CATHODE RAY OSCILLOSCOPE

CATHODE RAY OSCILLOSCOPE

Definition:

It is an equipment used to display signals by employing modulation of an electron beam prior to impact on a fluorescent screen. It is abbreviated as CRO.­

Introduction:

The cathode ray oscilloscope is a vital piece of diagnostic lab equipment for observing and measuring electrical signals at frequencies ranging from dc to GHz. Oscilloscopes are used to observe the change of an electrical signal over time, such that voltage and time describe a shape which is continuously graphed against a calibrated scale. 

The observed waveform can be analyzed for such properties as amplitude, frequency, rise time, time interval, distortion and others. Modern digital instruments may calculate and display these properties directly. Originally, calculation of these values required manually measuring the waveform against the scales built into the screen of the instrument. The oscilloscope can be adjusted so that repetitive signals can be observed as a continuous shape on the screen. A storage oscilloscope allows single events to be captured by the instrument and displayed for a relatively long time, allowing human observation of events too fast to be directly perceptible.

Analog and Digital:

Electronic equipment's can be divided into two types: analog and digital. Analog equipment works with continuously variable voltages, while digital equipment works with binary numbers (1 and 0’s) that may represent voltage samples. Oscilloscopes also come in analog and digital types. An analog oscilloscope works by directly applying a voltage being measured to an electron beam moving across the oscilloscope screen. The voltage deflects the beam up and down proportionally, tracing the waveform on the screen. This gives an immediate picture of the waveform. In contrast, a digital oscilloscope samples the waveform and uses an analog-to-digital converter (or ADC) to convert the voltage being measured into digital information. It then uses this digital information to reconstruct the waveform on the screen.

Construction:

Other features of this are given below: At the center of the instrument is a highly evacuated cathode ray tube with the following features:

●    A heated cathode to produce a beam of electrons (a typical beam current is of the order of 0.1mA.

●    A grid to control the brightness of the beam.

●    An accelerating anode (a typical potential difference between two anodes and the cathode would be about +1000V.

●    A pair of vertical deflection plates to deflect the beam in vertical direction.

●    A pair of horizontal plates.

●    A fluorescent screen

Electron Gun:

The electron gun creates the electron beam and adjusts the intensity and width of the beam moving to the screen. It is located next to the base of the CRT and consists of five major parts: heater, cathode, control grid, focusing anode, and accelerating anode. Electrons are produced by thermionic emission. Essentially a cathode (negative electrode) is heated and electrons boil off the surface to be attracted by a series of anodes (positive electrodes). The anodes accelerate the electrons and collimator them into a narrow beam.

Deflection System:

The deflection plates are simply pairs of oppositely charged metal plates. There are two sets of deflection plates: vertical and horizontal. Each set of plates is parallel and located at the neck of the tube. The vertical deflection plates lie horizontally but control the vertical position of the beam. The horizontal plates are positioned at right angles to the vertical plates and control the horizontal position of the beam. External electric circuits are used to control and change the amount of charge on these plates and the electric field between them. The  electron beam passes between each pair of plates, and is attracted to the positively charged side and repelled by the negatively charged side. In this way,the plates control the path of the electron beam and where the beam hits the screen.

To display a waveform, a repetitive reversing voltage is applied to the X-plates. This causes the electron beam to be slowly repelled from the left-hand plate and attracted towards the right-hand plate. On the CRO screen this translates as an illuminated dot moving from left to right.

The voltage is then reversed and increased rapidly. The effect is to move the dot very quickly from right to left.
The applied voltage is called the time-base. The curve has the general shape of a 'saw-tooth' and is often referred to by this name. The p.d. applied to the Y-plates is the signal to be examined. With the p.d. across the X-plates (the time-base) switched off, a sinusoidal signal makes the dot go up and down, executing simple harmonic motion. With the time-base on, a sine waves is displayed.

Fluorescent Screen:

The fluorescent screen is the display on the bulb. The display screen is coated on the inside with a very thin layer of a phosphor called cadmium sulphide. Electrons emerging from the deflection plates strike the screen and the phosphorous converts the energy in the electron beam into photons of visible light. This results in a spot of light on the display,with brightness proportional to the intensity of the beam. The element on the screen is also phosphorescent, meaning that item its energy as light gradually instead of instantaneously. This allows us to see lines on the screen instead of a moving dot. This line is maintained by rapid,repetitive tracing.

Use of oscillator in merging technology:

The primary uses of CRO are to measure voltage,frequency and phase

Measuring Voltage:

●    Because of its effectively infinite resistance the CRO makes an excellent voltmeter.It has relatively low sensitivity but this can be improved by the use of an internal voltage amplifier. The oscilloscope must first be callibrated by connected D.C. Source of known E.M.F. To the Yplates and measuring the deflection of the spot on the screen. This should be repeated for a rang of values,so that the linearity of the deflection maybe checked. The known E.M.F. Is then connected and its values found from the deflection produced. Most oscilloscope have a previously calibrated screen giving the deflection sensitivity in volts per scale division. In this case a calibration by a D.C. Source may be considered unnecessary.

Measuring frequency

Using the calibrated as the input signal of unknown frequency maybe frozen and its frequency found directly by comparison with the scale divisions. Alternative the internal time base  may be switched off and asignal of known frequency applied to the X-input.If the signal of unknown frequency applied to the Y-input. Thus the computrized figures are formed on thhe screen are formed on the screen .Analysis of the peaks gives the value of the unknown frequency.

 Measuring Phase:

The internal time base is switched off as above and two signals are applied as before.The frequency of the known signal is adjusted until it is the same as that of the unknown signal. An ellipse will then be formed on the formed on the screen and the angle of the elllipse will denote the phase difference between the two signals

Some other uses:

●    Waveform analysis in communication system design.

●    Output analysis designed electronic circuits and their response to wards different voltage and current inputs.

●    In determination of time constant, phase shift, frequency, pulses width, and amplitude variations of the signal under test.

●    Study of noise spectrum in received signals at the receiver section of communication systems.

●    For study of wave propagation of in laboratories.

●    For visualization of physical quantities after their conversion into proper electrical form so that the changes occurring over there may be observe and study may become even simpler.

Disadvantages of cathode ray oscilloscope

Although these are proven to be very useful by the virtue of their application in wave form analysis but still them come up with their own embedded limitations. Some of them can be summarized as followed:

1.  It is a very sensitive device and is often noise prone i.e. upon application small signals to them noise may enter in the system through open wiring, exposed metallic components and unprotected parts. So they require complete isolation from noise prone sources.

2.  When it comes to analyse very high frequency signals a general cathode ray oscilloscope becomes incapable to produce some reliable result because that much of variations are not supported by normal electronic components used in them.

3.  Very sudden changes cannot be observed with normal CRO, particularly when they are occurring at very high frequency because they occur for a very small instant of time and human eye remains incapable of observing them.

4.  Cathode ray oscilloscope cannot be used for study of high voltage signals and in order to study them they first need to be converted to low voltage, this puts a limiting mark upon application of these instruments.

5.  They are a lot of control terminals over the control panel that leads to a greater complexity of the device making it difficult to use.