Cathode ray oscilloscope (CRO) for voltage (AC AND DC)

AIM: 1. Use of cathode ray oscilloscope (CRO) for voltage (AC AND DC) and frequency measurement (direct method) 2. To determine the frequency of unknown source using Lissajous pattern.

                                            Cathode Ray Oscilloscope (CRO)

         Cathode Ray Oscilloscope (CRO) is a versatile measuring instrument. The electrical parameters associated with ac signals such as voltage, current, frequency, phase etc. can be most conveniently measured using CRO. It not only measures these parameters but also displays the nature of the applied signal. (Analogue and digital meters are incapable of displaying the nature of the applied signal. Moreover, these meters do not measure peak value but they measure RMS values). Apart from the use of CRO for direct measurements of an electrical signal, it can also be used for measuring different physical quantities such as temperature, pressure, light intensity etc. with the help of transducers (the transducer is a device which converts physical quantities into electrical signals). These features make CRO an ideal instrument useful for analysis in different field viz. physical sciences, medical field, and engineering.

APPARATUS: CRO, step down transformer, function generator, chords

FORMULAE:  1. f_x = ( n_y/  n_x ) . f_y                                                                                                    

          f_{x   =}  frequency of the signal applied at  X  i/p

          f_{y  =}   frequency of the signal applied at  Y  i/p

          n_{x  =}  number of points in which horizontal tangent cuts pattern.

          n_{y =}  number of points in which vertical tangent cuts the   pattern

        

 INTRODUCTION

            Cathode Ray Oscilloscope (CRO) is a versatile measuring instrument. The electrical parameters associated with ac signals such as voltage, current, frequency, phase etc. can be most conveniently measured using CRO. It not only measures these parameters but also displays the nature of the applied signal. (Analogue and digital meters are incapable of displaying the nature of the applied signal. Moreover, these meters do not measure peak value but they measure rms values). Apart from the use of CRO for direct measurements of an electrical signal, it can also be used for measuring different physical quantities such as temperature, pressure, light intensity etc. with the help of transducers (the transducer is a device which converts physical quantities into electrical signals). These features make CRO an ideal instrument useful for analysis in different field viz. physical sciences, medical field, and engineering.

             The working of CRO is based on the principle that the electron beam gets deviated from its path if subjected to an external electric field. The extent of deviation depends on the magnitude and direction of the applied field. Thus the electron beam is used as pointer just similar to the mechanical pointer in analogue meters. CRO can be used for high frequency signal measurements also since the electron beam pointer can follow even very rapid changes in ac signal due to its negligible inertial mass.

The different Basic parts of CRO are -

1. Cathode Ray Tube (CRT):

    it is used to generate, accelerate and focuses the electron beam.

2. Horizontal circuits:

      i/p to X deflecting plates is applied through these circuits.

3. Vertical circuits:

       i/p to Y deflecting plates is applied through these circuits.

4. Time Base circuit: true shape of signal is displayed on screen.
5. Trigger circuit: To display the stationary wave pattern, it provides pulse to time base circuit.
6. High voltage power supply: it is supplied to Cathode ray tube (CRT).
7. Low voltage power supply: different circuits like time base, trigger, horizontal and vertical circuits get supply through this.

BLOCK DIAGRAM OF CRO

 

1. BLOCK DIAGRAM OF CRT

                                                                                             

                  

                  

 A CRT consists of three parts:

A. Electron gun

B. Deflection System

C. fluorescent Screen      

A. Electron Gun 

Principle : It makes use of the fact that non-uniform electic field causes bending of the electron path, leading to focusing of electron. Electron gun consist of - 

cathode K, a filament heater F, A control grid G, and Anodes A1,A2,A3.

(i) Cathode: K is a short hollow nickel cylinder and encloses the filament heater F. The front face of cathode is coated with tungsten/barium/strontium oxides. The coating helps thermionic emission of electrons to occur at moderate temperatures of about 700 to 900⁰C

(ii) Control Grid: G is held at negative potential with respect to cathode and controls the number of electron passing through it. Intensity of luminous spot on screen depends on the number of electrons striking the screen. Voltage on grid controls this and hence intensity of luminous spot on screen.                

(iii) A₁: Anode held at high +ve potential, Accelerates the electron.

(iv) A₂: Anode A2 and Grid forms first lens system. It Focuses electron beam to a fine point.

