Voltage Rating

Probe Compensation
 The compensation capacitance of the 10 probe must be adjusted to suit the input
capacitance of the oscilloscope. This requires adjusting the compensation each time
the probe is moved to a different scope.
 To compensate the probe, it is driven with a square wave4. The resultant scope
display will appear as one of the three waveforms in figure .
 The upper display is under-compensated; the compensation capacitance is too small.
In the lower display, it is overcompensated, the compensation capacitance is too
large. The centre display is the Goldilocks version: just right.
 These square waves indicate the response of the probe over a range of frequencies.
The upper waveform, with its undershoot, corresponds to a depressed high-
frequency response.
 The lower waveform corresponds to an emphasized high-frequency response. The
centre waveform corresponds to a flat frequency response.
 The compensation capacitance may typically be found in one of three locations: As
a screwdriver adjustment in the body of the probe (visible as the small dot on the
probe body in figure 1). As a screwdriver adjustment in the base of the probe cable,
where the cable plugs into the scope.
 As a coaxial capacitor adjustment in the probe. In this case, the probe body
includes a locking ring, which is rotated to unlock the compensation adjustment.
Part of the probe is then rotated to adjust the probe compensation. When the
compensation is correct, the locking ring is re-tightened.

Passive Probes
 Passive voltage probes consist of passive components:
wires, connectors, resistors, and capacitors.
 There are no active components – transistors or amplifiers – in
the probe, and thus no need to supply power to the probe.
 The advantages of passive probes are:
 Relatively Inexpensive
 Mechanically Rugged
 Wide Dynamic Range
 High Input Resistance
 The primary disadvantage of passive probes is:
 High Input Capacitance

 Active voltage probes include active components such as
transistors and amplifiers and require power to operate.
 Compared to passive probes, the advantages of active
probes are:
 Low Input Capacitance
 Wide Bandwidth
 High Input Resistance
 Better Signal Fidelity
 Compared to passive probes, the primary disadvantages of
active probes are:
 Higher Cost
 Limited Dynamic Range
 Mechanically Less Rugged

MEASUREMENTS USING THE CATHODE-RAY OSCILLOSCOPE:
1) Measurement of Frequency:

MEASUREMENTS USING THE CATHODE-RAY OSCILLOSCOPE:
 2) Measurement of Phase:
 3 Measurement of Phase Using Lissajous Figures:

Measurement of Phase Using Lissajous Figures:

The continuous sweep is of limited use in displaying
periodic signals of constant frequency and amplitude.
When attempting 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.
A triggered sweep can display such signals, and those
of short duration, e.g. narrow pulses.
In triggered mode, the input signal is used to generate
substantial pulses that trigger the sweep.
Thus ensuring that the sweep is always in step with the
signal that drives it.

 resistance R3 and R 4 form a voltage divider such that the voltage Vo at
the cathode of the diode is below the peak voltage i/o for UJT conduction.
When the circuit is switched on, the UJT is in
the non-conducting stage, and C, charges exponentially through RT.
towards VBB until the
diode becomes forward biased and conducts; the capacitor voltage never
reaches the peak
voltage required for UJT conduction but is clamped at Vo. If now a –ve
pulse of sufficient
amplitude is applied to the base and the peak voltage Vp is momentarily
lowered, the UJT fires.
As a result, capacitor C. discharges rapidly through the UJT until the
maintaining voltage of the
UJT is reached; at this point the UJT switches off and capacitor CT
charges towards VBB, until
it is clamped again at VD fig 2.2 shows the output waveform

There are two separate vertical input channels A and B.
These use two separate attenuators and preamplifiers
stages.
Therefore the amplitude of each input as viewed on
the oscilloscope can be individually controlled.
After pre-amplification the two channels meet at an
electronic switch.
This has the ability to pass one channel at time into
the vertical amplifier, via the delay line.
The dual trace oscilloscope is basically operated in
alternate and chop modes.
The operating modes are selected form the instrument
front panel.

In alternate mode the electronic switch alternates
between channels A and B, letting each through one
cycles of the horizontal sweep display is balanced
during the fly back and hold off periods.
Provided the sweep speed is much greater than the
delay time of the CRT phosphor, the screen will show a
stable display of both the waveforms at channel A and
B.
In chopped mode the electronic switch free runs at a high
frequency of the order of 100 KHz to 500 KHz.
The result is that small segments from channel A and B
are connected alternately to the vertical amplifier and
displayed on the screen.

