Electronic Instrumentation

Transcription

1 June - 0 Third Semester B.E. Degree Examination Time: 3 hrs. Max. Marks: 00 Note:. Answer any FIVE full questions, selecting at least TWO questions from each part. PAT - A. (a) De ne the following terms: (06 Marks) (i) Gross error and systematic error (ii) Absolute error and relative error i) Gross and Systematic errors: The gross errors occur mainly due to the carelessness/lack of experience of a human being involved in the measurement of any quantity. These errors are due to incorrect adjustment of measuring instruments mathematically and these errors cannot be recti ed completely However, Gross errors can be controlled by taking proper care while reading, calculating and recording a quantity under measurement; at least 3 to 4 repeated measurements of the same quantity should be taken. The systematic errors occur mainly due to the defective parts and ageing of the meter and also environmental effects. Systematic errors includes the friction in bearings, moving parts, variation in air gap, loading effects of the measuring instrument. Another reason for systematic errors in an instrument is the environmental factors which are external to the instrument; they are temperature changes, thermal E.M.F, stray capacitance, etc. These errors can be reduced by hermetically sealing the instrument and using a magnetic electrostatic shields. (ii) Absolute errors and relative errors. Absolute error may be de ned as the difference between the true value and the measured value. Absolute error represent the amount of physical error in a measurement. As an example, consider the true value of voltage measurement is, say, 5V and the measured value is 5. V. Then the absolute error is 5-5. V = 0.V. elative error is de ned as the ratio of the absolute error produced in a given measurement to the measured value. That is Absolute error elative error = Measured value Considering the above example 0. relative error = = =.96% 5. (b) Explain the working of true MS voltmeter, with a neat block diagram. (08 Marks) True MS voltmeter: Any complex waveforms can be accurately measured using true rms - voltmeter, these instruments sense the signal heating power and produces meter indication proportional to square or rms value of voltage. This heating power is ampli ed and fed to thermocouple and then the measured output voltage is proportional to square of rms - value. Electronic Instru. June/July 0 6-June-July-0.indd 0 8//03 :08: PM

2 June 0 June - To measure true rms voltage two thermocouples connected forming a bridge network is used as shown in gure. The input voltage to be measured is applied to the heater element of the thermocouple. The heating effect of heater is measured by thermocouple and generates corresponding voltage called V. The input voltage is ampli ed and then given to the heater element of measuring thermocouple to produce enough heating so that V = V. The two thermocouples (balancing and measuring) form a bridge network and when V = V the bridge is balanced, output voltage V A( V V ) = sine A is gain of dc ampli er () V = V. 0 V = K V irms, V = KV, V = V 0 V irms = rms value of the input = K.V = V irms 0 V = V 0 irms (c) Convert a basic D Arsonal movement with an internal resistance of 00 and a full scale de ection of 0 ma into a multi range dc voltmeter with ranges from 0 5 V, 0 50V and 0-00V. (06 Marks) V V V 3 V ma M=00 Ans (i) V 3 = ( 3 + m ) I 5V = ( ) June-July-0.indd 03 8//03 :08:5 PM

3 June - June 0 5V = = = = 400Ω (ii) V = I ( m ) 50 V = 00mA ( ) 50 V = 0 ma ( ) 50V 00mA = = = 4500Ω (iii) V l = I ( m ) 00V = 0 ma ( ) 00V 0mA = = 5000Ω. (a) A 4½ digit voltmeter is used for voltage measurement (07 Marks) (i) Find its resolution (ii) How would.98 V be displayed on 0 V range? (iii) How would V be displayed V and 0 V range? Given data 4½ digit display (i) Full digit display N = 4 esolution = 4 0 = (ii) There are 5 digit places in a 4½ digit display. To display.98v on a 0V range it displays as.980. (iii) In V range, V will be displayed as , since the resolution in V range is V = 0.000V In 0V range the resolution is 0V = 0.00V Therefore 0.557V will be displayed instead of V.. (b) Explain the working principle of successive approximation digital voltmeter, with the help of block diagram. (07 Marks) 6-June-July-0.indd 04 8//03 :08:5 PM

