EE205
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Title: The Oscilloscope
Aim: To Introduce the Oscilloscope as a
measuring instrument to the students and particularly, to;
I.
Find the deflection sensitivity of ab oscilloscope
II.
Measure DC and AC Voltages and
III.
Measure frequency of mains supply and that of an oscillator
Apparatus:
1. Oscilloscope (Type
CCA8040)
2. DC power supply unit
3. 2A Variac
5. A Signal Genarator
(Oscillator)
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THEORY:
The oscilloscope, or scope for short, is a
device for drawing calibrated graphs of voltage vs time very quickly and
conveniently. Such an instrument is obviously useful for the design and repair
of circuits in which voltages and currents are changing with time. There are
also many devices, called transducers, which convert some non-electrical quantity
such as pressure, sound, light intensity, or position to a voltage. By using a
transducer the scope can make a plot of the changes in almost any measurable
quantity. This capability is widely used in science and technology.
The heart of the oscilloscope is a cathode ray
tube or CRT, of the sort you have already studied. Looking at the face of the
instrument, you are viewing the screen that the electron beam strikes.
Electronic circuits in the scope apply voltages to one set of deflection plates
to sweep the beam across the screen from left to right at a constant rate,
thereby providing the time axis. Other circuits amplify or attenuate the input
signal as needed, and apply voltages to the other set of deflection plates to
move the beam vertically, providing the voltage axis.
Fig. 1
Controls are provided to select the time and
voltage scales needed for any given situation. At the end of each sweep, the
beam is shut off and the horizontal deflection voltage is reset so the beam
would start at the left edge of the screen again.
The Oscilloscope Displays Four Types Of
Waveforms But some limited versions display only three like the type you have in your laboratory.
Uses of the
Oscilloscope
An oscilloscope allows signal voltages to be
viewed, usually as a two-dimensional graph of one or more electrical potential
differences (vertical axis) plotted as a function of time or of some other
voltage (horizontal axis). Although an oscilloscope displays voltage on its
vertical axis, any other quantity that can be converted to a voltage can be
displayed as well. In most instances, oscilloscopes show events that repeat
with either no change, or change slowly. The oscilloscope is one of the most
versatile and widely-used electronic instruments.
Oscilloscopes
are commonly used when it is desired to observe the exact wave shape of an
electrical signal. In addition to the amplitude of the signal, an oscilloscope
can show distortion and measure frequency, time between two events (such as
pulse width or pulse rise time), and relative timing of two related signals.
Some modern digital oscilloscopes can analyze and display the spectrum of a
repetitive event. Special-purpose oscilloscopes, called spectrum analyzers,
have sensitive inputs and can display spectra well into the GHz range. A few
oscilloscopes that accept plug-ins can display spectra in the audio
range.
Oscilloscopes
are used in the sciences, medicine, engineering, telecommunications, and
industry. General-purpose instruments are used for maintenance of electronic
equipment and laboratory work. Special-purpose oscilloscopes may be used for
such purposes as analyzing an automotive ignition system, or to display the waveform
of the heartbeat as an electrocardiogram.
Originally all oscilloscopes used cathode ray tubes as their display element
and linear amplifiers for signal processing, but modern oscilloscopes can have
LCD or LED screens, fast analog-to-digital converters and digital signal
processors.
Although not as commonplace, some oscilloscopes
used storage CRTs to display single events for a limited time. Oscilloscope
peripheral modules for general purpose laptop or desktop personal computers use
the computer's display, and can convert them into useful and flexible test
instruments.
For
Procedures see your Electrical Laboratory Manual on page 20
Fill Up
Tables in Your Manual and Continue To….
PRECAUTIONS:
Answers
to Questions:
1.
Sources of error apart from human error include
a. Fluctuating Voltage
suppy from the mains
b. Calibration error
c. Time base center
frequency accuracy,
e. Vertical noise,
f. Bandwidth effects,
g. Interpolation
(aliasing) error.
2. Expressions for
Voltage.
a. When the voltage is DC,
V=SD (where
S=Deflection and D=Deflection sensitivity).See Table 205.1 in your manual.
b. When Voltage is AC,
V=DL/2
(where L=length of trace) See Table 205.2 in your manual.
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