## new

ece4uplp Normal incidence on a perfect conductor

## Reconstruction filter(Low Pass Filter)

Reconstruction filter (Low Pass Filter) Procedure to reconstruct actual signal from sampled signal:-

Low Pass Filter is used to recover original signal from it’s samples. This is also known as interpolation filter.

An LPF is that type of filter which passes only low frequencies up to cut-off frequency and rejects all other frequencies above cut-off frequency. For an ideal LPF, there is a sharp change in the response at cut-off frequency as shown in the figure. i.e, Amplitude response becomes suddenly zero at cut-off frequency which is not possible practically that means an ideal LPF is not physically realizable.

i.e, in place of an  ideal LPF a practical filter is used.

In case of a practical filter, the amplitude response decreases slowly to zero (this is one of the reason why we choose  ) This means that there exists a transition band in case of practical Low Pass Filter in the reconstruction of original signal from its samples. Signal Reconstruction (Interpolation function):-

The process of reconstructing a Continuous Time signal x(t) from it’s samples is known as interpolation.

Interpolation gives either approximate (or) exact reconstruction (or) recovery of CT signal.

One of the simplest interpolation procedures is known as zero-order hold.

Another procedure is linear interpolation. In linear interpolation the adjacent samples (or) sample points are connected by straight lines.

We may also use higher order interpolation formula for reconstructing the CT signal from its sample values.

If we use the above process (Higher order interpolation) the sample points are connected by higher order polynomials (or) other mathematical functions.

For a Band limited signal, if the sampling instants are sufficiently large then the signal may be reconstructed exactly by using a LPF.

In this case an exact interpolation can be carried out between sample points.

Mathematical analysis:-

A Band limited signal x(t) can be reconstructed completely from its samples, which has higher frequency component fm Hz.

If we pass the sampled signal through a LPF having cut-off frequency of  fm  Hz.

From sampling theorem

.

.

g(t)     has a multiplication factor  . To reconstruct  x(t)  (or)  X(f) , the sampled signal must be passed through an ideal LPF of Band Width of    Hz and gain   .

.

.

. If sampling is done at Nyquist rate , then Nyquist interval is  .

therefore  .

h(t) = 0.      at all Nyquist instants    , when    g(t)    is applied at the input to this filter the output will be  x(t)  .

Each sample in g(t)  results a sinc pulse having amplitude equal to the strength of sample. If we add all these sinc pulses that gives the original signal  x(t) . .

.

.

.

This is known as interpolation formula

It is assumed that the signal  x(t) is strictly band limited but in general an information signal may contain a wide range of frequencies and can not be strictly band limited this means that the maximum frequency in the signal can not be predictable.

then it is not possible to select suitable sampling frequency  fs  . ## aliasing effect in Sampling

Effect of under sampling (aliasing effect):-

When a Continuous Time  band-limited signal is sampled at, then the successive cycles of the spectrum of the sampled signal overlap with each other as shown below Some aliasing is produced in the signal this is due to under-sampling.

aliasing is the phenomenon in which a high-frequency component in the frequency spectrum of the signal takes as a low-frequency component in the spectrum of the sampled signal.

Because of aliasing, it is not possible to reconstruct x(t) from g(t) by low pass filtering.

The spectral components are in the overlapping regions and hence the signal is distorted.

Since any information signal contains a large no.of frequencies so the decision of sampling frequency always becomes a problem.

A signal is first passed through LPF  before sampling.

i.e, it is band limited by this LPF which is known as a pre-alias filter.

To avoid aliasing

1. Pre-alias filter must be used to limit the bandwidth of the signal to   Hz.
2. Sampling frequency must be  .

Pre-alias filter means before sampling is passed through an LPF to make a perfect band-limited signal.

## FSK Generator /BFSK generator

we know that the input to the FSK Generator is a binary sequence 1010…etc.

FSK generator uses two product modulators upper-modulator and lower-modulator with carriers and .