4 June 0 June - Successive Approximation DVM is based on the principle of simple weighing technique used in practice. The basic block diagram of a successive approximation DVM is shown. When a +ve start pulse is applied to multi-vibrator which activates the control circuit, SA is cleared to and V Cout of DAC is 0V. Sample and Hold Clock if V in > V Cout then comparator output is positive during rst clock pulse of the counter and D 7 = and all other Vref registers to 0 and V Cout jumps to. Now V Cout > V in then comparator output is negative during second clock pulse of the counter and D 7 = 0, D 6 = Vref and all other register to 0 and V Cout jumps to 4. Similarly, the rest of bits beginning from D 7 to D 0 are set and tested. Therefore in 8 clock cycles the measurement is completed and the content in SA is the actual digital output. (c) With a basic block diagram, explain the method used for digital measurement of time period. (6 Marks) To get good accuracy for low frequency measurement, it is a practice to measure the time period rather than make direct frequency measurement. The gure below shows a simple block diagram of time measurement. Input signal Attenuator Amplifier Schmitt trigger Flip Flop Display Input signal Attenuator Amplifier Schmitt trigger Crystall Oscillator Schmitt trigger decade counting 00 display The gating signal is derived from unknown input signal,that controls opening and closing of ip- op. The 6-June-July-0.indd 05 8//03 :08:6 PM

5 June - June 0 number of pulses which occur during one period of unknown signal are counted and displayed by the decade counting assemblies. The displayed time value is taken to calculate the unknown frequency using the formula, F = T 3. (a) Explain the working of dual trace oscilloscope, with a neat block diagram and necessary waveforms. (0 Marks) Dual-trace oscilloscopes are those that can perform (more or less) the same operations as that of a dual-beam oscilloscope, yet use only one electron gun and one set of vertical and horizontal de ection plates for their operations. Figure shows the simpli ed block diagram of a dual-trace oscilloscope. Y input Y ampli er Ampli er Y-plate Y input Y ampli er X-plate Electronic Switch Beam Simpli ed block diagram of a dual-trace CO In this scheme, the electron beam is switched between positions and of an electronic switch at a very rapid rate. The electronic switch is connected to the Y input, when it is in position and to the Y input, when it is in position. Thus the switch connects inputs and to the vertical de ection plates so that the same beam will be subjected to the de ection by two different input voltages in an alternating fashion as shown below. This switching produces the waveforms corresponding to inputs and, respectively. Since the switching is done at a fast rate, we observe that the two waveforms appear to be stationary and independent. 6-June-July-0.indd 06 8//03 :08:6 PM

6 June 0 June - (b) With the help of basic block diagram and circuit diagram, explain the working principle of electronic switch. (08 Marks) Operation of electronic switch in oscilloscope: The electronic switch is used in cathode ray oscilloscope to display two channels at a time. The electronic switch basically consists of two separate gain control and gate stages. These gate stages are alternating biased to cut-off by a square wave signal applied to the gate stage. This technique allows only one gate is in a condition to pass its signal at any given time. The outputs of these gate stages are directly coupled through a capacitor to VDF of CT. To vertical de ection of CT (c) Brie y explain about the focus control knob available on the CO panel. (0 Marks) The focus knob actually controls the grid voltage of CT, which in turn control the ow of electrons passing through the focusing anodes. Focusing anodes works like a lens and focus the beam to a ne point on the screen. This helps the viewer to observe a clear wave form on the screen. 4. (a) Describe the working of oscilloscope delayed time base system, with the help of block diagram and associated waveforms. (0 Marks) CT Delayed Time base 6-June-July-0.indd 07 8//03 :08:6 PM

