cathode ray oscilloscope &function generator


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cathode ray oscilloscope &function generator

  1. 1. My presentation on CRO -MEGHA AGRAWAL Ece 2nd year,mnit jaipur. Cathode ray oscilloscope:OBJECTIVE: To learn how to operate a cathode-ray oscilloscope.APPARATUS: Cathode-ray oscilloscope, multimeter, and oscillator.INTRODUCTION: The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accuratetime and aplitude measurements of voltage signals over a wide range of frequencies. Its reliability, stability, andease of operation make it suitable as a general purpose laboratory instrument. The heart of the CRO is a cathode-raytube shown schematically in Fig. 1. The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) andaccelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and acceleratinganode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control itsintensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates - one orientedto provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. Theseplates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflectionsallows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, thephosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us towrite with points or lines of light on an otherwise darkened screen. In the most common use of the oscilloscope the signal to be studied is first amplified and then applied to thevertical (deflection) plates to deflect the beam vertically and at the same time a voltage that increases linearly withtime is applied to the horizontal (deflection) plates thus causing the beam to be deflected horizontally at a uniform(constant> rate. The signal applied to the verical plates is thus displayed on the screen as a function of time. Thehorizontal axis serves as a uniform time scale.
  2. 2. The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep generator that isincorporated in the oscilloscope circuitry. The voltage output of such a generator is that of a sawtooth wave asshown in Fig. 2. Application of one cycle of this voltage difference, which increases linearly with time, to thehorizontal plates causes the beam to be deflected linearly with time across the tube face. When the voltage suddenlyfalls to zero, as at points (a) (b) (c), etc...., the end of each sweep - the beam flies back to its initial position. Thehorizontal deflection of the beam is repeated periodically, the frequency of this periodicity is adjustable by externalcontrols. To obtain steady traces on the tube face, an internal number of cycles of the unknown signal that is applied tothe vertical plates must be associated with each cycle of the sweep generator. Thus, with such a matching ofsynchronization of the two deflections, the pattern on the tube face repeats itself and hence appears to remainstationary. The persistance of vision in the human eye and of the glow of the fluorescent screen aids in producing astationary pattern. In addition, the electron beam is cut off (blanked) during flyback so that the retrace sweep is notobserved.CRO Operation: A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, theinstrument is operated in the following manner. The signal to be displayed is amplified by the vertical amplifier andapplied to the verical deflection plates of the CRT. A portion of the signal in the vertical amplifier is applied tothe sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with a selected point inthe cycle of the triggering signal. This pulse turns on the sweep generator, initiating the sawtooth wave form. Thesawtooth wave is amplified by the horizontal amplifier and applied to the horizontal deflection plates. Usually,additional provisions signal are made for appliying an external triggering signal or utilizing the 60 Hz line fortriggering. Also the sweep generator may be bypassed and an external signal applied directly to the horizontalamplifier.Cro Description:-Display and general external appearanceThe basic oscilloscope, as shown in the illustration, is typically divided into four sections: the display,vertical controls, horizontal controls and trigger controls. The display is usually a CRT or LCD panel which is laid out with both horizontal and vertical referencelines referred to as the graticule. In addition to the screen, most display sections are equipped with threebasic controls, a focus knob, an intensity knob and a beam finder button.The vertical section 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 forthe instrument. Additionally, this section is typically equipped with the vertical beam position knob.The horizontal section controls the time base or “sweep” of the instrument. The primary control is theSeconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-Yaxis signals. The horizontal beam position knob is generally located in this section.
  3. 3. The trigger section controls the start event of the sweep. The trigger can be set to automatically restartafter each sweep or it can be configured to respond to an internal or external event. The principal controlsof this section will be the source and coupling selector switches. An external trigger input (EXT Input) andlevel adjustment will also be included.CRO Controls The controls available on most oscilloscopes provide a wide range of operating conditions and thus make theinstrument especially versatile. Since many of these controls are common to most oscilloscopes a brief descriptionof them follows.CATHODE-RAY TUBEPower and Scale Illumination: Turns instrument on and controls illumination of the graticule.Focus: Focus the spot or trace on the screen.Intensity: Regulates the brightness of the spot or trace.VERTICAL AMPLIFIER SECTIONPosition: Controls vertical positioning of oscilloscope display.Sensitivity: Selects the sensitivity of the vertical amplifier in calibrated steps.Variable Sensitivity: Provides a continuous range of sensitivities between the calibrated steps. Normally thesensitivity is calibrated only when the variable knob is in the fully clockwise position.