(v)  A₃: Anode causes further acceleration. Further, Anode A2 and A3 forms second lens system, which focuses the electron beam to a fine point on the screen. The focus of the beam is adjusted by varying the positive potential on A2.

B. Deflection System (Electrostatic Type)

(i)  Y: Y-Plates (VDP): Two vertical metal plates parallel to each other.

                                    Cause vertical deflection of the beam.

(ii) X: X-Plates (HDP): Two horizontal metal plates parallel to each other.

                                     Cause horizontal deflection of the beam.

X and Y-Plates are perpendicular to each other and also to the path of electrons.

C. Fluorescent Screen: The interior surface of the front face of the CRT is coated with fluorescent material, which glows (usually with green or blue colour) when electrons strike on it and the path of the electron beam can be visualized. The screen is provided with acquadag coating that avoids automatic fading of display.

Acquadag coating

It is a conducting coating used to complete the electrical circuit. If CRO is operated continuously; intensity of spot on screen automatically   decreases gradually due to two reasons;

1. Electrons striking the screen gives negative charge to the screen. Therefore ,further coming electron beam will be repel by screen, causing decrease in the brightness of the glow. 
2. Also they may produce secondary electrons near screen. Thus cloud of electrons are deposited near the screen tends to charge negatively and repel arriving electrons.

   The electrons therefore must be conducted away from screen. This is done by completing the circuit from screen to cathode using aquadag (graphite) coating. The aquadag coating is connected to anode A₃. The electrons that remain accumulated near screen; are collected by aquadag coating and are returned to cathode through ground. 

2,3. Horizontal circuits and Vertical circuits:

These circuits mainly consist of attenuator and amplifier circuits. The signal to be tested is applied to either X or Y deflecting plates through these circuits. The amplitude of the signal is adjusted according to needs, therefore amplified (increased) before applying to the deflecting plates so that the display is visual. The amplitude is decreased if the signal is too large and it is increased if the signal is too weak.

4. Time Base Circuit

Time base circuit consists of time base generator which is a variable frequency oscillator which produces ramp voltage as shown below.

 This circuit is used to obtain the true shape of the signal on the screen. It generates a ramp voltage that increases linearly with time from

 -V_max to +V_max and suddenly drops to again –V_max as shown in fig. This process is repeated. Due to its resemblance with the shape of teeth of a saw; it is also called saw tooth wave.

 This voltage is applied to X plate internally when required. If it is applied to X plate; the spot sweeps screen continuously from left to right and suddenly flies back when the voltage suddenly changes to –V_max. Hence this voltage is also called sweep voltage.

Time taken by sweep voltage to reach to +V_max from –V_min is called trace time or sweep time  and time taken to reach from +V_max to –V_min is called retrace time or fly back time. Thus total time period of sweep voltage is given as

T = T_trace+ T_retrace.

However T_retrace is negligible, so T ≈    T_trace.

Thus X-axis of the screen not only denotes the amount of horizontal deflection but also the time elapsed. Due to this reason, the ramp voltage is also called as time base.

     Blanking: The retrace path of the electron is not displayed on the screen otherwise it will give bad visual effect. Therefore, retrace time should be made equal to zero.  This is done by applying very high –ve voltage pulse to control grid during T_retrace.

4. Trigger Circuit

Display on the screen continuously moves from left to right if inputs to X and Y deflecting plates do not operate in synchronization (i.e. if they do not start at same instance).

To display a stationary wave pattern on the CRO screen, the horizontal deflection should start at the same point of the input signal in each sweep cycle. When such happen it is said that horizontal sweep voltage is synchronized with input signal.

Trigger circuit is used to provide a pulse to time base circuit so that it starts at the same instance as that of signal at Y i/p. The signal at Y is synchronized with sweep at X when its frequency equals to or is integral multiple of sweep frequency i.e. 

F_sweep   = n F_signal  or T_sweep = n T_signal 

Part of the signal voltage is fed to trigger circuit. Trigger circuit generates a pulse at predetermined level of the i/p signal. This pulse then operates time base circuit. Thus sweep voltage is not developed until the time base circuit does not get pulse from trigger circuit.

THEORY

 1. Determination of unknown frequency:

             The method used here is based on principle of superposition of two orthogonal simple harmonic waves. Well-defined closed loop patterns called Lissajous figures are formed due to superposition. The nature of pattern depends on the ratio of frequencies and phase difference of two signals. When the tangents are drawn to this pattern in horizontal and vertical directions; the ratio of number of points in which they cut the pattern gives the ratio of the applied frequencies. If n_x and n_y   are the number of points along X and Y directions; then the ratio nx / n_y = f_y / f_{x .;} from which unknown frequency can be calculated.

