CATHODE RAY OSCILLOSCOPE CRO

What is a CRO?

The cathode ray oscilloscope is an electronic test instrument, it is used to obtain waveforms when the different input signals are given. In the early days, it is called as an Oscillograph.

The oscilloscope observes the changes in the electrical signals over time, thus the voltage and time describe a shape and it is continuously graphed beside a scale. By seeing the waveform, we can analyze some properties like amplitude, frequency, rise time, distortion, time interval and etc.

Block Diagram of CRO

The following block diagram shows the general purpose CRO contraction. The CRO recruit the cathode ray tube and acts as a heat of the oscilloscope. In an oscilloscope, the CRT produces the electron beam which is accelerated to a high velocity and brings to the focal point on a fluorescent screen. Thus, the screen produces a visible spot where the electron beam strikes with it. By detecting the beam above the screen in reply to the electrical signal, the electrons can act as an electrical pencil of light which produces a light where it strikes.

To complete this task we need various electrical signals and voltages. This provides the power supply circuit of the oscilloscope. Here we will use high voltage and low voltage. The low voltage is used for the heater of the electron gun to generate the electron beam. The high voltage is required for the cathode ray tube to speed up the beam. The normal voltage supply is necessary for other control units of the oscilloscope.

The horizontal and vertical plates are placed between the electron gun and the screen, thus it can detect the beam according to the input signal. Just before detecting the electron beam on the screen in the horizontal direction which is in X-axis a constant time-dependent rate, a time base generator is given by the oscillator. The signals are passed from the vertical deflection plate through the vertical amplifier. Thus, it can amplify the signal to a level will be provided the deflection of the electron beam.

If the electron beam is detected in the X-axis and the Y- axis a trigger circuit is given for the synchronizing these two types detections. Hence the horizontal deflection starts at the same point of the input signal.

Working of CRO

The following circuit diagram shows the basic circuit of a cathode ray oscilloscope. In this, we will discuss important parts of the oscilloscope.

Vertical Deflection System

The main function of this amplifier is to amplify the weak signal so that the amplified signal can produce the desired signal. To examine the input signals are penetrated to the vertical deflection plates through the input attenuator and number of amplifier stages.

Horizontal Deflection System

The vertical and horizontal system consists of horizontal amplifiers to amplify the weak input signals, but it is different to the vertical deflection system. The horizontal deflection plates are penetrated by a sweep voltage that gives a time base. By seeing the circuit diagram the sawtooth sweep generator is triggered by the synchronizing amplifier while the sweep selector switches in the internal position. So the trigger saw tooth generator gives the input to the horizontal amplifier by following the mechanism.Here we will discuss the four types of sweeps.

Recurrent Sweep

As the name, itself says that the saw tooth is respective that is a new sweep is started immodestly at the end of the previous sweep.

Triggered Sweep

Sometimes the waveform should be observed that it may not be predicted, thus the desired that the sweep circuit remains inoperative and the sweep should be initiated by the waveform under the examination. In these cases, we will use the triggered sweep.

Driven Sweep

In general, the drive sweep is used when the sweep is a free running but it is a triggered by the signal under the test.

Non-Saw Tooth Sweep

This sweep is used to find the difference between the two voltages. By using the non-sawtooth sweep we can compare the frequency of the input voltages.

Synchronization

The synchronization is done to produce the stationary pattern. The synchronization is between the sweep and the signal should measure. There are some sources of synchronization which can be selected by the synchronization selector. Which are discussed below.

Internal

In this the signal is measured by the vertical amplifier and the trigger is abstained by the signal.

External

In the external trigger, the external trigger should be present.

Line

The line trigger is produced by the power supply.

Intensity Modulation.

This modulation is produced by inserting the signal between the ground and cathode. This modulation causes by brightening the display.

Positioning Control

By applying the small independent internal direct voltage source to the detecting plates through the potentiometer the position can be controlled and also we can control the position of the signal.

Intensity Control

The intensity has a difference by changing the grid potential with respect to the cathode.

Applications of CRO

• Voltage measurement
• Current measurement
• Examination of waveform
• Measurement of phase and frequency

Uses of CRO

In laboratory, the CRO can be used as

• It can display different types of waveforms
• It can measure short time interval
• In voltmeter, it can measure potential difference
  

  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.

  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 V_m 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 (V_RMS) read on the AC voltmeter is

  V_RMS = 0.707 V_m (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 V_m may be taken as 1/2 the peak to peak signal V_pp

  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).

  • Measure short time interval: A CRO can be used to measure time interval between the pulses in a signal which are displayed on the screen. By counting the units of separation of the pulses and multiplying it with the set time/div reading of the knob, one can precisely state the time interval
  
  • Measure potential difference (as a voltmeter): A CRO provides infinite (practically very high) input impedance just a usual voltmeter while measuring the potential difference. The only difference between them is that the CRO utilizes its screen to display the nature of the voltage being measured, which enables the user to observe the characteristics of the signal.
  
  • Current measurement: Voltage measurements can be used to calculate current flow through a device or through any portion of a circuit. From Ohm’s Law[2] :
    E = I x R
    Where E = electromotive force in volts
    I = current in amps
    R = resistance in ohms
    Solving for I:
    I = E/R
  
  • Examination of Waveform: Once, the corresponding signal has been measured and displayed, it can be analyzed to obtain its various parameters such as peak-peak voltage, this period et