A level shifter is there in which the output of the level shifter is when the input is a binary ‘1’ and ‘0’ volts for the input ‘0’ level shifter.

i.e,

The working of the FSK generator is as follows when the input binary sequence is ‘1’

on the upper modulator 1 has been shifted to a voltage so that the output of product modulator 1 is   and on the lower modulator input ‘1’ is passed through an inverter and if the output of the inverter is ‘0’ then the output of the level shifter will not change it remains at ‘0’ volt itself.

then the product modulator 2 output is then the overall output  similarly, when the input sequence is a binary ‘0’

## Frequency Shift Keying (FSK)/BFSK

In a Binary FSK system, symbols 1 and 0 are distinguished from each other by transmitting one of two sinusoidal waves that differ in frequency by a fixed amount.

(or)

The frequency of the carrier signal shifted to two frequencies  for symbols ‘1’ and ‘0’ transmission.

The equation for FSK signal is

$S_{FSK}(t)= \sqrt{\frac{2E_{b}}{T_{b}}} \ \ cos (2\pi f_{c_{i}}t),\ 0\leq t\leq T_{b} , \ i=1,2$

$S_{FSK}(t)= 0 elsewhere$

i.e, where is generally a high frequency. is a low frequency and vice-versa is also true. <

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## Delta Modulation Receiver The encoded DM output from the transmitter is sampled through the channel which is given as input to the DM Receiver.

The encoded DM output is first decoded into  which will be given as input accumulator so that the resultant output is approximated  (or) quantized signal of .

∴ The original signal can be reconstructed using a Low Pass Filter from

## UNIT 1-QUIZ1

1. Find the modulation index if the amplitude of message signal is Two thirds of the amplitude of carrier signal ———————.
2. The diagonal clipping in Amplitude Demodulation (using envelope detector) can be avoided if RC time-constant of the envelope detector satisfies the following condition, (here W is message bandwidth and ω is carrier frequency both in rad/sec) ( )
a. RC<1/W             b. RC>1/W                  c. None of the above         d. RC>1/ω.
3. Given AM signal is  XAM(t)= 10 (1+0.25 sin 2πfmt) cos 2πfct , then The average side band power given for the above AM signal is ( )
a. 25W                    b. 12.5W               c.1.5625W                    d.3.125W.
4. Given AM signal is XAM(t) = 100 (1+0.85 cos 2πfmt) ,then The total power required for the above AM signal is ( )
a. 25W                   b. 12.5W               c.6.806KW                       d. None of the above.
5. Consider the AM signal Ac cos Wct + 2 cos Wct cos Wmt   for the demodulation of the signal using envelope detector the minimum value of Ac should be ( ).
a. 2                          b. 1                         c. 0.5                                    d. 0.
6. Given AM signal is SAM(t) = 100 (1+0.3 cos 2πfmt +0.4 sin 2πfmt) cos 2πfct i. i.The total power required for the above AM signal is ( )
a. 5.625W              b. 5.625KW             c.6.806KW                  d. None of the above.
ii.Modulation index is ( )
a. 0.53                     b. 0.5                         c. 0.2                               d. None of the above.
iii.The carrier power is ( )
a. 5KW                    b.6KW                       c. 7KW                           d.100KW.
iv. Total current flowing through the transmitter if carrier current is 5A ( ).
a. 5mA                      b.5.303A                 c. none of these          d. 25A.
7. If the band width of message signal is 5KHz and the carrier frequency is 200KHz then upper sideband frequencies are( )
a. 205KHz,190KHz                                                b.205KHz,-205KHz
c. 205 KHz,-195 KHz                                           d. None of the above.
8. If the highest frequency of message signal is 5KHz and the carrier frequency is 200KHz then lower sideband frequencies are( )
a. 205KHz,190KHz                                                b.205KHz,-205KHz
c. 195 KHz,-195 KHz                                            d. None of the above.
9. If the bandwidth of message signal is 500Hz then the bandwidth of Amplitude Modulated signal is ( )
a. 500Hz                    b. 1000Hz                    c.2KHz                     d. None of the above.
10. If the message m(t)= 10cos⁡2000πt and carrier signal is                                      C(t) = 25 cos 200000 πt then draw the amplitude spectrum of AM signal————— .