  4. 4. AC-DC-GND: Selects desired coupling (ac or dc) for incoming signal applied to vertical amplifier, or grounds theamplifier input. Selecting dc couples the input directly to the amplifier; selecting ac send the signal through acapacitor before going to the amplifier thus blocking any constant component.HORIZONTAL-SWEEP SECTIONSweep time/cm: Selects desired sweep rate from calibrated steps or admits external signal to horizontal amplifier.Sweep time/cm Variable: Provides continuously variable sweep rates. Calibrated position is fully clockwise.Position: Controls horizontal position of trace on screen.Horizontal Variable: Controls the attenuation (reduction) of signal applied to horizontal aplifier through Ext. Horiz.connector.TRIGGERThe trigger selects the timing of the beginning of the horizontal sweep.Slope: Selects whether triggering occurs on an increasing (+) or decreasing (-) portion of trigger signal.Coupling: Selects whether triggering occurs at a specific dc or ac level.Source: Selects the source of the triggering signal. INT - (internal) - from signal on vertical amplifier EXT - (external) - from an external signal inserted at the EXT. TRIG. INPUT. LINE - 60 cycle trigerLevel: Selects the voltage point on the triggering signal at which sweep is triggered. It also allows automatic (auto)triggering of allows sweep to run free (free run).CONNECTIONS FOR THE OSCILLOSCOPEVertical Input: A pair of jacks for connecting the signal under study to the Y (or vertical) amplifier. The lower jackis grounded to the case.Horizontal Input: A pair of jacks for connecting an external signal to the horizontal amplifier. The lower terminal isgraounted to the case of the oscilloscope.External Tigger Input: Input connector for external trigger signal.Cal. Out: Provides amplitude calibrated square waves of 25 and 500 millivolts for use in calibrating the gain of theamplifiers. Accuracy of the vertical deflection is + 3%. Sensitivity is variable.BandwidthBandwidth is a measure of the range of frequencies that can be displayed; it refers primarily to the verticalamplifier, although the horizontal deflection amplifier has to be fast enough to handle the fastest sweeps.The bandwidth of the oscilloscope is limited by the vertical amplifiers and the CRT (in analog instruments)
  5. 5. or by the sampling rate of the analog to digital converter in digital instruments. The bandwidth is definedas the frequency at which the sensitivity is 0.707 of the sensitivity at lower frequency (a drop of 3 dB).The rise time of the fastest pulse that can be resolved by the scope is related to its bandwidthapproximately:Bandwidth in Hz x rise time in seconds = 0.35For example, an oscilloscope intended to resolve pulses with a rise time of 1 nanosecond would have abandwidth of 350 MHz.For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten times thehighest frequency desired to resolve; for example a 20 megasample/second rate would be applicable formeasuring signals up to about 2 megahertz. Horizontal sweep should be accurate to within 3%. Range of sweep is variable.Operating Instructions: Before plugging the oscilloscope into a wall receptacle, set the controls as follows: (a) Power switch at off (b) Intensity fully counter clockwise (c) Vertical centering in the center of range (d) Horizontal centering in the center of range (e) Vertical at 0.2 (f) Sweep times 1Plug line cord into a standard ac wall recepticle (nominally 118 V). Turn power on. Do not advance the IntensityControl.Allow the scope to warm up for approximately two minutes, then turn the Intensity Control until the beam is visibleon the screen.WARNING: Never advance the Intensity Control so far that an excessively bright spot appears. Bright spots implyburning of the screen. A sharp focused spot of high intensity (great brightness) should never be allowed to remainfixed in one position on the screen for any length of time as damage to the screen may occur.Adjust Horizontal and Vertical Centering Controls. Adjust the focus to give a sharp trace. Set trigger to internal,level to auto.PROCEDURE:I. Set the signal generator to a frequency of 1000 cycles per second. Connect the output from the gererator to thevertical input of the oscilloscope. Establish a steady trace of this input signal on the scope. Adjust (play with)all ofthe scope and signal generator controls until you become familiar with the functionof each. The purpose fo such"playing" is to allow the student to become so familiar with the oscilloscope that it becomes an aid (tool) in makingmeasurements in other experiments and not as a formidable obstacle. Note: If the vertical gain is set too low, it maynot be possible to obtain a steady trace.
