Instrumentation and Measurements Eng. Ahlam Damati

Oscilloscopes

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Introduction:

The cathode Ray Oscilloscope or mostly called as CRO, shown in Figure.1, is an electronic device used for giving the visual indication of a signal waveform. It displays the amplitude of electrical signals as a function of time.

Fig.1: Block diagram of a CRO

The major subsystem of a general purpose CRO consists of the following

1. Cathode ray tube2. Vertical amplifier3. Delay line4. Time base circuit5. Horizontal amplifier6. Trigger circuit7. Power supply.

Functions of the components:

1. Cathode Ray Tube     It is the heart of the oscilloscope. It generates the electron beam, accelerates it to high velocity, and deflects it to create an image on the phosphor screen of the CRT. A visual signal is displayed on the CRT.

2. Vertical Amplifier     The vertical amplifier receives the input from the signal which is to be measured. It then amplifies the

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signal and supplies it to the vertical deflection plates. The input signals are amplified by the vertical amplifier.

3. Delay Line    As the name suggests, this circuit is used to delay the signal for a period of time in the vertical section of CRT. The input signal is not applied directly to the vertical plates because the part of the signal gets lost, when the delay time not used. This is because of all electronic circuitry in the oscilloscope (attenuators, amplifiers, pulse shapers, generators…); therefore, the input signal is delayed by a period of time (on order of 80 ns or so).

4. Time Base GeneratorThe saw tooth voltage produced by the time base circuit is required to deflect the beam in the horizontal section. The spot is deflected at a constant time dependent rate.

5. Horizontal Amplifier   The horizontal amplifier generates the signal which provides voltage to horizontal deflection plates. The horizontal deflection plates deflect the beam along the horizontal direction. This is helpful to create the waveform along with the time domain. The saw tooth voltage produced by the time base circuit is amplified by the horizontal amplifier before it is applied to horizontal deflection plates.

6. Trigger Circuit    This circuit synchronizes the horizontal deflection with the vertical input signal each time it sweeps.

7. Power supply:    The voltages required by CRT, horizontal amplifier and vertical amplifier are provided by the power supply block. Power supply block of oscilloscope is classified in to two types:

(1) Negative high voltage supply. The voltages of negative high voltage supply is from -1000V to -1500V.

(2) Positive low voltage supply. The range of positive voltage supply is from 300V to 400V.

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Cathode Ray Oscilloscope Principles

Figure 2 shows the structure, and the main components of a cathode ray tube (CRT). Also it shows the face plane of the CRO screen.

Fig.2: the main components of a cathode ray tube (CRT)

Electron beam generated by the electron gun first deflected by the deflection plates, and then directed onto the fluorescent coating of the CRO screen, which produces a visible light spot on the face plane of the oscilloscope screen.

A detailed representation of a CRT is given in Figure 3

Fig.3: A detailed representation of a CRT

The CRT is composed of two main parts:

1. Electron Gun:

Electron Gun provides a sharply focused electron beam directed toward the fluorescent-coated screen. The thermally heated cathode emits electrons in many directions. The control grid controls the number and speed of electrons in the beam. The momentum of the electrons determines the intensity, or brightness, of the light emitted from the fluorescent coating due to the electron bombardment. Since the electron beam consists of many electrons, the beam tends to

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diverge. This is because of the electrons repulsion of each other. To compensate for such repulsion forces, an adjustable electrostatic field is created between two cylindrical anodes, called the focusing anodes. The variable positive voltage on the second anode cylinder is used to adjust the focus or sharpness of the bright spot.

2. Deflection System

The deflection system consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. One of the plates in each set is permanently connected to the ground (zero volt), whereas the other plate of each set is connected to input signals or triggering signal of the CRO.

As shown in Figure 3, the electron beam passes through the deflection plates. In reference to the schematic diagram in Figure 2, a positive voltage applied to the Y input terminal causes the electron beam to deflect vertically upward, due to attraction forces, while a negative voltage applied to the Y input terminal causes the electron beam to deflect vertically downward, due to repulsion forces. Similarly, a positive voltage applied to the X input terminal will cause the electron beam to deflect horizontally toward the right, while a negative voltage applied to the X input terminal will cause the electron beam to deflect horizontally toward the left of the screen.

In order to find out the expression for the deflection, let us consider a system as shown below in Figure 4.

