SAMPLE PAPER FOR ELECTRICAL AND ELECTRONICS MEASUREMENTS
ELECTRICAL AND ELECTRONICS MEASUREMENTS
Q.1 is compulsory and contains 20 marks (2 marks for each bit) .
Q.2 – Q.7( do any five from the rest) contains 10 marks each
(a) Distinguish between Reliability and Repeatability?
Ans:- The reliability gives the resultant lower stresses (electric and thermal) which leads to greater life and stability in the instrument whereas the variation of scale reading and its randomness in nature gives the repeatability.
(b) Give two example of the types of standard capacitors used in schering bridge.
Ans:- The 2 eg. of the type of standard capacitors is
(i) Mica-capacitor for general measurement work.
(ii) Air-capacitor for insulation measurement.
(c) Why wagner ground connection is used in a.c. bridge.
Ans:- Because, it control the stray capacitances by shielding its arms and connecting the shields to the ground.
(d) Write the working of the voltmeter in two to three sentences.
Ans:- To give the peak rectified voltage across R and C. Its mainly work with series type and compensate shunt type voltmeter which provide the stabilization in the output meter.
(e) What is the purpose of delay line in the vertical amplifier section of an oscilloscope.
Ans:- The purpose of delay line the vertical amplifier section of an oscilloscope is to delayed the signal waveform by at least same amount of time.
(f) Explain the types of probes required for a.c. voltage and dc voltage measurement in an oscilloscope.
Ans:- There are 3 different types of probes used for the measurement:-
(i) Direct reading probe:- It is used to avoids the stray pick-ups and it uses a shielded co-axial cable.
(ii) Circuit isolation probe:- It is used to avoids the undesirable circuit loading effects of the shielded probe.
(iii) Detector probe:- It is used to separate the modulation component from the high ferquency carrier.
(g)Draw the block diagram of phase-locked loop.
(h) What is the dynamic range of spectrum analyzer with a third-order intercept point of +25 dBm and a noise level of -85 dBm?
Ans:- By formula,
dynamic range = 2/3 (Ip – MDS)
2/3 [25- (-85)] = 73 dB
(i) Suggest a transducer for the measurement of displacement in the order of one-tenth of mm. Write the basic principle of measurement using two three sentences.
Ans:- Piezoelectric transducer
Its basic principle is use in high frequency accelerometer. The device needs no external power source so its a self generating.
(j) Why the service request (SRQ) signal is used by device in IEEE-449 GPIB Bus!
Ans:- It is use to minimize the cross talk the SRQ signal which is twisted with ground.
2. Explain in brief, the causes and remedies of the following types of measurement error in frequency meter.
(i) Gating error:-
It occurs whenever frequency and period measurement are made. For frequency measurement the main gate is opened, and closed by the oscillator output pulse. This allow the input signal to pass through the gate and b counted by the decade counters.If gating error will be shown in waveform then it 3 waveform assume a, b, and c will be represented w.r.t. the gating signal where waveform a, b shows the error of +-1 count which is an ambiguity in the measurement.
Signals with frequency lower than to should be measured n the “period” mode. The accuracy degradation at to caused by the +-1 count gating error.
(ii) Trigger level error:-
The accuracy with which the gate is opened and closed is a function of the trigger level error. The time at which the schmidt circuit trigger,it occurs a function of the input signal amplification and of its signal-to noise ratio.
The following points will be helpful to reduce this error:-
(i) The effect of the one-count gating error can be minimized by making frequency measurement above the clock frequency of the counter.
(ii) The accuracy of the time measurement is greatly affected by the slope of the incoming signal controlling the signal gate. large signal amplitude and fast ride-time assure maximum accuracy
3. Why the error distribution is Gaussian? Calculate the percentage of observation lie within the following ranges.
(i) Mean value +,- standard deviation.
(ii) Mean value +,- 2 (standard deviation)
(iii) Mean value +,- 3(standard deviation)
Ans:- The error distribution is Gaussian because it gives the occurence of deviation from average value of an infinite number of measurements and it is expressed as:
y = (n / √y) × exp (-h^2 x^2)
Where, x= magnitude of deviation from mean
y = number of readings at any deviation x
h= a constant called precision index.
We know the relation between mean value and standard deviation is as:
σm = (σ / √n)
Now putting the given values and find their ranges.
(i) σm = +- (σ / √n) = σm = +- σ
putting several n’s value get……..0,1,0.71…..
(ii) σm =+- (2σ / √n)
putting several values,ranges= 0,+-2 ,+- 1.414,+- 1.15, +-1,……..
