physics cro experiment viva questions with answers
#1

how are the signals fed to the y-input reproduced on the screen?
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#2
Nowadays the demand for oscilloscopes are increasing. PC based
oscilloscope is used to view the waveforms of various types of electrical signals.
This circuit conditions different signals of frequency below 1 kHz and displays
their waveforms on the computers screen. The circuit is used to the input
waveform and convert it to the digital format for interfacing to the PC. The
software for acquiring the data into the PC and displaying the same on its screen is
written in Visual basic.
The input waveform is first applied to a rectifier comprising op-amps and a
zero crossing detector. The full wave rectifier rectifies the input signal such that
negative half of the input signal is available in the positive side itself, so both the
care read as positive when it is given as input to the ADC. The zero crossing
detector detects whether the cycle is positive or negative. It is the most critical
part of the circuit and if it works improperly, the symmetry of the analogue signal
displayed in the PC monitor gets affected. At the zero-crossing instant when the
input signal transits to negative side the zero-crossing detector informs the PC by
taking pin 15 of 25 pin D connector of the parallel port high.
The data available in the parallel port is then read by the software. After
performing required calculations, waveform is plotted on the monitor screen by
the software.

INTRODUCTION

Oscilloscopes are tools that allow engineers to view signals graphically. The
oscilloscope, often just called the scope, displays the signals as a plot of magnitude versus
time. There are two types of oscilloscopes: analog and digital. Analog oscilloscopes use a
cathode raytube and display the signal much like a television set displays an image. Digital
oscilloscope sample the signals digitally and are more flexible in how they display,
manipulate, and store the signals. An oscilloscope is an instrument that measures voltage as a
function of time. The major elements of an oscilloscope include a cathode ray tube for visual
observation of the signal, a sweep generator, two amplifiers, and various power supplies.
The heart of an oscilloscope is the cathode ray tube (CRT). The interior of the tube is
highly evacuated (i.e., a vacuum). The cathode at the left end of the tube is raised to a high
temperature by a heater. This process liberates electrons from the surface of the cathode. The
accelerating anode is maintained at a high potential with respect to the cathode. The resultant
electric field causes the electrons liberated from the cathode to accelerate toward the
accelerating anode. A hole in the centre of the accelerating anode allows electrons travelling
along the axis of the tube to pass through. Once the electrons have passed through the
accelerating anode they travel with constant velocity to the right until striking the fluorescent
screen. This collision causes the screen to fluoresce, thus creating a bright spot on the
oscilloscope display.
The horizontal deflection plates control the vertical deflection of the electron beam.
As the beam passes through the region between the plates it experiences a deflection that is
proportional to the potential difference established between them. When a voltage signal is
fed into an oscilloscope it passes through an amplifier and is applied to these plates. The
setting on the amplifier determines the vertical deflection of the electron beam and hence its
position on the display screen. The sweep generator varies the voltage to the vertical plates in
such a manner that the electron beam sweeps across the display screen from left to right at a
uniform rate, then returns abruptly to the left side of the screen at the end of the sweep.

LITERATURE SURVEY OF PC BASED OSCILLOSCOPE

An oscilloscope (also known as a scope, CRO, DSO or, an O-scope) is a type
of electronic test instrument that allows observation of constantly varying signal voltages,
usually as a two-dimensional graph of one or more electrical potential differences using the
vertical or 'Y' axis, plotted as a function of time, (horizontal or 'x' 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.
Oscilloscopes are commonly used to observe the exact wave shape of an electrical
signal. In addition to the amplitude of the signal, an oscilloscope can show distortion, the
time between two events (such as pulse width, period, or rise time) and relative timing of two
related signals.Oscilloscopes are used in the sciences, medicine, engineering, and
telecommunications 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.

DUAL BEAM OSCILLOSCOPE

The dual-beam analog oscilloscope can display two signals simultaneously. A special
dual-beam CRT generates and deflects two separate beams. Although multi-trace analog
oscilloscopes can simulate a dual-beam display with chop and alternate sweeps, those
features do not provide simultaneous displays. (Real time digital oscilloscopes offer the same
benefits of a dual-beam oscilloscope, but they do not require a dual-beam display.)

ANALOG STORAGE OSCILLOSCOPE

Trace storage is an extra feature available on some analog scopes; they used directview
storage CRTs. Storage allows the trace pattern that normally decays in a fraction of a
second to remain on the screen for several minutes or longer. An electrical circuit can then be
deliberately activated to store and erase the trace on the screen.

FULL WAVE RECTIFIER

In order to establish a digital analog interface we need to use an analog to digital
converter (we are using a unipolar ADC for convenience. i.e, ADC we use can only convert
positive magnitude). So we make use of the full wave rectifier. Full wave rectifier will
convert the negative half of the signal to the positive side. It also act as a voltage limiting
circuit (i.e, voltage beyond a specific level is clipped out). Full wave rectifier section consists
of an inverting amplifier and a non inverting amplifier. When the positive portion occurs the
non inverting amplifier will handle the signal and when the opposite polarity occurs inverting amplifier will handle the signal and we get unipolar signal.

SAMPLER

The instantaneous amplitude of the signal is important so that we can study the
variations of the amplitude of the signal properly. Also it is difficult for the analog to digital
converter to make the conversion if the input voltage is swinging to a higher limit. So in order
to reduce the voltage swing we make use of the sampler circuit. So that the instantaneous
values are available for conversion to the analog to digital converter. We use a switch IC to
sample the amplitudes. The triggering signal/clock is issued by the PC through the parallel
port accordingly.

ANALOG TO DIGITAL CONVERTER (ADC)

The information regarding signals amplitude and phase should be conveyed to the
computer. And the computer only understands the digital language. So we need to convert the
analog data with us to digital format so that computer can analyse that data. This is done by
the help of an ADC. We use ADC0804 which is a single channel ADC. ADC converts the
analog voltage available to it to an equivalent eight bit binary number.

ZERO CROSSING DETECTOR (ZCD)

It is necessary to convey the information regarding signals amplitude and phase to the
computer. And the computer only understands the digital language. So we need to convert the
analog data with us to digital format so that computer can analyse that data. This is done by
the help of an ADC. We use ADC0804 which is a single channel ADC. ADC converts the
analog voltage available to it to an equivalent eight bit binary number.

DATA PROCESSING SOFTWARE

In order to interpret the available data properly so that we can reconstruct the
waveform with required accuracy, so that signal amplitude and frequency can be calculated.
For that a software has been developed using VB.net which continuously monitor the parallel
port and process the data available. The data obtained from the parallel port is weighted
properly ( because 255 in ADC is equal to 5V since the range of ADC is from 0v to 5v) and is
plotted on the computer screen.
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