## UNIT 2- QUIZ 2

1. Angle modulation is a technique in which the ————— of the is varied with respect to instantaneous values of ————————— by keeping as constant. 2M.
2. Write the expression for Angle Modulated signal –. 1M.
3. An Angle Modulated signal is given as 𝑥(𝑡) = 100 cos(2𝜋𝑓𝑐 𝑡 + 4 sin(1000𝜋𝑡)) where 𝑓𝑐 = 10 𝑀𝐻𝑧. 6M.
i. The Peak frequency deviation is ( )
a. 2K                           b. 4000                   c. 4π                      d. 8π.
ii. The Peak- phase deviation is ( ).
a. 4                              b. 6                           c. 0                        d. None of the above.
iii. The power of the Modulated signal is ( ).
a. 10KW.                   b. 5 W                       c. 5 KW                d. 50W.
4. The amount of change in carrier frequency produced by modulating signal is known as ( ). 1M.
a. phase deviation                                                            b. amplitude deviation
c. Frequency deviation                                                  d. none of the above.
5. The total Transmitted power in FM is equal to the power of ( ) 1M.
a. An AM signal.                                                            b. an unmodulated carrier
c. Message signal                                                         d. all of the above.
6. A 20 MHz carrier is frequency modulated by a sinusoidal signal with frequency 1KHz such that peak frequency deviation is 100KHz what will be the modulation index ( ) 2M.
a. 100                           b.101                                c. 99                         d.200.
7. The bandwidth for above FM system will be ( ) 2M.
a. 101 KHz                 b. 202 KHz                     c. 99 KHz              d. 100 KHz.
8. Which one of the following is an indirect method of generating FM ( ) 1M.
a. Armstrong method                                                   b. Varactor diode modulator
c. Reactance BJT modulator.                                     d. Reactance FET Modulator.
9. In which of the Modulation system when the modulating frequency is doubled the modulation index reduces to half while modulating voltage remains constant ( ) 2M.                                                                                                        a. Phase                   b. Amplitude                   c. Frequency       d. None of the above.
10. In FM, the frequency deviation is ( ) 2M.                                                                         a. Proportional to modulating frequency.                                                                         b. Proportional to amplitude of modulating signal.                                                     c. Constant.                                                                                                                                   d. Zero.
11. In indirect method of FM generation FM is obtained from ( ) 1M.                         a. AM                    b. PM                 c. DSB                              d. FM
12. Write Carson’s rule –. 1M.
13. The Bandwidth of NBFM is given as –. 1M.
14. A 25 MHz carrier is modulated by a 400Hz audio sine wave. The carrier voltage is 4V and the maximum deviation is 10 KHz. The modulation index will be( ) 2M.                                                                                                                                  a. 2.5                  b. 5                     c. 15                        d. 25
15. For the above problem write the expression of FM wave will be———————————————————————————————–.1M.
16. For the problem in 14 write the expression of PM wave———————————————————————————————————.1M.
17. Standard FM broadcast stations uses a maximum bandwidth of ( ) 1M.              a. 150 KHz                   b.75KHz.            c. 200KHz         d. 15KHz.
18. Which type of oscillators are preferred for carrier generation because of their good frequency stability ( ) 1M.                                                                                 a. LR               b.LC                       c. Crystal                         d. RC.
19. The oscillator whose frequency is varied by an input voltage is called as ————————————————. 1M.
20. Maximum deviation results at what point on modulating signal if the system is FM( ) 1M.                                                                                                                      a. Zero crossing of m(t)                                                                                                              b. Peak negative amplitude and peak positive amplitude of m (t).                        c. None of the above.                                                                                                                d. Both a and b.