  6. 6. II. Measurements ofVoltage: Consider the circuit inFig. 4(a). Thesignal generatoris used toproduce a 1000hertz sine wave.The ACvoltmeter andthe leads to theverticle input ofthe oscilloscopeare connectedacross thegeneratorsoutput. Byadjusting theHorizontal Sweep time/cm and trigger, a steady trace of the sine wave may be displayed on the screen. The tracerepresents a plot of voltage vs. time, where the vertical deflection of the trace about the line of symmetry CD isproportional to the magnitude of the voltage at any instant of time. To determine the size of the voltage signal appearing at the output of terminals of the signal generator, an AC(Alternating Current) voltmeter is connected in parallel across these terminals (Fig. 4a). The AC voltmeter isdesigned to read the dc "effective value" of the voltage. This effective value is also known as the "Root MeanSquare value" (RMS) value of the voltage. The peak or maximum voltage seen on the scope face (Fig. 4b) is V m volts and is represented by the distancefrom the symmetry line CD to the maximum deflection. The relationship between the magnitude of the peak voltagedisplayed on the scope and the effective or RMS voltage (VRMS) read on the AC voltmeter is VRMS = 0.707 Vm (for a sine or cosine wave).Thus Agreement is expected between the voltage reading of the multimeter and that of the oscilloscope. For asymmetric wave (sine or cosine) the value of V m may be taken as 1/2 the peak to peak signal VppThe variable sensitivity control a signal may be used to adjust the display to fill a concenient range of the scope face.In this position, the trace is no longer calibrated so that you can not just read the size of the signal by counting thenumber of divisions and multiplying by the scale factor. However, you can figure out what the new calibration is anuse it as long as the variable control remains unchanged.Caution: The mathematical prescription given for RMS signals is valid only for sinusoidal signals. The meter willnot indicate the correct voltage when used to measure non-sinusoidal signals.
  7. 7. III. Frequency Measurements: When the horizontal sweep voltage is applied, voltage measurements can still betaken from the vertical deflection. Moreover, the signal is displayed as a function of time. If the time base (i.e.sweep) is calibrated, such measurements as pulse duration or signal period can be made. Frequencies can then bedetermined as reciprocal of the periods. Set the oscillator to 1000 Hz. Display the signal on the CRO and measure the period of the oscillations. Usethe horizontal distance between two points such as C to D in Fig. 4b. Set the horizontal gain so that only one complete wave form is displayed. Then reset the horizontal until 5 waves are seen. Keep the time base control in a calibrated position. Measurethe distance (and hence time) for 5 complete cycles and calculate the frequency from this measurement. Compareyou result with the value determined above. Repeat your measurements for other frequencies of 150 Hz, 5 kHz, 50 kHz as set on the signal generator.IV. Lissajous Figures: When sine-wave signals of different frequencies are input to the horizontal and verticalamplifiers a stationary pattern is formed on the CRT when the ratio of the two frequencies is an intergral fractionsuch as 1/2, 2/3, 4/3, 1/5, etc. These stationary patterns are known as Lissajous figures and can be used forcomparison measurement of frequencies. Use two oscillators to generate some simple Lissajous figures like those shown in Fig. 5. You will find itdifficult to maintain the Lissajous figures in a fixed configuration because the two oscillators are not phase andfrequency locked. Their frequencies and phase drift slowly causing the two different signals to change slightly withrespect to each other.V. Testing what you have learned: Your instructor will provide you with a small oscillator circuit. Examine theinput to the circuit and output of the circuit using your oscilloscope. Measure such quantities as the voltage andfrequence of the signals. Specify if they are sinusoidal or of some other wave character. If square wave, measure thefrequency of the wave. Also, for square waves, measure the on time (when the voltage is high) and off time (when itis low).Examples of use:-
  8. 8. Lissajous figures on an oscilloscope, with 90 degrees phase difference between x and y inputs.One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment. One of theadvantages of a scope is that it can graphically show signals: where a voltmeter may show a totally unexpectedvoltage, a scope may reveal that the circuit is oscillating. In other cases the precise shape or timing of a pulse isimportant.In a piece of electronic equipment, for example, the connections between stages (e.g. electronic mixers, electronicoscillators, amplifiers) may be probed for the expected signal, using the scope as a simple signal tracer. If theexpected signal is absent or incorrect, some preceding stage of the electronics is not operating correctly. Since mostfailures occur because of a single faulty component, each measurement can prove that half of the stages of acomplex piece of equipment either work, or probably did not cause the fault.Once the faulty stage is found, further probing can usually tell a skilled technician exactly which component hasfailed. Once the component is replaced, the unit can be restored to service, or at least the next fault can be isolated.This sort of troubleshooting is typical of radio and TV receivers, as well as audio amplifiers, but can apply to quite-different devices such as electronic motor drives.Another use is to check newly designed circuitry. Very often a newly designed circuit will misbehave because ofdesign errors, bad voltage levels, electrical noise etc. Digital electronics usually operate from a clock, so a dual-tracescope which shows both the clock signal and a test signal dependent upon the clock is useful. Storage scopes arehelpful for "capturing" rare electronic events that cause defective operation. Function generator
