In this article we will discuss about:- 1. Introduction to C-R Oscillograph 2. Construction of CR Tube Used in C-R Oscillograph 3. Power Supply 4. Time Base or Sweep Circuits 5. Frequency and Phase Measurements.
Contents:
- Introduction to C-R Oscillograph
- Construction of CR Tube Used in C-R Oscillograph
- Power Supply for C-R Oscillograph
- Time Base or Sweep Circuits Used in C-R Oscillograph
- Frequency and Phase Measurements of C-R Oscillograph
1. Introduction to C-R Oscillograph:
Now-a-days perhaps, all of us are familiar with at least one type of cathode-ray tube used in the television picture tube known as kinescope. However, cathode-ray tubes are of many types, sizes and shapes. The modern cathode-ray tube is, in fact, an important modification of the Crookes tube discovered in 1879.
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The CR tube is now widely used in laboratory to study the electric circuit phenomena. It is also extensively used as a viewing device for the reception of radar signals and television images.
The CR tube is, in general, used in an assemblage of apparatus called a cathode-ray oscillograph; the essential components of which are:
(i) Cathode-ray tube,
(ii) Deflection voltage amplifiers,
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(iii) Power supplies, and
(iv) Time base circuit.
2. Construction
of CR Tube Used in C-R Oscillograph:
A cathode-ray tube, has the following main components:
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i. The electron gun for producing and focussing the emitted electrons in a narrow beam,
ii. A deflection system, for deflecting the electron beam either electrostatically or magnetically, and
iii. A fluorescent screen, upon which the electron beam impinges to create a visible light spot.
A tube base is there to which connections from the elements inside the tube are made. The components of a CR tube are mounted carefully inside a highly evacuated glass envelope.
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In an electrostatically focussed CR tube, the electron gun consists of- (a) an indirectly heated cathode, (b) a control grid, (c) m accelerating electrode, (d) a first focussing anode and (e) a second, or final accelerating anode.
The cylindrical cathode is usually coated with barium and strontium oxides for providing a plentiful supply of electrons. By a tungsten heater wire this cathode is indirectly heated. The control grid is a cylinder with a tiny aperture at its centre and is kept enveloping the cathode.
Adjacent to the control grid there are first and second anodes, followed by the accelerating grid, which are also cylindrical in shape with small apertures at the centre.
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The indirectly heated cathode emits a stream of electrons whose density is controlled by the bias voltage applied between the control grid and the cathode. In order to focus the beam of electrons properly, the voltage applied on the first anode is utilized. The second anode which is kept at a high positive d.c. voltage relative to the cathode, accelerates the electron beam.
The anode potentials in a CR tube are adjusted by considering the required beam power. The first anode is always kept at an appreciably lower potential than the second one. In television camera tube of iconoscope type the first anode potential is of the order of 300 volts while that of the second anode potential is 1000 volts.
If the beam of electrons coming from the electron gun is not deflected it would form a luminous spot of light at the centre of the fluorescent screen. Though the control grid is usually used to control the intensity of this spot, but it is also dependent on the fluorescent material. The fluorescent material coated inside the tube is termed as phosphor. Some commonly used phosphors are- zinc silicate, zinc oxide, zinc sulphide, etc.
The beam of electrons in a CR tube is deflected by two different means:
(i) Electrostatic Deflection:
The electrostatic deflection sensitivity is the amount of deflection of electron spot produced on the screen when a voltage of one volt from a d.c. source is applied to the deflection plates. The unit of deflection sensitivity is then expressed in inch/volt or mm/volt.
The geometry of the electrostatic deflection system is shown in Fig. 9.3. In the figure, Vd is the potential applied between two plates, each of length l and spacing S. Let the plates be kept at a distance D from the screen and on entering the deflection plates let an electron move with a velocity v.
For a given CR tube, l, L and S are fixed and so varying the voltage applied to the plates, Vd, or the final anode voltage Va, one may alter the deflection spot d. It is also seen from equation (9.9) that the deflection sensitivity is inversely proportional to the final anode voltage.
(ii) Magnetostatic Deflection:
The magnetic deflection sensitivity is defined as the amount of deflection of electron spot produced when one milliampere current flows through the deflection coil. The unit of deflection sensitivity is then expressed in inch/mA or mm/mA. In this system the magnetic field is produced by passing current through the coil, whose axis is kept normal to the direction of the electron beam.
The configuration of the magnetic deflection system is shown in Fig. 9.5. As shown in the figure, the electron travels in an arc MN of a circle inside the magnetic field and thereafter it moves in a straight line.