7 June - June 0 The delayed time base system is as shown in g. (a). This feature increases the versatility of the instrument by making it possible only to magnify the required portion of an undelayed sweep, measure the rise time/jitter and many more applications. When the delay time in CO is not used then the initial part of the signal is lost in the display. In order to counter act this disadvantage the delay line of 000 sec is used and the signal is indirectly applied to vertical depletion plates as shown in g. (a). This gives nite time for the sweep generator to start at the Horizontal plates before any input signal is applied to CO, and the signal has reached the vertical plates. The Trigger pulse has off time of t 0 sec after which the input signal from input has passed through main ampli er. The sweep generator generally delay the Horizontal sweep to Horizontal Ampli er only after t n sec so that the Horizontal sweep start earlier to the vertical signal. The vertical sweep starts at t n sec and by this approach the input signal is not lost. (b) Explain the basic operation of digital storage oscilloscope with the help of block schematic and associated waveforms. (0 Marks) In a digital storage Oscilloscope, the waveform to be displayed and stored is converted into binary digits (s and 0s), stored in a random access memory, and retrieved for display on screen. The stored wave form may be continuously displayed by repeatedly scanning the stored waveform and, therefore, a conventional oscilloscope tube can be used for the display. The stored data can be displayed inde nitely as long as power is applied to the memory. The digitized waveform can be analyzed by either the oscilloscope itself or by using a digital computer connected to it. Figure 4.3 shows the block diagram of a digital storage oscilloscope. The input is ampli ed and attenuated with input ampli ers as in any oscilloscope. The digital storage oscilloscope uses the same types of input circuitry as a conventional oscilloscope and can operate in a conventional mode, bypassing the digitizing and storing features. 6-June-July-0.indd 08 8//03 :08:7 PM

8 June 0 June - Input signal Input Vertical amplifier S/H circuit A/D converter Memory D/A converter Trigger circuit Control logic D/A converter Vertical deflection amplifier Horizontal deflection amplifier CT deflection plants Fig. 4.3 Block diagram of DSO As shown in the gure, the input signal ampli ed by the vertical ampli er-attenuator combination is applied to an analog-to-digital converter, which then drives a random-access memory (AM). This temporarily stores the digitized input data. A control logic circuit is used to control the operations of the ADC and the memory. The output of the memory is applied to a digital-to-analog (DA) converter, which in turn is used to drive the vertical de ection ampli er and vertical de ection plates. The control logic also drives the horizontal-sweep DAC and the horizontal de ection ampli er. The combined action of the de ection plates, as in the conventional oscilloscope produces display on the screen. PAT - B 5. (a) With the help of block diagram, explain the working of modren laboratory signal generator. (0 Marks) The above gure shown is a +ve feed back system. It basically consists of ampli er with gain (A) and feedback n/w whose feedback fraction is. When the product of the loop gain A = and total phase shift around the loop is 360 sustained oscillations are generated called barkhausen criteria. 6-June-July-0.indd 09 8//03 :08:7 PM

9 June - June 0 Block diagram of standard signal generator Ans: Fig. 4(a) Signal generators are extensively used in testing of radio receivers and transmitters. Signal generator is basically a radio frequency signal generator generating sinusoidal signal whose frequency is greater than MHz. The block diagram is shown in gure 4(a). The F oscillator is used to generate oscillation depending on range and frequency selected. It is an LC-tank-circuit oscillator generating a stable F signal and fed to wide - band ampli er. The modulation may be done by a sine wave, square wave or triangular wave using the setting ON the front - panel indicating carrier frequency for frequency modulation. The output of the wide band ampli er is a modulated signal where the output is fed to attenuator which helps in selecting required attenuation and output signal level is controlled for high frequency modulation we require isolate oscillator from output circuit by using buffer ampli er. (b) Explain the working principle of frequency synthesizer, with a neat block diagram. (0 Marks) Figure 5.e shows a frequency multiplier using a PLL (phase locked loop). It consists of a PLL with a divide by N counter connected in its feedback path. Let the input frequency be f i and the fedback input f 0. These two are compared in the phase detector, which produces an output voltage that is proportional to the phase difference between f i and f 0. The low pass filter removes the AC content in the voltage and produces an almost pure DC voltage which drives a voltage controlled oscillator (VCO). The VCO in turn produces a frequency that is proportional to the input DC voltage. Thus we find that the output frequency is proportional to the phase difference between f i and f 0. f i Phase detector Low pass filter VCO Nf 0 f 0 Divide by N Figure 5.e Phase locked loop frequency multiplier Now with the divide by N circuit introduced in the feedback path, we observe that the output frequency is really Nf 0. This is because, with output frequency equal to Nf 0 the input frequency becomes Nf 0 /N = f 0. This principle is used in frequency synthesizer circuits lo produce frequencies of all values and ranges. The theory of the frequency multiplier using PLL may be extended to synthesize (artificially produce) oscillations in any desire frequency range. Figure 5.f shows a typical PLL frequency synthesizer. It consists of a crystal oscillator that produces a fixed frequency of, say, I MHz. This is divided in a frequency counter by M, 6-June-July-0.indd 0 8//03 :08:7 PM