Provided the chopping rate is much faster than the
horizontal sweep rate, the display will show a continuous
line for each channel.
The time base circuit of dual trace CRO to similar to that
of single trace CRO.
Switch S2 allows the circuit to be triggered on either the A
or B channel waveforms or an external signal.
The horizontal amplifier can be fed from the sweep
generator or the external horizontal input through channel
B via switch S1.
This is the X-Y mode with channel A as the vertical signal
and channel B as the horizontal signal. Several other
operating modes of dual trace CRO are channel A only
channel B only, channel A and B as two traces and signals
A + B, A – B, B – A or – (A + B) as a single trace.

 The dual-beam oscilloscope emits two electron beams that are
displayed simultaneously on a single scope, which could be
individually or jointly controlled.
 The construction and working of the dual beam oscilloscope
are completely different from dual trace oscilloscope.
 The tubes are more complicated to build, and the whole thing is
more expensive.
 A special type of double beam oscilloscope can display two
electrons beam by generating or deflecting beams.
 Now a days, double beam oscilloscope is outdated, as this
function could be performed by the digital scope with greater
efficiency and they do not require a dual-beam display.
 The digital scope captures a single beam of electron and
simultaneously and splits into many channels

 There is two individual vertical input channel for two
electron beams coming from different sources. Each
channel has its own attenuator and pre-amplifier.
Therefore, the amplitude of each channel can be
controlled eventually.
 The two channels may have common or independent time
base circuits which allow different sweep rates. Each
beam passes through different channels for separate
vertical deflection before it crosses a single set of
horizontal plate.
 The horizontal amplifier is compiled by sweep generator
to drive the plate which gives common horizontal
deflection. The horizontal plates allow both the electron
beams across the screen at the same time.

 Dual beam oscilloscope can generate the two electron beams
within the cathode ray tube either by using double electron gun
tube or by splitting beam. In this method, the brightness and
focus each beam are controlled separately. But two tubes
increases the size and weight of the oscilloscope and it looks
bulky.
 The other method is split beam tube, a single electron gun is
used in this method. There is a horizontal splitter plate between
the Y deflection plate and last anode. The potential of the plate
is same as that of the last anode and it goes along the length of
the tube between the two vertical deflection plates. Therefore,
it isolates the two channels. As the single beam is split into
two, its brightness of the resultant beam is half of the original.
At high-frequency operation, it works as a disadvantage. The
alternative way to improve the brightness of resultant beam is
to have two sources in the last anode instead of one so that
beams emerge from it.

 As it name suggests, it collects samples from several successive waveform and
construct a complete picture of the waveform from the assembled data.
 The resultant waveform is amplified with low band pass filter and then shown on the
screen.
 This waveform is made with the joining of many dots associated with each other to
construct the whole shape.
 Each dot of the wave is the vertical deflection of the point of the progressive layer in
each successive cycle of a staircase waveform. They are used to monitor high-
frequency signals up to 50 GHz or more.
 The frequency of the displayed waveform is higher than the sample rate of the scope.
It is about 10 pieces per division or more along with large bandwidth of amplifier
about 15 GHz.
 At sampling stage, signals have low-frequency and to achieve large band-width it
combines with an attenuator. Though, it reduces the dynamic range of the
instrument.
 The sampling oscilloscope is limited to repetitive signals and not responsive to
transient events. They only display high frequency within the range limit.

Sampling Method
 Before each sampling cycle, the trigger pulse activates an
oscillator and liner voltage is generated.
 When the amplitude of two voltages is equal, the
staircase move one step and a sampling pulse is generated
and it opens the sampling gate for a sample of the input
voltage.
 The resolution of the waveform depends upon the
dimension of the steps of the staircase generator.
 There are different ways of sample taking but two are
commonly used. One is real-time sample and other is
equivalent sample method.

Real-Time Sample Method
 In real-time method digitizer works at high-speed so it
can register maximum points in one sweep.
 It’s main purpose to capture high-frequency transient
events with accuracy.
 The transient waveform is so unique that its voltage or
current level at any instant of time cannot be associated
with its nearest ones.
 These events do not repeat themselves, so it must be
registered in the same time frame as they occur.
 The frequency of samples is very high about 500 MHz
and sample rate is about 100 samples per second. To store
such a high-frequency waveform, a high-speed memory
is required.

Equivalent Sample Method
 Sampling in equivalent method works upon the
principle of prophecy and estimation which is
possible only with the repetitive waveform.
 In equivalent method digitizer get samples from many
repetitions of signals.
 It may take one or more samples from each repetition.
By doing so, the accuracy in capturing signal gets
increases.
 The frequency of the resultant waveform is much
higher than the scope sample rate.
 This type of sampling can be done by two methods;
Random method and sequential method.