Thus, the total deflection of the spot can be divided into two parts:
(i) Deflection within the magnetic field, d1 and
(ii) Deflection outside the field, d2.
Equation (9.19) shows that the deflection sensitivity is directly proportional to the square root of the ratio of the charge to mass of an electron but inversely proportional to the square root of the final anode potential.
3. Power Supply
for C-R Oscillograph:
In a CR tube the final anode potential is generally of the order of 1000 to 2000 volts. In television and radar circuits, however, much higher anode potentials are required and so the power supply in such cases must be designed accordingly.
In practice the final anode is often kept at the earth potential while a negative voltage is applied to the other electrode of the CR tube. This avoids the danger of electrical shock which may come if the final anode is kept at a high positive voltage as this terminal is frequently required to be handled during the study of waveforms, etc., using a CRO.
4. Time Base or Sweep Circuits
Used in C-R Oscillograph:
On the front panel of a cathode-ray oscillograph there are various d.c. voltage controls which are named as:
(i) Focus control,
(ii) Intensity control,
(iii) Horizontal position control, and
(iv) Vertical position control.
Using these controls the beam is first focussed to a spot and then it can be placed at any desired position on the screen.
In addition to all these d.c. voltage controls, a source of sweep voltage is internally provided. The waveform of this sweep generator output is of the shape of saw-tooth, as shown in Fig. 9.6, and so this voltage is termed as saw-tooth voltage. As seen from the figure, the voltage during each cycle increases linearly with time up to ti and then falls to zero in a shorter time tƒ.
Using a switch the saw-tooth generator may be connected to the horizontal deflecting plates. In the most common form, since the deflection of the spot in the horizontal direction (or vertical or sometimes circular) is proportional to time, it is called time base and the retracing time tf is known as the flyback time.
Some common types of time base circuits are:
(i) Neon time base,
(ii) Puckle’s hard valve time base,
(iii) Thyratron time base, etc.
The property of neon tube is applied in a neon time base circuit, tube conducts at a definite potential called the striking potential. But, if that potential is gradually reduced, a stage is reached called extinguishing potential, when the conduction by ionisation ceases. This behaviour is important in the operation of neon time base. A basic neon time base circuit is shown in Fig. 9.7.
In the figure, a voltage source Vd is connected with a high resistance R and a capacitor C. When the battery circuit is made on, the voltage Vc across the capacitor increases exponentially with time by a relation-
Equation (9.21) shows that the voltage across the condenser Vc is approximately proportional to the time. The value of Vc depends on the time constant CR of the charging circuit. When the value of C.R is low, the rate of charging becomes more rapid. At the striking potential neon tube starts to conduct and its resistance decreases to a very small value, say r ohms.
The condenser C then discharges exponentially through the neon tube with a time constant Cr. The charging and discharging curves of a condenser is shown in Fig. 9.8. Since the value of r is small, the discharge of condenser becomes more rapid.
The time period T of a neon time base potential is the sum of time of charge ti and time of discharge (flyback interval) tƒ. Referring to Fig. 9.8, the extinguishing potential Ve, at time ti is given by-
The sweep-voltage curve departs considerably from linearity in a neon time base circuit. This is achieved by utilizing the constant current property of a pentode.
5. Frequency and Phase Measurements
of C-R Oscillograph:
If suitable alternating voltages are applied to the two sets of deflection plates of a CRO, various patterns may be obtained on the screen in the form of closed loop or straight line. These figures are known as ‘Lissajous Figures’. If vx and vy are the two instantaneous voltages applied to the x and y plates, we can write-
Where Vx and Vy are the voltage amplitudes; ωx, ωy are the angular frequencies and α is the phase angle for vy voltage. By adjusting all these parameters the so-called ‘Lissajous Figures’ are obtained on the screen.
In order to measure the frequencies of an alternating voltage, it is applied to one set of deflection plates while at the other set sinusoidal voltage is applied from a variable frequency standard oscillator which is properly calibrated.
The oscillator is allowed to work for some- time to get a stabilized temperature and then the frequency of this oscillator is altered until a single loop stationary pattern is obtained. Now, the frequency of the oscillator voltage is noted from the calibrated dial of the oscillator which is equal to the frequency of the alternating voltage applied.
To measure relative phase angle of two alternating voltages of identical amplitude and frequency, they are applied to the x and y deflection plates of a CRO. Usually an ellipse is obtained on the screen from which values of P and Q are noted, as illustrated in Fig. 9.10, and then the phase angle a is calculated by using the relation-
α = sin-1 (P/Q) … (9.33)