10 June 0 June - an integer of appropriate value. This forms the f i of the PLL. whose feedback frequency is, as before f 0, which is obtained by frequency division, as shown in Fig. 5.e. Thus we observe that the frequency output of the PLL depends on the ratio N/M and by suitably choosing this ratio, we can obtain several frequencies, which are all crystal controlled frequencies, and hence are stable. By using several crystal oscillators of different frequencies, and several PLL units, we can produce frequencies in all ranges, values and amplitudes. Crystal Osc Frequency counter Phase detector Low pass filter VCO Nf 0 f i f 0 Divide by N Figure 5.f Phase locked loop frequency synthesizer 6. (a) Mention the limitations of wheatstone s bridge. Derive the balance equation for Kelvin s double bridge. (0 Marks) The limitations of Wheatstone s bridge are :. The resistance of the leads and contacts becomes signi cant in the low resistance measurement that introduces error.. In the measurement of high resistive values, the resistance presented by the Wheatsotone s bridge becomes so large that the galvanometer will be insensitive due to imbalance. 3. The heating effect of the current that rise the temperature which in turn causes a change in the value of resistance in bridge arms. Excessive current cause a permanent change in the value of resistance that affects the measurement. Kelvin s double bridge is a modi cation of Wheatstone s bridge and provides increased accuracy in the measurement of low value resistance typically below. The term double bridge is used because the circuit contains a second set of ratio arms as shown in Fig. 6 (a). This second sets of arms, labeled A and B in the diagram, connects the galvanometer to a point P at the appropriate potential between M and N, and eliminates the effect of the yoke resistance Y. The initial condition is that the resistance ratio of A and B is the same as the ratio of and. Unbalanced wheatstone s bridge E K L O 3 B A X M Y N Fig. Kelvin s double bridge 6-June-July-0.indd 8//03 :08:8 PM

11 June - June 0 The galvanometer indicates zero, when the potential at point K equals the potential at P. That is E KL = E LMP, where ( A + B) Y ( ) EKL = E = I 3 + x and B ( A + B) Y ELMP = I 3 + ( A + B) ( A + B + Y) Equating E KL and E LMP, we get A B Y ( ) B ( ) ( ) ( ) ( ) A + B Y A + B X I 3 + X + = I A + B + Y A + B A + B + Y Simplifying we get ( A + B) Y BY ( + + ) + ( + + ) + + = + 3 X 3 A B Y A B Y Expanding the HS term results ( A + B) Y 3 BY ( + + ) ( + + ) + + = x 3 A B Y A B Y Solving for X, we get ( A + B) Y ( ) 3 B Y B Y x = + + ( A + B + Y) ( A + B + Y) A + B + Y Therefore, 3 B Y A x = ( ) A B Y B Using the assumed initial condition, x = 3 A =, we see that the above equation reduces to B (b) A capacitance comparison bridge is used to measure a capacitive impedance at a frequency of khz. The bridge constants at balance are C 3 = 00 F. = 0 K. = 50k and 3 = 00K. Find the equivalent circuit of the unknown impedance. (04 Marks) 3 00kΩ 50kΩ x = = 0kΩ x = C x = C 3 500k Ω 6 = k 50k 6-June-July-0.indd 8//03 :08:8 PM