Fig.4: Electrostatic deflection

The deflection (D) on the screen (in meters) is given by:

D=L ldEd2d Ea

Where,

L: the distance from the center of deflection plates to screen

ld : the effective length of deflection plates

d: the distance between deflection plates

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Ed : the deflection voltage

Ea : the accelerating voltage

The deflection sensitivity (S) is defined as:

S= DEd

=Lld

2d Ea

S here is measured in m/V, typically 0.1mm/V to 1mm/V

Example:

A CRT has an anode voltage of 2000 V and parallel deflecting plates 2 cm long and 5 mm apart. The screen is 30 cm from the center of the plates. The input voltage is applied to the deflecting plates through amplifier having an average gain of 100. Find:

a) The deflection sensitivity of the CRT b) The input voltage required to deflect the beam by 3 cm

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Time Base Generator (Sweep Generator)

Generally, it is required to display a periodical voltage waveform as a function of time. By applying the voltage to be displayed on the CRO, to the vertical deflection plates, the

vertical deflection of the beam spot will be proportional to the magnitude of this voltage. It is then necessary to convert the x axis (horizontal deflection) into a time axis. If the signal to be observed is periodic, then a periodic voltage waveform that varies

linearly with time, as shown in figure 5 below, is applied to the horizontal deflection plates. This type of waveform is called the saw tooth waveform. When s(t) is zero volt, the bright spot is at the extreme left-hand position, and when s(t) is

maximum, the bright spot is at the extreme right position. Therefore, the bright spot travelsfrom extreme left to extreme right in a time equal to the sweep time T s.

During the retrace time T r, which is usually very short compared to trace/sweep time, The electron beam is turned off to prevent retrace path from being displayed.

Fig.5: Typical Saw tooth waveform applied to the horizontal deflection plates

Trigger Circuit (Sweep Synchronization):

Suppose the input Vy(t) and s(t) shown in Figure 6 are applied to the vertical and horizontaldeflection plates of the CRO respectively. Note that, at the beginning of each sweep cycle,

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(i.e when the bright spot is at extreme left) Vy(t) gets exactly the same value (The pointsindicated by red circles). Therefore the bright spot is following exactly the same path in eachsweep cycle. Thus, we can observe a steady waveform on CRO screen. Notice that the timebetween the beginning of two consecutive sweep cycles is a multiple of input signal period.(τ = n T. T, as shown in figure 6, n is a positive integer.)

Fig.6: Steady waveform

Figure 7 shows an unsteady waveform. The bright spot is following different paths in different sweep cycles; therefore we cannot obtain a steady waveform on CRO screen.In order to obtain stable and stationary waveform displays, the saw tooth signal should be applied to the horizontal deflection plates, in synchronism with the waveform being displayed.

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Fig.7: Waveform is not steady

The trigger signal is initiated when two conditions are met:

1. When the input signal reaches a certain voltage value determined by level control.2. When the input signal has either positive/negative slope determined by slope control.

So, after the end of one sweep cycle, the time base generator waits until the input signal reaches the specified level of voltage and the specified slope to start the next sweep cycle.

Measurement TechniquesThe oscilloscope displays the voltage value of the waveform as a function of time. The oscilloscope screen is partitioned into the grids, which divides both the horizontal axis (time) and the vertical axis (voltage) into divisions which will be helpful in making the measurements as shown in Figure 8.

Fig.8: Oscilloscope screen

In order to know the time or the voltage values corresponding to each division to make accurate calculation, these values are determined by two variables: the time/div and the volt/div which can be adjusted from the relevant buttons available on the front panel of the oscilloscope. Typical quantities, which are of primer interest when observing a signal with the scope, areshown in Figure 9.

Fig.9: Sinusoidal signal on Oscilloscope screen

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The phase difference is expressed in terms of radians or degrees. In Dual Mode of the oscilloscope the phase difference can be calculated easily as follows.Given the two signals having the same frequency, as shown in Figure 10

Fig.10: Two signals displayed in dual mode

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The phase difference between the signals can also be determined in XY mode of the oscilloscope. In the XY mode, the x-axis data is taken from one channel, y-axis data is taken from the other. In that way, Channel I vs Channel II graph can be obtained, so that the variation of a signal with respect to another can be observed. Figure 11 shows a typical graph in XY mode, of two signals having a constant phase difference.

Fig.11: Phase difference calculation in XY mode

Figure 12 shows typical graphs in XY mode corresponding to different values of phase difference.

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Fig.12: Different Phase angles in XY mode

Advantages of General Purpose Oscilloscope

1. It provides a graphical display of the amplitude of a signal as a function of time. Hence, it is

   used to measure various electrical parameters.

2. Amplitude of signals like voltage, current, power etc., can be measured by the oscilloscope.

3. Apart from amplitude measurement, it can measure frequency, phase angle, time delay of the

    signal, time between two events, and relative timing of two related signals.

4. It has an advantage over electro-mechanical measuring devices that it can respond very well

    to high frequency signals because it is completely an electronic device.

5. General purpose CRO's are used for maintenance of electronic equipment and laboratory

    work.

6. It can also be used to measure capacitance, inductance, etc

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