  9. 9. Definition:- A function generator is usually a piece of electronic test equipment or software used to generatedifferent types of electrical waveforms over a wide range of frequencies. Some of the most common waveformsproduced by the function generator are the sine, square, triangular and sawtooth shapes. These waveforms can beeither repetitive or single-shot (which requires an internal or external trigger source). Integrated circuits used togenerate waveforms may also be described as function generator ICs.Feature:- Other important features of the function generator are continuous tuning over wide bands with max-minfrequency ratios of 10:1 or more, a wide range of frequencies from a few Hz to a fewMHz, a flat output amplitude and modulation capabilities like frequency sweeping, frequency modulation andamplitude modulation.Although function generators cover both audio and RF frequencies, they are usually not suitable for applications thatneed low distortion or stable frequency signals. When those traits are required, other signal generators would bemore appropriate.Uses:- Function generators are used in the development, test and repair of electronic equipment. For example, theymay be used as a signal source to test amplifiers or to introduce an error signal into a control loop.WorkingSimple function generators usually generate triangular waveform whose frequency can be controlled smoothly aswell as in steps.[4] This triangular wave is used as the basis for all of its other outputs. The triangular wave isgenerated by repeatedly charging and discharging a capacitor from a constant current source. This producesa linearly ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the chargingand discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and thesize of the capacitor, different frequencies may be obtained. Sawtooth waves can be produced by charging thecapacitor slowly, using a current, but using a diode over the current source to discharge quickly - the polarity of thediode changes the polarity of the resulting sawtooth, i.e. slow rise and fast fall, or fast rise and slow fall.A 50% duty cycle square wave is easily obtained by noting whether the capacitor is being charged or discharged,which is reflected in the current switching comparator output. Other duty cycles (theoretically from 0% to 100%)can be obtained by using a comparator and the sawtooth or triangle signal. Most function generators also contain anon-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave by roundingoff the corners of the triangle wave in a process similar to clipping in audio systems.A typical function generator can provide frequencies up to 20 MHz. RF generators for higher frequencies are notfunction generators in the strict sense since they typically produce pure or modulated sine signals only.
  10. 10. Function generators, like most signal generators, may also contain an attenuator, various means of modulating theoutput waveform, and often the ability to automatically and repetitively "sweep" the frequency of the outputwaveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capabilitymakes it very easy to evaluate the frequency response of a given electronic circuit.Some function generators can also generate white or pink noise.More advanced function generators are called arbitrary waveform generators (AWG). They use direct digitalsynthesis (DDS) techniques to generate any waveform that can be described by a table of amplitudes.SpecificationsTypical specifications for a general-purpose function generator are: Produces sine, square, triangular, sawtooth (ramp), and pulse output. Arbitrary waveform generators can produce waves of any shape. It can generate a wide range of frequencies. For example, the Tektronix FG 502 (ca 1974) covers 0.1 Hz to 11 MHz. Frequency stability of 0.1 percent per hour for analog generators or 500ppm for a digital generator. Maximum sinewave distortion of about 1% (accuracy of diode shaping network) for analog generators. Arbitrary waveform generators may have distortion less than -55dB below 50 kHz and less than - 40dB above 50 kHz. Some function generators can be phase locked to an external signal source, which may be a frequency reference or another function generator. AM or FM modulation may be supported. Output amplitude up to 10V peak-to-peak. Amplitude can be modified, usually by a calibrated attenuator with decade steps and continuous adjustment within each decade. Some generators provide a DC offset voltage, e.g. adjustable between -5V to +5V. An output impedance of 50 ohms. Generator Control Panel - What it is and how it’s used The Control Panel – what is it? Visually, a control panel is a set of displays that indicate the measurement of various parameters like voltage, current and frequency, through gauges