12 June 0 June - C x = 0 μ f The equivalent circuit is capacitor 0 f in series with 500k (c) Derive an expression for frequency of the wein bridge circuit. (06 Marks) Wein bridge in its basic form is designed to measure frequency but also can be used to measure the unknown capacitance. Figure shown a wein bridge, which is combination of a series C in one arm and parallel C in another arm. C Detector 3 4 C 3 From figure, impedance of one arm is Z = f ωc The admittance of the parallel arm is Y = + jωc Using bridge equation, Z Z 4 = Z Z 3 Z i.e, Z Z = and Z = Z Z Y Y3 j = 4 + jωc3 ωc 3 j C = + jω C ωc3 C Equality the real and imaginary parts, we have C = + and ω C = C ωc 3 C Therefore, C ωc ω = 3 = + and = ω CC 3 3 C 3 C = + j ωc C ωc3 6-June-July-0.indd 3 8//03 :08:8 PM

13 June - June 0 or ω= CC 3 3 Since ω= πf, weget f = π CC 3 3 In most Wein bridge circuit, the components are chosen such that = = and C = C = C, then the above equation becomes. f = π C which is the general equation for frequency of the bridge circuit. 7. (a) Explain the construction and working of bonded resistance wire strain gauge and semiconductor strain gauge. (0 Marks) Bonded resistance wire strain gauge: Fine wire Direction of strain Leads A ne metallic wire element about 5 m in diameter when looped fourth on back carriers base, and cemented to the external member under going stress is called bonded wire strain gauge is as shown in the gure. The cemented carrier is a thin sheet of backlight the metallic wire is wound back and fourth on cemented base so that it is not mechanically damaged. The metallic wire spreading should be uniform to as so permit uniform distribution of stress, the cemented base permits good transfer of strain from carrier to metallic wire, the tensile stress permits the wire to elongate and decrease in C/S area and increasing wire resistance. = ρl a The strain gauge is connected to one arm of the bridge and bridge is activated. The other arms of the bridge comprises of a constant resistance. When stress is applied to the strain gauge the resistance increases the bridge is brought to unbalanced condition by measuring the current through the galvanometer, which is proportional to the strain on the strain gauge. Semi conductor strain gauge: 6-June-July-0.indd 4 8//03 :08:8 PM

14 June 0 June - When very high gauge factor is required of the order 50 then we use semiconductor stain gauges. The semi conductor gauge consists of Base material like silicon or germanium and gold leads are used to make contacts with base material and two electrodes are taken out from the base semi conductor strain gauge depends upon the piezo resistance that is the change value due to change in resistants of semiconductor in these gauges can be cascaded with op-amp which can act as pressure sensor but these gauges are very sensitive to change in temperature. (b) With necessary sketches, explain the construction and working principle of LVDT. (0 Marks) LVDT Figure shows the construction of the linear variable differential transformer (LVDT). The differential transformer consists of single primary winding and two secondary windings wound on a hallow cylindrical former. The two secondary windings have equal number of turns and are placed on both sides of the primary winding. The primary winding is excited by an ac source. A movable soft iron core slides in & out the hallow former effecting the magnetic coupling between primary and two secondary windings. When the core is at the normal position (exactly at the middle of the former) the secondary voltages induced are equal and hence the output voltage is the difference of these two voltages, V o = E E = 0. V0 = 0V. When the core is moved to the bottom more ux links S than S the output voltage is E, the output voltage V0 = E E. when the moved to bottom most, the output voltage is very negligible (almost zero). E = 0 sov0 E Similarly, when the core is moved in the opposite position V o = E =E. At the extreme end, E = 0 V0 = E Thus, we find that, as the position of the core changes within the former, the voltages induced in the individual secondary coils differ; this produces an output voltage that is linearly proportional to the position of the core; hence the name linear variable differential transformer. The transfer curve of the LVDT is shown in Fig. 7 (c). The transfer characteristic shows a fairly linear operation of the LVDT. 6-June-July-0.indd 5 8//03 :08:9 PM

