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 accurate time and aplitude measurements of voltage signals over a wide range of frequencies. Its reliability, stability, and ease of operation make it suitable as a general purpose laboratory instrument. The heart of the CRO is a cathode-ray tube shown schematically in Fig. 1.

   The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) and accelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflections allows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us to write 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 the vertical (deflection) plates to deflect the beam vertically and at the same time a voltage that increases linearly with time 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. The horizontal axis serves as a uniform time scale.

          The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep generator that is incorporated in the oscilloscope circuitry. The voltage output of such a generator is that of a sawtooth wave as shown in Fig. 2. Application of one cycle of this voltage difference, which increases linearly with time, to the horizontal plates causes the beam to be deflected linearly with time across the tube face. When the voltage suddenly falls to zero, as at points (a) (b) (c), etc...., the end of each sweep - the beam flies back to its initial position. The horizontal deflection of the beam is repeated periodically, the frequency of this periodicity is adjustable by external controls.

          To obtain steady traces on the tube face, an internal number of cycles of the unknown signal that is applied to the vertical plates must be associated with each cycle of the sweep generator. Thus, with such a matching of synchronization of the two deflections, the pattern on the tube face repeats itself and hence appears to remain stationary. The persistance of vision in the human eye and of the glow of the fluorescent screen aids in producing a stationary pattern. In addition, the electron beam is cut off (blanked) during flyback so that the retrace sweep is not observed.

CRO Operation:  A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, the instrument is operated in the following manner. The signal to be displayed is amplified by the vertical amplifier and applied to the verical deflection plates of the CRT. A portion of the signal in the vertical amplifier is applied to the sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with a selected point in the cycle of the triggering signal. This pulse turns on the sweep generator, initiating the sawtooth wave form. The sawtooth 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 for triggering. Also the sweep generator may be bypassed and an external signal applied directly to the horizontal amplifier.

Cro Description:-

Display and general external appearance

The 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 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.

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 for the 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 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.

The trigger section controls the start event of the sweep. The trigger can be set to automatically restart after each sweep or it can be configured to respond to an internal or external event. The principal controls of this section will be the source and coupling selector switches. An external trigger input (EXT Input) and level adjustment will also be included.

CRO Controls

          The controls available on most oscilloscopes provide a wide range of operating conditions and thus make the instrument especially versatile. Since many of these controls are common to most oscilloscopes a brief description of them follows.

CATHODE-RAY TUBE

Power 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 SECTION

Position:  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 the sensitivity is calibrated only when the variable knob is in the fully clockwise position.

AC-DC-GND:  Selects desired coupling (ac or dc) for incoming signal applied to vertical amplifier, or grounds the amplifier input. Selecting dc couples the input directly to the amplifier; selecting ac send the signal through a capacitor before going to the amplifier thus blocking any constant component.

HORIZONTAL-SWEEP SECTION

Sweep 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.

TRIGGER

The 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 triger

Level:  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 OSCILLOSCOPE

Vertical Input:  A pair of jacks for connecting the signal under study to the Y (or vertical) amplifier. The lower jack is grounded to the case.

Horizontal Input:  A pair of jacks for connecting an external signal to the horizontal amplifier. The lower terminal is graounted 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 the amplifiers.

          Accuracy of the vertical deflection is + 3%. Sensitivity is variable.

Bandwidth

Bandwidth is a measure of the range of frequencies that can be displayed; it refers primarily to the vertical amplifier, 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)

or by the sampling rate of the analog to digital converter in digital instruments. The bandwidth is defined as 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 bandwidth approximately:

Bandwidth in Hz x rise time in seconds = 0.35 

For example, an oscilloscope intended to resolve pulses with a rise time of 1 nanosecond would have a bandwidth of 350 MHz.

For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten times the highest frequency desired to resolve; for example a 20 megasample/second rate would be applicable for measuring 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 1

Plug line cord into a standard ac wall recepticle (nominally 118 V). Turn power on. Do not advance the Intensity Control.

Allow the scope to warm up for approximately two minutes, then turn the Intensity Control until the beam is visible on the screen.

          

WARNING:   Never advance the Intensity Control so far that an excessively bright spot appears. Bright spots imply burning of the screen. A sharp focused spot of high intensity (great brightness) should never be allowed to remain fixed 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 the vertical input of the oscilloscope. Establish a steady trace of this input signal on the scope. Adjust (play with)all of the 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 making measurements in other experiments and not as a formidable obstacle. Note: If the vertical gain is set too low, it may not be possible to obtain a steady trace.

II. Measurements of Voltage:  Consider the circuit in Fig. 4(a). The signal generator is used to produce a 1000 hertz sine wave. The AC voltmeter and the leads to the verticle input of the oscilloscope are connected across the generator's output. By adjusting the Horizontal Sweep time/cm and trigger, a steady trace of the sine wave may be displayed on the screen. The trace represents a plot of voltage vs. time, where the vertical deflection of the trace about the line of symmetry CD is proportional 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 is designed to read the dc "effective value" of the voltage. This effective value is also known as the "Root Mean Square value" (RMS) value of the voltage.