and meters. These meters and gauges are set in a metallic body, usually corrosion proof, to protect from the effect of rain or snow. The panel may be set up on the body of the generator itself, which is usually the case with small generators. If they are mounted on the generator, they typically have vibration proof pads that help isolate the control panel from shocks. Control panels for a larger industrial generators can be completely separate from the generator and are typically large enough to stand upon their own. These units may also be shelf-mounted or wall-mounted next to the generator, which is common inside an enclosure or intental application like a data center. Control panels are usually fitted with buttons or switches that help to operate the generator such as a switch-off button or turn-on key. The switches and gauges are usually grouped on the basis of functionality. This makes the panel friendly and safe for use since it minimizes the possibility of an operator accidentally selecting or executing the wrong control. Imagine trying to shut down a vibrating generator with a spring loaded lever in the middle of the night and you will appreciate why having a simple cut of switch at the control panel makes sense. • The front panel is divided into six major control groups: 1) Frequency Selection Group;
  11. 11. 2) Sweep Group; 3) Amplitude Modulation Group; 4) DC Offset Group; 5) Function, or Waveform Group; and 6) Output Group. front panel of function generator • The power switch is on the upper left-hand corner of the unit. The green LED will indicate that the unit is on. • The three most important groups for this lab are the frequency, function, and output groups. The remaining three groups, (sweep, amplitude modulation, and DC offset) will be briefly covered in the lab setup procedures. Should you desire more detailed descriptions of these groups, the Leader Function Generator manual is available in the lab. (1) Frequency Selection Group:- These controls are used to select the operating frequency of the function generator. This group consists of the frequency control knob and the eight frequency multiplier selection buttons. (2) Output Group:- 1. These controls are used to adjust the amplitude of the generators output signal. The group consists of the amplitude-control knob, the three attenuation buttons and the fused 50 ohm BNC connector. Although the amplitude knob is not indexed, the amplitude ranges from a few millivolts to approximately 20 volts. We will set the amplitude levels by aligning the white line on the amplitude knob to the three oclock position (90 degrees right), the nine oclock position (90 degrees left), or the twelve oclock position (straight up). Notice that rotating the knob fully to the left does not result in a zero amplitude signal. • The attenuation buttons are used to attenuate (decrease) the amplitude of the signal by a factor measured indecibels. The following relationship will assist in working with the attenuation buttons: (dB) = -10 * log10 (Pout / Pin) (if power is the unit of measurement) or (dB) = -20 * log10(Vout / Vin) (if voltage is the unit of measurement) (3) Function/Waveform Selection Group:- This group is used to select the shape of the generated waveform. The group is made up of the six wave-selector buttons. The six waveforms that the function generator can
  12. 12. produce are the sine wave, the square wave, the triangle wave, two sawtooth waves, and the variable-width pulse wave. How does it work? The control panel is becoming an increasingly complex piece of electronics with a microprocessor that can manipulate input from sensors to help give feedback to the machine to manage itself. One such feedback could be the temperature, indicating overheating, other examples would be over/under speed and low/high oil pressure. Typically, a heat sensor inside the generator would sense the build up of heat in the generator body and pass this to the microprocessor in the control panel. The microprocessor will then take effective measures to regulate the performance of the machine including shutdowns if, for example, the oil pressure is too low or the coolant temperature is too high, leading to buildup of heat. In industrial situations, this functionality of control panels is becoming increasingly critical. The microprocessor or microcontroller is embedded in the circuitry inside the control panel and is programmed to take in the sensor input and react to that with the programmed control rules.Control panels can be combined with an Automatic Transfer Switch (ATS) to maintain the continuity of electricalpower. The ATS detects an outage of power when your local grid fails. It signals the control panel to start thegenerator. Depending on the type of generator being used, the control panel may activate glow plugs (for diesel) foran adjustable length of time. It will then start the generator using an automatic starter, similar to the one you engagewhen you turn the keys in the ignition of your car in the morning. As soon as the engine of the generator reaches anoptimum speed, the starter is disengaged. The ATS then switches to the generator power, and you can go back tobusiness as usual, without having to frantically scramble to figure out what caused power loss. This aspect of acontrol panel makes it extremely useful in homes during bad weather and in industrial situations for ensuringmission-critical continuity.