(iii) σm = (3σ / √n)
putting several values, ranges = 0, +- 3, +- 2.12, +- 1.732,+-1.5,…….
4. Draw a block diagram showing basic elements of a function generator. Explain how sine, triangular, square and saw-tooth waves are generated?
The upper current sources supplies a constant current to the triangle integrator whose output voltage increase linearly with time. Its e.m.f. output is come with the relation
e(out) = -(1 / C ) integration of i w.r.t. dt
An increase or a decrease in the current supplied by the upper current source increases or decreases the slope of the output voltage. The voltage comparator multivibrator change state at a predetermined level on the positive slope of the integrator’s output voltage. The lower current source supplies a reverse current to the integrator. so, that it output decreases linearly with time. When the output voltage reaches a predetermined level on the -ve slope of the o/p waveform the voltage comparator again switches and cut-off the lower current source while at the same time switching on the upper source again.
The voltage at the output of the integrator has a triangular waveform whose frequency is determined by the magnitude of the current supplied by the constant current source. The comparator delivers a square wave o/p. The 3rd o/p waveform is delivered from the triangular waveform which is synthesized in to sine wave by a diode resistance network.
5. Explain briefly?
(a) why Maxwell’s bridge is limited to the measurement of medium Q coils (1<Q<10)?
Ans:- The above statement can be understand easily by the figure as:-
Fig:- Maxwell bridge
The sum of the phase angles of one pair of opposite arms must be equal to the sum of the phase angles of the pair. Since the phase angles of the resistive elements in arm 2 and arm 3 add up to 0 degree, the sum of the angles of arm 1 and arm 4 must also add up to 0. The phase angle of the capacitive arm must also be very nearly 90 degree(-ve).
(b) Hay bridge is suitable for the measurement of high Q coils (Q>10)?
As from the fig:-
Ls = (R2 R3 C1) / 1 + (1 / Q2) ^2
so,for a value (Q> 10),
the terms (1/ Q)^2 will be smaller than (1/100) and can be neglected. Hence the Hay bridge is more suitable than Maxwell bridge and vice versa.
6. What are the advantages of an oscilloscope as compared to the electronic voltmeter? Explain the term ”Deflection sensitivity “ and “Deflection factor” of an oscilloscope. Why CRT is called as “ Linear voltage-indicating device?
Ans:- The advantages of an oscilloscope are:-
(i) The oscilloscope allows the amplitude of electrical signal to displayed primarily as a function of time
(ii) It depends on the movement of an electron beam which when focus on the phosphor screen produces a visible spot.
(iii) It is capable of displaying events that takes place over periods of microseconds and nanoseconds.
Deflection Sensitivity:- It is defined as the deflection on the screen (in meters) per volt of deflection voltage. It is denoted by ‘S’.
So, S = (D / Ed) = (Lld / 2dEa) m/v.
Deflection factor:- It is defined as the reciprocal of deflection sensitivity. It is denoted by G.
therefore, G= 1 / s = (2dEa / Lld ) (m/v)
CRT is called “linear voltage” – “indicating device” because for a given accelerating voltage Ea and for the particular dimensions of the CRT, the deflection of the electron beam on the screen is directly proportional to the deflection voltage Ed.
.7.Describe the working of a stable or free running multivibration for the generation of pupes.
Ans:- Astable Multivibrator
The Astable Multivibrator is used to produce either square wave or pulses. This shows in fig. above . The circuit consists of a two stage RC-coupled amplifier, with the output of the second state (Q2) coupled back to the input of the first stage (Q1) via capacitor C1. Similarly,the output between the two transistors is taken from the collectors, the circuit is known as a collector coupled Astable multivibrator.
When the power is applied to the circuit, both transistors start conducting, because of small differences in their operating characteristics, one of the transistors will conduct slightly more than other.
Assuming Q1 initially conducts more than Q2. It means the collector voltage of Q1 (ec1) drops more rapidly than the collector voltage of Q2(ec2). The decrease in ec1 is applied to the R2C2 network and because the charge on C2 cannot change, the full negative going change appears across R2. This decreases the forward bias on Q2 which in turn decrease the collector current of Q2 (iC2) and the collector voltage of Q2 rises. This rise in Q2 collector voltage is applied via the R1C1 network to the base of Q1, increases its forward bias.
The switching action on begins, C2 begins to discharge exponentially through R2. when the charge on C2 reaches 0 v,C2 attempts to charge up to the value of FVbb the base supply voltage.