1. Radio receivers are classified into how many types ( ). 2M.
a. Three                   b. two                      c. four                             d. none of the above.
2. The ability of a radio receiver to amplify weak signals is called as ( ). 2M.
a. Fidelity             b. Selectivity            c. sensitivity                  d. all of the above.
3. The phenomenon of Picking up of same short wave station at two nearby points on the receiver dial is known as ( ). 2M.
a. Fidelity            b. sensitivity               c. Double spotting               d. selectivity.
4. The ability of a receiver to reject unwanted signals is called ( ). 2M.
a. Selectivity           b. Fidelity                  c. sensitivity                  d. Double spotting
5. Standard broadcast AM receivers tuned in the frequency range of 540 KHz to 1640 KHz has an intermediate frequency of ( ). 2M.
a. 455 KHz                 b.1MHz                      c. 20Hz                      d. 200Hz.
6. Standard broadcast FM receivers tuned in the frequency range of 88MHz -108 MHz has an intermediate frequency of ( ). 2M.
a. 455 KHz                    b.1MHz                   c. 20Hz                      d. 10.7MHz.
7. Television receivers in the VHF band(54MHz-223MHz) and in the UHF band(470MHz-940MHz) use an IF between 26MHz and 46MHz with the two most popular values ( ). 3M.
a. 36 MHz and 46 MHz                                                b. 455 KHz and 46 kHz.
c. 36 KHz and 46 KHz.                                                 d. none of the above.
8. In a broadcast FM receiver if the local oscillator is tuned to 98.7 MHz then the image frequency is ( ). 3M.
a. 88MHz              b. 109.4MHz                   c. 96 MHz                d. none of the above.
9. In a broadcast AM receiver if the signal is tuned to 530 KHz then Intermediate frequency, local oscillator frequency and image frequency are( ). 3M.
a. 200 kHz, 730 KHz and 1000 kHz.
b. 10.7MHz, 15.37MHz and 1000 KHz.
c. 455 KHz, 985 KHz and 1440 KHz.
d. None of the above.
10. In communications, Audio frequency range is —————-. 2M.
11. In communications, Radio frequency range is—————–. 2M.
12. Draw the radio frequency spectrum with detailed values starting from Very Low Frequencies (VLF) to Extreme High Frequencies (EHF). 5M.
13. Draw the block diagram of TRF receiver (only diagram). 5M.

## UNIT5-Quiz5Pulse Modulation Techniques

1. In Pulse Position Modulation, the drawbacks are ( ) 2M.                 a.Synchronization is required between transmitter and receiver                       b. Large bandwidth is required as compared to PAM                                                 c. None of the above                                                                                                                 d. Both a and b.
2. In PWM signal reception, the Schmitt trigger circuit is used ( ) 2M.                    a. To remove noise                                                                                                                    b. To produce ramp signal                                                                                                      c. For synchronization                                                                                                            d. None of the above.
3. In pulse width modulation, ( ) 2M.                                                                                    a. Synchronization is not required between transmitter and receiver              b. Amplitude of the carrier pulse is varied                                                                      c. Instantaneous power at the transmitter is constant                                              d. None of the above.
4. In different types of Pulse Width Modulation, ( ) 2M.                                                a. Leading edge of the pulse is kept constant                                                                b. Trailing edge of the pulse is kept constant                                                                c. Centre of the pulse is kept constant                                                                              d. All of the above.
5. In Pulse time modulation (PTM), ( ) 2M.                                                                         a. Amplitude of the carrier is constant                                                                             b. Position or width of the carrier varies with modulating signal                           c. Pulse width modulation and pulse position modulation are the types of PTM                                                                                                                                                 d. All of the above.
6. Drawback of using PAM method is ( ) 2M.                                                                      a. Bandwidth is very large as compared to modulating signal                                  b. Varying amplitude of carrier varies the peak power required for transmission                                                                                                                                  c. Due to varying amplitude of carrier, it is difficult to remove noise at receiver                                                                                                                                          d. All of the above.
7. Pulse time modulation (PTM) includes ( ) 2M                                                              a. Pulse width modulation                                                                                                        b. Pulse position modulation                                                                                                  c. Pulse amplitude modulation                                                                                              d. Both a and b.
8. Calculate the Nyquist rate for sampling when a continuous time signal is given by  x(t) = 5 cos 100πt +10 cos 200πt – 15 cos 300πt ( ) 3M.                            a. 300Hz                                                                                                                                            b. 600Hz                                                                                                                                            c. 150Hz                                                                                                                                          d. 200Hz.
9. Calculate the minimum sampling rate to avoid aliasing when a continuous time signal is given by x(t) = 5 cos 400πt ( ) 3M.                                                              a. 100 Hz                                                                                                                                            b. 200 Hz                                                                                                                                          c. 400 Hz                                                                                                                                          d. 250 Hz.
10. A distorted signal of frequency fm is recovered from a sampled signal if the sampling frequency fs is ( ) 2M.                                                                                          a. fs > 2fm                                                                                                                                        b. fs < 2fm                                                                                                                                      c. fs = 2fm                                                                                                                                        d. fs ≥ 2fm.
11. The desired signal of maximum frequency wm centered at frequency w=0 may be recovered if ( ) 2M.                                                                                                    a. The sampled signal is passed through low pass filter                                              b. Filter has the cut off frequency wm                                                                              c. Both a and b                                                                                                                              d. None of the above.
12. The frequency spectrum of x(t) is X(f) is given as follows 6M.

