Unit 05 ELECTRONIC INSTRUMENTATION in Electronic Engineering

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*Unit-03    *Unit-04 
*Unit-05    *Short-Q/Ans
*Question-Paper with solution 21-22 

Q1. Explain the basic principle of digital voltmeter with the help of block diagram. What are the characteristics of DVM ? 

Ans. Digital Voltmeter : 

• 1. The digital voltmeter (DVM) is a tool for indication. DVM is mostly used to measure the voltage between two locations. It shows discrete numerals for either DC or AC voltage. 
• 2. It is a helpful laboratory tool with a variety of uses. Additionally, it is a valuable component of digital instrumentation systems. In systems for processing data, the DVM is frequently utilised.
• 3. In order to prevent it from consuming any current from the circuit, an ideal voltmeter has an infinitely high input resistance. 
• 4. Think about a metre with a 1000 Ω low input resistance. Because the metre shunts the resistance, it is unable to provide a correct reading for the voltage across a resistance of the same magnitude. 
• 5. As a result, it’s crucial to calculate the ohms per volt loading impact of a voltmeter. 
• 6. The block diagram of a digital voltmeter is shown in Fig. It has three stages : 
  • a. Signal preparation, 
  • b. Analog to digital conversion, and 
  • c. Display unit. 

• 7. The signal preparation stage, also known as the input circuit, changes the signal’s amplitude in accordance with the needs and guards against loading the source. 
• 8. Here, the big incoming signal is attenuated using a resistive attenuator, and the small incoming signal is amplified to observable levels using an amplifier. 
• 9. In essence, digital voltmeters are analogue to digital converters with digital displays that show the voltage being measured.
• 10. Digital voltmeter can be categorized as follows :  

General characteristics : 

• 1. Discrete numbers representing the measurement of OC or AC voltage are shown on the digital voltmeter (DVM).
• 2. This is beneficial in many applications since it decreases interpolation and human reading errors, speeds up reading, and produces output in digital format.
• 3. DVM has following typical operating and performance features : 

Q2. Explain with the help of neat diagram, working and characteristics curve of ramp type digital voltmeter. 

Ans.

• 1. The operation of a ramp type DVM is based on measuring how long it takes a linear ramp voltage to increase or fall from the level of the input voltage to zero volts, respectively.

• 2. A ramp voltage is started at the beginning of the measurement cycle; this voltage may be positive going or negative going. The input voltage is compared to the downward ramp (unknown). 
• 3. The comparator circuit emits a pulse that opens a gate at the precise moment the ramp voltage equals the unknown voltage. In Fig., this gate is depicted. 
• 4. When the ramp voltage finally hits zero volts (or ground potential), a second comparator provides an output pulse that closes the gate.  
• 5. A number of decade counting units (DCUs) track the number of clock pulses that have passed through the gate after they are generated by an oscillator.
• 6. The input voltage’s magnitude is indicated by a decimal number that is shown via indicator tubes connected to the DCUs. 

Characteristic curve : 

Q3. What is CRO? Explain its basic principle. 

Ans.

• 1. An electrical tool called a Cathode Ray Oscilloscope (CRO) provides a visual representation of a signal waveform. 
• 2. It is frequently employed in laboratory work as well as for troubleshooting radio and television receivers. 
• 3. It can also be used for measuring voltage, frequency and phase shift.  

Basic principle: 

• 1. In a CRO, electrons are accelerated to a high speed and focused on a fluorescent screen as they are emitted from a cathode.
• 2. Where the electron beam collides with the screen, a visible spot is created. 
• 3. Electrons can be made to act as an electrical pencil of light that produces a spot of light wherever it strikes by deflecting the electron beam over the screen in response to an electrical signal.
•  4. Due to their extremely low mass, electrons react almost instantly to electrical signals and can almost always trace even the fastest electrical fluctuations.
• 5..A cathode ray tube and the accompanying power supplies are what make up a CRO. 

Q4. With the help of neat block diagram, explain the working of a CRO. 

Ans.

• 1.The cathode ray oscilloscope is a very useful and adaptable laboratory tool for measuring voltage, current, power, and frequency as well as for examining the waveforms of alternating currents and voltages.
• 2. The block diagram ofCRO is shown in Fig.

Block Diagram : 

• 1. The CRO employs a cathode ray tube (CRT). 
• 2. It produces an electron beam, accelerates it to a high speed, deflects it to produce a picture, and then eventually makes the electron beam visible on a phosphor screen.
• 3. High voltage, of the order of a few thousand volts, is needed for the CRT to accelerate the beam while low voltage supply is needed for heating the electron gun for beam generation. 
• 4. Between the electron gun and the screen are deflection plates that allow the beam to be bent based on the input signal. 
• 5. Electron beam strikes the screen and creates a visible spot. 
• 6. On the screen, this point is refracted at a constant time-dependent rate in the X direction. 
• 7. Through the vertical amplifier, which elevates the input signal’s potential to a level that will allow for usable electron beam deflection, the input signal is delivered to the vertical deflection plates.
• 8. Now electron beam deflects in 2 directions, horizontal on X-axis and vertical on Y-axis. 

Function of each block :

1. CRT : This cathode ray tube produces electrons, which impact an internal phosphor screen to provide a visual display of the signal.  

2. Vertical amplifier : Signals in the vertical part are amplified using this broad band amplifier.

3. Delay line : In the vertical parts, it is utilized to slightly delay the signal. 

4. Time base : It is utilized to provide the sawtooth voltage needed to deflect the horizontal section of the beam. 

5. Horizontal amplifier : Before being applied to horizontal deflection plates, this is utilized to enhance the sawtooth voltage.

6. Trigger circuit : This is used to convert the incoming signal into trigger pulses so that the input signal and the sweep frequency can be synchronized. 

7. Power supply : 

• a. A negative High Voltage (HV) supply and a positive Low Voltage (LV) supply are the two power sources.
• b. In the CRO, there are two voltages produced. The range of the positive voltage supply is + 300 to 400 V. The low-voltage supply ranges from -1000 to – 1500 V.
• c. This voltage is passed through a bleeder resistor at a few mA.  
• d. For controls over intensity, focus, and placement, the bleeder resistor provides the intermediate voltages. 

Q5. Explain digital storage oscilloscope with block diagram. 

Ans.

• 1. The input signal is digitised by a digital oscilloscope, making all subsequent signals digital.
• 2. Storage takes place in electronic digital memory, which is shown on a traditional CRT. 
• 3. Fig. shows a block diagram of a basic digital storage oscilloscope. 
• 4. The input signal is converted to digital form and stored in memory. It can be analysed in this state to provide a range of various types of information. 
• 5. The data from memory is reconverted into analogue form in order to view the display on the CRT.

• 6. A sample of the input waveform is talced at regular intervals to digitize it.
• 7. According to sampling theory, the sampling rate must be at least twice as rapid as the input signal’s maximum frequency in order to prevent information loss. Aliasing will happen as a result if this is not done, as demonstrated in Fig.
• 8. The digitiser, an analogue to digital converter, must have a quick conversion rate due to the requirement for a high sampling rate.· 
• 9. Costly flash analogue to digital converters are typically needed for this, and their resolution degrades as sampling rates rise.
• 10. This is the reason why an analogue to digital converter typically imposes a limit on the bandwidth and resolution of a digital oscilloscope. 

Q6. Give the comparison of DSO with analog oscilloscope . 

Ans. 

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