To visualise the patterns on the CRO screen, the two signals are applied at X & Y inputs of CRO.

FRONT PANEL CONTROLS OF CRO:

S.N	Control  knob	function of knob & initial setting if any
1.	power	Switches on the power supply
2.	intensity	Controls brightness of the display on the screen. Should be kept preferably at middle position.
3.	focus	Controls the sharpness of the display. Should be kept preferably at middle position.
4.	ch1	Signal is connected to Y deflecting plates.
5.	ch2	Signal is connected to X deflecting plates
6.	hor-pos	Controls the horizontal position of display.
7.	ver-pos	Controls the vertical position of display.
8.	volt/div (2knobs for X and Y i/ps)	select voltage  sensitivity range for both X and Y inputs from 10mv/div to 5v/div. Should be preferably at 1v/div.
9.	ac-dc-gnd (2 knobs for X and Y i/ps)	Connects desired input to X or Y deflecting plates. if kept at `ac’,connects ac signal; if kept at ‘dc’, connects dc signal; if kept at `gnd’, disconnects the input.
10.	(time/div)/XY	if kept at `time/div’ sweep signal gets connected to  X plate internally. True waveform of signal applied at Y i/p is displayed. Selects sweep signal speed from 5µsec/div to 0.1sec/div. Should be preferably at 1msec/ div. If kept at X-Y: connects input to X plates externally.
11.	variable	selects speed of the sweep voltage. In `CAL’ (fully anticlockwise) position speed is equal to the selected speed. In fully clockwise position, the speed is approx. 2.5 times of the selected speed.
12.	Calibrator 0.5V	Provides a square wave of 0.5V Vp-p the at line frequency.
13.	mode	selects the vertical display mode if kept at: 1. Ch1: trigger signal is automatically applied to Ch1. 2. Ch2(X-Y): trigger signal is applied to Ch2.       It is used for X-Y operation. 3. Dual: for simultaneous ch1, ch2 operation. Triggersignal  gets connected to ch1 automatically hence triggering is done with the signal applied at Ch1. Initially should be set at Ch1.

PROCEDURE 

1.      Measurement of DC Voltage

1. The DC power supply is connected to Y input of CRO taking care that positive lead of the cable is  connected to +ve terminal and negative to the –ve of the dc power supply.

2.      By setting the Volt/div  knob to 10V/div , the dc power supply is switched ON. Obtain a sufficiently large display of signal (vertical line) on the CRO screen by varying the power supply voltage.

3.      The vertical length of the waves is read on the graphic scale of the screen.

4.       This reading (in div.) is multiplied by the volt/div knob reading to give DC voltage.

   2. Measurement of AC voltage

1. The sine wave (50 Hz) is supplied to the Y input of CRO using function generator.

 2. By adjusting the Volt/div  knob , obtain a sufficiently large display of signal on the

     CRO screen.

 3.  The vertical length of the waves from the negative maximum to the positive maximum is read on the graphic scale of the screen.

  4. This reading (in div.) is multiplied by the volt/div knob reading to give peak to peak voltage V_p-p.

  5. The   voltage V_p-p  is divide by 2 to give peak ac voltage of the signal.

3. Determination of frequency (Direct Method)

1. The sine wave (50 Hz) is given to the Y input of CRO using function generator.

2. By adjusting the time/div knob , obtain a sufficiently large display of signal on

     the CRO screen. Note down the reading.

3. Measure the width of one full wave (in divisions).

4. Multiply this measured division with reading of time /div knob(ms/div).

    This gives time period of applied signal.

5. Reciprocal of time period will be the frequency of the applied signal.

4.Determination of frequency (Lissajious Figure)

1.      Make connections as shown in block diagram shown in Fig.1.

2.      Set function generator for obtaining sine wave. Adjust frequency to 50 Hz. Connect output of function generator to Y i/p of CRO.

3.      Set the CRO for operation as per the panel control settings as described above.

4.      Connect secondary of step down transformer to X i/p of CRO.

5.      After getting connections checked by the teacher, put ON the CRO, function generator and transformer.

6.      Keep the `mode’ control knob on `dual’ position so that two sign waves are obtained.

7.      Change the mode’ selector to `X-Y’ mode so that an ellipse or a circle is obtained.