In this chapter, we are going to learn about the cathode ray oscilloscope (CRO), the Block diagram of CRO, the construction and working of CRO, and the applications of CRO. So let’s discuss from the beginning the basic concept of CRO.
Cathode Ray Oscilloscope (CRO)
- the cathode ray oscilloscope (CRO) is generally referred to as an oscilloscope or simply scope. It is a basic electronic test instrument that allows observations of constantly varying signal voltages usually as a two-dimensional graph of one or more electrical potential differences as shown in Fig.1. It allows an electronic engineer to ‘observe’ the signal in various parts of the electronic circuit.
- By ‘observing’ the signal waveforms, the engineers or technicians can correct errors, understand mistakes in the circuit design and thus make suitable adjustments. The circuit symbol of an oscilloscope is shown in Fig.2.
- A cathode ray oscilloscope or simply CRO or oscilloscope is one of the extremely useful and the most versatile tool used in the sciences, medicine, engineering, and telecommunication industry. These 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 the relative timing of two related signals.
- Originally all oscilloscopes used cathode ray tubes(CRTs) as their display element and linear amplifiers for signal processing. However, modern oscilloscopes have LCD or LED screens, fast analog-to-digital converters, and digital signal processors. Some oscilloscopes use storage CRTs to display single events for a limited time.
- These days oscilloscope peripheral modules are available for general-purpose laptop or desktop personal computers which allows the laptop or desktop computers to be used as test instruments.
- An oscilloscope allows us to observe the constantly varying signal, 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. Whatever the type of oscilloscope, whether CRT or LCD, or LED screen, its front panel normally has control sections divided into Vertical, Horizontal, and Trigger sections. There are also display controls and input connectors.
- An oscilloscope can display and measure many electrical quantities like ac/dc voltage, time, phase relationships, frequency, and a wide range of waveform characteristics like rise-time, fall- time and overshoot, etc. Non-electrical quantities like pressure, strain, temperature, acceleration, etc. can also be measured by using different transducers to first convert them into an equivalent voltage.
Block Diagram of CRO
Fig.3 shows the block diagram of a CRO. As seen in the diagram, it consists of the following major sub-systems:
|1.||Display||All modern oscilloscopes it is usually LCD panels. In the old oscilloscopes, the display was a cathode ray tube or CRT. The display whether LCD panel or CRT is laid out with both horizontal and vertical reference lines referred to as the graticule. In addition to the screen, most display sections are equipped with three basic controls, a focus knob, an intensity knob, and a beam finder button.|
|2.||Cathode Ray Tube (CRT)||This is the cathode ray tube that emits electrons that strikes the phosphor screen internally to provide a visual display of the signal. It displays the quantity being measured.|
|3.||Vertical amplifier||It amplifies the signal waveform to be viewed.|
|4.||Delay Line||It is used to delay the signal for some time in the vertical sections.|
|5.||Horizontal amplifier||This is used to amplify the sawtooth voltage which is then applied to the X-plates.|
|6.||Sweep generator||Produces sawtooth voltage waveform used for horizontal deflection of the electron beam.|
|7.||Trigger circuit||Produces trigger pulses to start the horizontal sweep. It converts the incoming signal into trigger pulses so that the input signal and the sweep frequency can be synchronized.|
|8.||High and low Voltage power supply||Two voltages are generated in CRO|
(a) The low voltage supply is from +300 to 400 V.
(b) The high voltage supply is from –1000 to –1500 V.
Operation Control of Oscilloscope
The operating controls of a basic oscilloscope are shown in Fig.4. The different terminals provide.