          The peak or maximum voltage seen on the scope face (Fig. 4b) is Vm volts and is represented by the distance from the symmetry line CD to the maximum deflection. The relationship between the magnitude of the peak voltage displayed 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 a symmetric wave (sine or cosine) the value of Vm may be taken as 1/2 the peak to peak signal Vpp

The 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 the number of divisions and multiplying by the scale factor. However, you can figure out what the new calibration is an use 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 will not indicate the correct voltage when used to measure non-sinusoidal signals.

III. Frequency Measurements:  When the horizontal sweep voltage is applied, voltage measurements can still be taken 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 be determined as reciprocal of the periods.

          Set the oscillator to 1000 Hz. Display the signal on the CRO and measure the period of the oscillations. Use the 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. Measure the distance (and hence time) for 5 complete cycles and calculate the frequency from this measurement. Compare you 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 vertical amplifiers a stationary pattern is formed on the CRT when the ratio of the two frequencies is an intergral fraction such as 1/2, 2/3, 4/3, 1/5, etc. These stationary patterns are known as Lissajous figures and can be used for comparison measurement of frequencies.

          Use two oscillators to generate some simple Lissajous figures like those shown in Fig. 5. You will find it difficult to maintain the Lissajous figures in a fixed configuration because the two oscillators are not phase and frequency locked. Their frequencies and phase drift slowly causing the two different signals to change slightly with respect to each other.

V. Testing what you have learned:  Your instructor will provide you with a small oscillator circuit. Examine the input to the circuit and output of the circuit using your oscilloscope. Measure such quantities as the voltage and frequence of the signals. Specify if they are sinusoidal or of some other wave character. If square wave, measure the frequency of the wave. Also, for square waves, measure the on time (when the voltage is high) and off time (when it is low).

Examples of use:-

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 the advantages of a scope is that it can graphically show signals: where a voltmeter may show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other cases the precise shape or timing of a pulse is important.

In a piece of electronic equipment, for example, the connections between stages (e.g. electronic mixers, electronic oscillators, amplifiers) may be 'probed' for the expected signal, using the scope as a simple signal tracer. If the expected signal is absent or incorrect, some preceding stage of the electronics is not operating correctly. Since most failures occur because of a single faulty component, each measurement can prove that half of the stages of a complex 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 has failed. 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 of design errors, bad voltage levels, electrical noise etc. Digital electronics usually operate from a clock, so a dual-trace scope which shows both the clock signal and a test signal dependent upon the clock is useful. Storage scopes are helpful for "capturing" rare electronic events that cause defective operation.

Function generator

Definition:- A function generator is usually a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. Some of the most common waveforms produced by the function generator are the sine, square, triangular and sawtooth shapes. These waveforms can be either repetitive or single-shot (which requires an internal or external trigger source). Integrated circuits used to generate 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-min

frequency ratios of 10:1 or more, a wide range

of frequencies from a few Hz to a few MHz, a flat output amplitude and modulation capabilities like frequency sweeping, frequency modulation and amplitude modulation.

Although function generators cover both audio and RF frequencies, they are usually not suitable for applications that need low distortion or stable frequency signals. When those traits are required, other signal generators would be more appropriate.

Uses:- Function generators are used in the development, test and repair of electronic equipment. For example, they may be used as a signal source to test amplifiers or to introduce an error signal into a control loop.

Working

Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps.[4] This triangular wave is used as the basis for all of its other outputs. The triangular wave is generated by repeatedly charging and discharging a capacitor from a constant current source. This produces a linearly ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the charging and discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequencies may be obtained. Sawtooth waves can be produced by charging the capacitor slowly, using a current, but using a diode over the current source to discharge quickly - the polarity of the diode 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 a non-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave by rounding off 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 not function generators in the strict sense since they typically produce pure or modulated sine signals only.

Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes 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 digital synthesis (DDS) techniques to generate any waveform that can be described by a table of amplitudes.

Specifications

Typical 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;

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 generator's 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 o'clock position (90 degrees right), the nine o'clock position (90 degrees left), or the twelve o'clock 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 in decibels. 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 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 electrical power. The ATS detects an outage of power when your local grid fails. It signals the control panel to start the generator. Depending on the type of generator being used, the control panel may activate glow plugs (for diesel) for an adjustable length of time. It will then start the generator using an automatic starter, similar to the one you engage when you turn the keys in the ignition of your car in the morning. As soon as the engine of the generator reaches an optimum speed, the starter is disengaged. The ATS then switches to the generator power, and you can go back to business as usual, without having to frantically scramble to figure out what caused power loss. This aspect of a control panel makes it extremely useful in homes during bad weather and in industrial situations for ensuring mission-critical continuity.