Random Method of Sampling
 Random method of sampling is the most common
method of sampling.
 It uses an internal clock which adjusted in such a way
that it runs with respect to input signals and the signal
trigger samples are taken continuously, no matter
where it was triggered.
 Samples those are collected are regular with respect to
time but random with respect to trigger.

Sequential Method of Sampling
 In this technique, samples are taken with respect to
triggered and it is independent of time setting.
 Whenever the trigger is detected, the sample is recorded
with a small delay.
 Make sure that the delay should be very short but well
defined. When next trigger occurred, it gets registered with
a little incremental time delay with respect to previous one.
 The delayed sweep can have the range from few
microsecond too few seconds.
 Let us suppose delay for the first time is ‘t” then the delay
for the second time will be little more than ‘t’ and in this
manner samples are being taken many times with added
delay until the time window is filled.

Advantages
 Very High Frequency performance can be achieved.
 High speed electrical signals can be analysed.
 A clear display is produced.
 Allows design of oscilloscope with wide bandwidth, high
sensitivity even for low duty cycle pulses.
LIMITATIONS:
 It can’t be used to display the transient waveforms.

 A digital oscilloscope is an instrument which stores a digital copy of the waveform
in the digital memory which it analyses further using digital signal processing
techniques rather than using analogue techniques. It captures the non-repetitive
signals and displays it consciously until the device gets reset. In digital storage
oscilloscope, signals are received, stored and then displayed. The maximum
frequency measured by digital oscilloscope depends upon two things: one is
sampling rate of the scope, and the other is the nature of the converter. Converter is
either analogue or digital. The traces in digital oscilloscope are bright, highly
defined, and displayed within seconds as they are non-stored traces. The main
advantage of the digital oscilloscope is that it can display visual as well as numerical
values by analyzing the stored traces. The displayed trace on the flat panel could be
magnified and also we can change the brightness of the traces, and minute detailing
can be done as per requirement after an acquisition.

Digital Oscilloscope Technology
 First the waveforms are conditioned by some analogue
circuits then enter in the second stage which involves
receiving the digital signals.
 To do so, samples have to pass through analogue - to –
digital converter and output signals get recorded in digital
memory at different interval of time.
 These recorded points together make a waveform. The set
of points in a waveform show its length.
 The rate of samples defines the design of the oscilloscope.
 The recorded traces are then processed by the processing
circuit and obtained traces are ready to display for visual
assessment.

Modes of operation:
 The digital storage oscilloscope has three modes of
operation:
 1. Roll mode
 ii) Store mode
 iii) Hold or save mode

Roll Mode:
 It is the most basic mode of operation which is similar to
that of a general purpose CRO.
 When an input is applied, its trace is displayed on the
screen.
 A user can use this mode to keep an eye on the waveform
and its various characteristics.
Store mode:
 The most commonly used and called refresh mode.
 It is used when the sample rate of a waveform becomes
too high, and when the waveform of interest is repetitive
or nearly so.

Hold or Save mode:
 This is called automatic Refresh Mode.

Uses of Digital Storage Oscilloscope
 Used for testing signal voltage in circuit debugging.
 Testing in manufacturing.
 Designing.
 Testing of signals voltage in radio broadcasting
equipment.
 In the field of research.
 Audio and video recording equipment.

Applications of DSO
 Take cursor and pulse width readings
 Measuring rise time and propagation delay
 Implementing math functions like subtraction and addition
 Acting as a simple signal tracer, a DSO enables technicians to
probe electronic device’s individual connections and
components, to determine the malfunctioning part.
 In measuring the functions of the individual component of the
device, the DSO locates where an expected signal is incorrect or
absent.
 The DSO can also measure components’ minor variations in
operations and alert the technician of the need for fine-tuning or
replacement.
 To prevent erroneous replacement of parts, the DSO also helps
technicians identify the parts that are still working.

LISSAJOUS PATTERNS
An oscilloscope can be used to observe and measure ac
waveforms. It may also be used to observe a voltage-
versus-voltage pattern by eliminating the time parameter.
The resulting pattern is referred to as a Lissajous pattern.

 Lissajous patterns for sinusoids of the same frequency,
but varying phase relationships.
 The Lissajous patterns shown in Fig. 7.2 are from
sinusoids of varying frequency and phase relationships

 To determine the frequency ratio, draw horizontal and
vertical lines through the center of the pattern as shown in
Fig. 7.3.
 The ratio of the number of horizontal axis crossings to
the number of vertical axis crossings determines the
frequency ratio. This ratio is given as
 where f x and f y are the frequencies of the two
waveforms.

DEPARTMENT OF ECE,
KUPPAM ENGINEERING COLLEGE,KUPPAM.

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