Oscilloscopes Introduction: In studying the various electronic, electrical networks and systems, signals which are functions of time, are often encountered. Such signals may be periodic or non periodic in nature. The device which allows, the amplitude of such signals, to be displayed primarily as " function of time, is called cathode ray oscilloscope, commonly known as C.R.O. The CR.O gives the visual representation of the time varying signals. The oscilloscope has become an universal instrument and is probably most versatile tool for the development of electronic circuits and systems. It is an integral part of electronic laboratories.Basic Principle: The electron beam can be deflected in two directions : the horizontal or x-direction and the vertical or y-direction. Thus an electron beam producing a spot can be used to produce two dimensional displays, Thus CRO. can be regarded as a fast x-y plotter. The x-axis and y-axis can be used to study the variation of one voltage as a function of another. Typically the x-axis of the oscilloscope represents the time while the y-axis represents variation of the input voltage signal. Thus if bhe input voltage signal applied to the y-axis of CRO. is sinusoidally varying and if x-axis represents the time axis, then the spot moves sinusoidally, and the familiar sinusoidal waveform can be seen on the screen of the oscilloscope. The oscilloscope is so fast device that it can display the periodic signals whose time period is as small as microseconds and even nanoseconds. The CRO. Basically operates on voltages, but it is possible to convert current, pressure, strain, acceleration and other physical quantities into the voltage using transducers and obtain their visual representations on the CRO.Cathode Ray Tube (CRT): The main parts of the CRT are: i) Electron gun ii) Deflection system iii) Fluorescent screen iv) Glass tube or envelope v) Base Beam deflection es gente rn ay tt on SS ok ----------- {}-- sae -----| J—> Emission UA AE ws r\t 4, Cathode | Focus Vertical tro! ‘Selection Horizontal sian [nd Accelerating plates” defection ‘anode plates Electron gun producing electron teamElectron Gun: The electron gun section of the cathode ray tube provides a sharply focused electron beam directed :towards the fluorescent-coated screen. This section starts from theq] lally heated cathode, limiting the electrons. The control grid is give! negative potential with respect to cathode de. This grid controls the number of electrons in the beam, going to the screen. Deflection System: When the electron beam is accelerated it passes through the deflection system, with which beam can be positioned anywhere on the screen. The deflection system of the cathode-ray-tube consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. One of the plates’ in each set is connected to ground (0 V), To the other plate of each set, the external deflection voltage is applied through an internal adjustable gain amplifier stage, To apply the deflection voltage extemally, an external terminal, called the Y input or the X input, is available.Fluorescent Sereen: The light produced by the screen does not disappear immediately when bombardment by electrons ceases, i.e., when the signal becomes zero. The time period for which the trace remains on the screen after the signal becomes zero is known as "persistence". The persistence may be jS ~en minutes. short as a few microsecond, or as long as tens of seconds Long persistence traces are used in the study.. of transients. Long persistence helps in the study of transients since the trace is still seen on the screen after the transient has disappeared. Phosphor sereen characteristics: Many phosphor materials having different excitation times and colours as well as different phosphorescence times are available. The type PI, P2, PII or P3I are the short persistence phosphors and are used for the general purpose oscilloscopeBlock diagram of simple oscilloscope: Vertical amplifier Input signal ‘Screen Electron. gun Trigger circuit High voltage_| Power Low supply voltage generator +ve To all circuitsCRT: This is the cathode ray tube which is the heart of CR.O. It is' used to emit the rlectrons required to strike the phosphor screen to produce the spot for the visual display of the signals. Vertical Amplifier: The input signals are generally not strong to provide the measurable deflection on the screen. Hence the vertical amplifier. stage is used Jo amplify the input signals. The amplifier stages used are generally wide band amplifiers so as to pass faithfully the entire band of frequencies to be measured. Similarly it contains the attenuator stages as well. The attenuators are used when very high voltage signals are to be examined, to bring the signals within the proper range of operation.inverterDelay line: The delay line is used to delay the signal for some time in the verticClI sections. When the delay line is not used, the part of the signal gets lost. Thus the input signal is not applied directly to the vertical plates but is delClyed by some time using a delay line cu-cuit as shown in the Fig. Main = time Delayed signal amplifier ; oxTrigger circuit: It is necessary that horizontal deflection starts at the same point of the input vertical signal, each time it sweeps. Hence to synchronize horizontal deflection with vertical deflection a synchronizing or triggering circuit is used. It converts the incoming signal into the triggering pulses, which are used for the synchronization. e base generator: The time base generator is used to generate the sawtooth voltage, required to deflect the beam in the horizontal section. This voltage deflects the spot at a constant time dependent rate. Thus the x- axis' on the screen can be represented as time, which, helps to display and analyse the time varying signals.Dual Beam Oscilloscope: Two sets of vertical plates Channel A, Delay line A Horizontal plates ia ertical amp, Channel 8 input amp é ‘and attenuator by Tager selector switch d Tagger |_| Sweep et Bs oLine Lcvcut | [generatoro— and Channel 8 eat Ex. Lie ——4 Dual beam CRO with separate time bases Vertical plates Set! Vertical plates Set2 Horizontal plates Horizontal, platesMultiple beam oscilloscopes: Multiple beam oscilloscope has a single tube but several beam producing systems inside. Each system has separate vertical deflecting pair of plates and generally (1 common time base system. The triggering can be done internally using eith.er of the multiple inputs or externally by an external signal or line voltages.Dual trace oscilloscope: The block diagram of dual trace oscilloscope is shown in the Fig Delay A Electronic Pre ‘Switch amplifier ‘ane attenuator Pre amplifier and attenuator Channel 8 Ext trigger tine F 82 Trigger selector ‘switch