15 June - June 0 V 0 = V 0 V 0 Position of the core ( X) X max +X max Position of the core (+X) V 0 = V 0 V 0 Fig. 7. c Transfer characteristic of LVDT 8. (a) Mention the advantages and limitations of TD. (04 Marks) Advantages :. Linearity over a wide range. Operation at high temperature is possible 3. Wide operating range 4. Better stability at high temperature Disadvantages:. Sensitivity is less. Affected by contact resistance, vibration and shock 3. Comparatively expensive with other temperature transducers 4. equires 4-wire operation to eliminate errors due to lead resistance. (b) Define the terms: (i) Seebeck effect. (ii) Peltier effect. (04 Marks) Seebeck effect: A thermocouple consists of a pair of dissimilar metals wires joined together at one end, called sensing or hot end and terminated at another end, which is called reference or cold end. When a temperature difference exists between the hot and cold junctions, an emf is produced causing a current in the circuit. This thermoelectric effect is known as the Seebeck effect, named after the German Physicist Thomas seebeck. Thermocouple works on the principle of Seebeck effect. Peltier effect: Even at the same temperature, conductors made up of different materials will have different densities of free-carriers. When two dissimilar conductors are joined together, electrons will diffuse across the junction from the conductor that has higher electron density. And the conductor, which loses electrons, will acquire positive potential with respect to other conductor. This phenomena is called Peltier effect. (c) Explain how bolometer bridge can be used for the measurement of power. Also discuss the application of unbalanced bolometer bridge. (04 Marks) Bolometer is a small temperature sensitive resistive element that is used to measure F power. The F power to be measured heats the bolometer and causes change in its electrical resistance, which is used as an indication of the magnitude of power. The bolometer is generally used in a bridge network so that even a small change is resistance can be detected easily and corresponding power can be measured. The bolometer bridge is shown in g. 8(c). 6-June-July-0.indd 6 8//03 :08:9 PM

16 June 0 June - F Input /4 stub for Bolometer Ground return Tapered Section for Impedance Matching Bolometer Element F Bypass Capacitor E dc Bias V low F supply Fig. 8(c) bolometer Bride To measure the unknown F power, a small value of known F power indicated by a voltage V is superimposed on the F test power. The dc current form the dc source E is adjusted by varying the resistance that heats the bolometer element until its resistance equals the value of, which is the value required to balance the bridge. Now the test power is turned OFF, which unbalances the bridge. estore the balance by increasing the AF voltage indicated by V. The unknown F power is calculated using the relation, F power = V V, 4 since the power delivered to the bolometer element is of the power fed to the bridge. 4 In a transmission system, where coaxial cable or wave guide is used, the bolometer should provide the necessary impedance matching. This is done by using of a tapered section as shown in Fig. 8(c). Schematic diagram of an unbalanced Bolometer Bridge is shown in Fig. 8 (d). When there is no F power input, the bridge is brought in to balance by adjusting the exciting source voltage and the balanced condition of zero will be indicated by the detector. The F test power is applied now to the bolometer element and the resistance of the bolometer element changes. This results in to the unbalancing of the bridge, the amount of which is indicated by the detector and gives the magnitude of the F power directly. 6-June-July-0.indd 7 8//03 :08:9 PM

17 June - June 0 Zero Adjust Detector F power (Bolometer Element) ac Exciting source Fig 8 (d) Unbalanced bolometer bridge The unbalanced method is the simplest means of measuring low F power by realizing a direct reading bridge. However there is one disadvantage that the bolometer impedance getting changed since the resistance of the bolometer is a function of F power level. This upset the impedance matching of the F system. (d) List the important features of LCD. (04 Marks). The electric eld required to active LCDs is typically of the order of 0 4 V/cm. NLC materials possess high sensitivity of the order of 0 0 and so the current required for scattering hight in an NLC is very small in the range of 0. A/cm 3. Light source for re ective LCD is only the ambient light and so the power requirement is only to cause tarbulence in the cell, that is very small, typically W/cm. 4. They are very slow devices; The turn On and OFF time are typically in the range of a few milli seconds and tens of milliseconds respectively. 6-June-July-0.indd 8 8//03 :08:9 PM