- Horizontal Amplifier Input
- Vertical Amplifier Input
- Synchronous Input
- External Sweep Input.
|1.||Intensity||For the correct brightness of the trace on the screen. This adjusts trace brightness. Slow traces on CRT scopes need less, and fast ones, especially if they don’t repeat very often, require more. On flat panels, however, trace brightness is essentially independent of sweep speed, because the internal signal processing effectively synthesizes the display from the digitized data.|
|2.||Focus||For the sharp focus of the trace. This control adjusts CRT focus to obtain the sharpest, most-detailed trace. In practice, the focus needs to be adjusted slightly when observing quite- different signals, which means that it needs to be an external control. Flat-panel displays do not need a focus control; their sharpness is always optimum.|
|3.||Horizontal centering||For moving the pattern right and left on the screen. The horizontal position control moves the display sidewise. It usually sets the left end of the trace at the left edge of the graticule, but it can displace the whole trace when desired. This control also moves the X–Y mode traces sidewise in some ‘scopes, and can compensate for a limited DC component for the vertical position|
|4.||Vertical centering||For moving the pattern up and down on the screen. The vertical position control moves the whole displayed trace up and down. It is used to set the no-input trace exactly on the center line of the graticule but also permits offsetting vertically by a limited amount. With direct coupling, adjustment of this control can compensate for a limited DC component of the input.|
|5.||Horizontal gain (also Time/div or Time/cm)||For adjusting pattern width. This section controls the time base or “sweep” of the instrument. The primary control is the Seconds- per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-Y axis signals. The horizontal beam position knob is generally located in this section.|
|6.||Vertical gain (also volt/div or volt/cm).||For adjusting pattern height. This controls the amplitude of the displayed signal. This section carries a Volts-per-Division (Volts/Div) selector knob, an AC/DC/Ground selector switch, and the vertical (primary) input for the instrument. Additionally, this section is typically equipped with a vertical beam position knob.|
|7.||Sweep frequency||For selecting a number of cycles in the pattern.|
|8.||Sync. Voltage amplitude.||For locking the pattern|
Working of CRO
- In the past, CRO consists mainly of a vacuum tube that contains a cathode, anode, grid, X & Y-plates, and a fluorescent screen. When the cathode is heated (by applying a small potential difference across its terminals), it emits electrons. Having a potential difference between the cathode and the anode (electrodes), accelerate the emitted electrons towards the anode, forming an electron beam, which passes to fall on the screen.
- When the fast electron beam strikes the fluorescent screen, a bright visible spot is produced. The grid, which is situated between the electrodes, controls the number of electrons passing through it thereby controlling the intensity of the electron beam. The X & Y plates are responsible for deflecting the electron beam horizontally and vertically.
- A sweep generator is connected to the X-plates, which moves the bright spot horizontally across the screen and repeats that at a certain frequency as the source of the signal. The voltage to be studied is applied to the Y-plates. The combined sweep and Y voltages produce a graph showing the variation of voltage with time.
Applications of a CRO
As stated earlier, no other instrument in the electronic industry is as versatile as a CRO. In fact, a modern oscilloscope is the most useful single piece of electronic equipment that not only removes the guesswork from technical troubleshooting but makes it possible to determine the trouble quickly. Some of its uses are as under:
In Radio Work:
- to trace and measure a signal throughout the RF, IF, and AF channels of radio and television receivers.
- it provides the only effective way of adjusting FM receivers, broadband high-frequency
- RF amplifiers and automatic frequency control circuits;
- to test AF circuits for different types of distortions and other spurious oscillations;
- to give a visual display of wave shapes such as sine waves, square waves, and their many different combinations;
- to trace transistor curves
- to visually show the composite synchronized TV signal
- to display the response of tuned circuits etc.
Scientific and Engineering Applications:
- measurement of ac/dc voltages,
- finding B/H curves for the hysteresis loop,
- for engine pressure analysis,
- for the study of stress, strain, torque, acceleration, etc.,
- frequency and phase determination by using Lissajous Figures,
- radiation patterns of antenna,
- amplifier gain,
- modulation percentage,
- complex waveform as a short-cut for Fourier analysis,
- Standing waves in transmission lines etc.
Frequently Asked Questions on CRO
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[…] is then amplified by the video amplifier to a level where it can be properly displayed usually on CRT (Cathode Ray Tubes) directly or via digital signal processors. Synchronizing pulses are applied by the trigger source […]