The cathode ray oscilloscope (commonly abbreviated as CRO) is an electronic device that is capable of giving a visual indication of a signal waveform. A CRO is a versatile instrument that can be used to measure voltage, time intervals, and the phase angle between two sinusoidal voltages of the same frequency. It is also used as a visual display of waveforms, and In more advanced CROs to display signals that occur only for short intervals of time. Although the size and shape are different, the operating principle is the same Inside the tube is a vacuum.
In a true sense, the cathode ray oscilloscope has been one of the most important tools in the design and development of modern electronic circuits. The CRO is also used as a troubleshooting tool and it acts like an 'Eye' for observing the various faults in electronic equipment for servicemen.
The oscilloscope can be used to :
1. Display the wave shape.
2. Measure the frequency.
3. Measure the peak-to-peak amplitude.
4. Phase difference measurement.
5. Pulse width measurement.
6. Delay time measurement.
7. Measurement of amplitude.
8. Measurement of current.
9. Measure
BLOCK DIAGRAM OF CATHODE RAY OSCILLOSCOPE(CRO)
The CRO consists of the following:
1. Cathode ray tube (CRT).
2. Vertical amplifier.
3. Delay line.
4. Trigger circuit.
5. Time-based generator.
6. Horizontal amplifier.
7. Power supply.
1. Cathode Ray Tube: It is the heart of the CRO. When the electrons emitted by the electron gun and passes through the two pairs of deflection plates, strike the phosphor screen of the CRT, a visual signal is displayed on the CRT.
2. Vertical Amplifier: The input signal is applied to the vertical amplifier, It is used to amplify the applied input signal. It consists of more than one stage to increase the gain. The amplified signal is applied to the vertical deflection plates through the Delay line.
3. Delay Line: Delay Line is the name suggests that 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 part of the signal gets lost when the delay Time is not used. Therefore, the input signal is delayed for some time.
4. Trigger Circuit: It takes some input voltage from the output of the vertical amplifier. It produces triggering pulses, used to trigger the time-base generator. The time-based generator starts only when the output of the triggering circuit is applied.
5. Time Base Generator: It produces a saw tooth wave. Thus, the saw tooth wave controls the horizontal deflection of the electron beam along the x-axis by using the Time/div control knob present on the front panel of CRO.
6. Horizontal Amplifier: The saw tooth voltage produced by the time base generator is amplified by the horizontal amplifier before it is applied to horizontal deflection plates.
7. HV/LV Power Supply: The High Voltage power supply is used to provide power to the electrodes of CRT and the Low Voltage power supply is used to provide power to the other circuits in CRO.
NECESSITY OF DEFLECTION AMPLIFIERS IN CRO
The necessity of deflection amplifiers: Any voltage that is to produce deflection of the electron beam must be converted into two equal and opposite voltages +V/2 and -V/2. This requires an amplifier that accepts AC or de-input and provides differential output.
The amplifier is used with the vertical deflection plates, and the attenuator resistors are selected to produce a wide range of deflection sensitivity. At the most sensitive position of the attenuator switch (least attenuation), a 2 mV input typically produces one division of deflection on the screen. At the greatest attenuation position of the switch, a 10 V input is required to produce one division of deflection. The input to the attenuator may be capacitor-coupled (AC) or direct-coupled (DC),
Working of Deflection Amplifiers :
Transistors Q2 and Q3 form an emitter-coupled amplifier. Q₁ and Q4 are emitter followers to provide high input resistance.
• When the input voltage to the attenuator is zero, the base of Q₁ is at ground level. If the Q4 base is also adjusted to ground level, Q2 and Q3 bases are both at the same negative potential with respect to ground (-VB2=VB3). Also, Ic2 = Ic3, and the voltage drops across R3 and R6 set the collectors of Q2 and Q3 at ground level. These collectors are the amplifier outputs, and they are connected directly to the deflection plates.
• An input voltage that is to produce vertical deflection is coupled to the attenuator of the amplifier feeding the vertical plates Fig. 2.3. The attenuated voltage appears at the base of transistor Q1, where it is further attenuated (via., R₁ and R2) and then applied to Q₂ base.
• A positive-going input produces a positive going voltage at the Q2 base and causes lc2 to increase and Ic3 to decrease. The Ic2 Increase causes output VC2 to fall below its normal ground level, and the Ic3 decrease makes Vc3 rise above the ground. If the change in Vc2 is AVc2 = 1 V, then AVc3 +1 V.
• When the input to the attenuator is a negative-going quantity, 1c2 decreases and Ic3 increases. Now A VC2 is positive and Δ Vc3 is an equal and opposite negative voltage. Where the amplifier is used with the vertical deflection plates, the attenuator resistors are selected to produce a wide range of deflection sensitivity. At the most sensitive position of the attenuator switch (least attenuation), a 2 mV input typically produces one division of deflection on the screen. At the greatest attenuation position of the switch, a 10 V input is required to produce one division of deflection. The input to the attenuator may be capacitor-coupled (ac) or direct-coupled (dc), according to the selected position of switch S1.
It is a de-shift control that serves to adjust the voltage at the base of Q4. When the moving contact Is centralized, the Q4 base is at ground level. By adjusting the moving contact of R10, either a positive or a negative DC potential is applied to the base of Q4. When VB4 is positive, Q3 base voltage is raised positively, so that Ic3 increases and Ic2 decreases. This causes VC3 to fall and Vc2 to rise.
A differential DC voltage is thus applied to the deflecting plates to deflect (or shift) the electron beam above the center of the screen. When R10 is adjusted to produce a negative voltage at the Q4 base, the electron beam is adjusted below the screen center This DC shift does not affect a waveform to be displayed, which is applied to the attenuator input. However, the R10 adjustment shifts the displayed waveform up or down on the screen as desired by the operator.