Draw the frequency spectrum of sampled signal by assuming suitable values for sampling frequency under the following conditions
i. Over sampling                      ii. Under sampling                         iii. fs = 2fm.

# EXPERIMENT WISE VIVA QUESTIONSANALOG COMMUNICATION LAB

## Amplitude Modulation:

1. What is meant by Modulation? What is the need for modulation?
2. What are different types of analog modulation techniques?
3. What are the other names of message signal? What are the other names of carrier signal?
4. Write the equation of AM signal and explain each parameter in that equation?
5. Define Amplitude Modulation? Define modulation depth or modulation index?
6. What is the range of Audio frequency signals? What is the range of Radio frequency signal?
7. What are the applications of Amplitude modulation?
8. How many generation methods are there to generate an AM wave? What are the methods of demodulation of an AM wave?
9. Explain the operation of diode detector circuit?
10. Write the formula for modulation index? Differentiate under, over and perfect modulation in AM?
11. As the amplitude of message signal increases, modulation index increases or decreases?
12. Define single tone modulation? In laboratory type of AM is single tone modulation or not?
13. Draw the frequency spectrum of AM wave?
14. If modulation index is 100%, calculate the ratio of total power to carrier power of an AM wave?
15. If µ=1 in an AM wave what is the amount of power saving in an AM wave? What is the band width of an AM wave?
16. Explain the operation of AM modulator? Explain the operation of 8038 circuit in AM modulator?
17. Explain the procedure of Amplitude modulation? What is the significance of Emax and Emin points in AM wave?
18. Plot message, carrier and AM signals?
19. What is meant by envelope detector?
20. The frequency of AM wave follows — (message signal frequency or carrier frequency)?
21. The amplitude of AM wave at fc +fm is— and The amplitude of AM wave at fc -fm is—–
22. In amplitude modulation the amplitude of ——— is changing with respect to ——
23. Envelope of AM signal follows————– (message signal/ carrier signal)?
24. What are the advantages and disadvantages of AM?
25. How demodulated signal differs from original signal in AM?
26. The two important distortions that can appear in the demodulated output of an envelope detector are————– and—————————- –.
27. Differentiate high-level and low-level modulations in AM?
28. What is trapezoidal rule?

## Balanced Modulator:

1. What are the disadvantages of AM?
2. Most of the power in AM spectrum is carried by ————
3. Define DSBSC modulation?
4. How DSBSC is more efficient than AM in terms power saving, explain?
5. What is meant by frequency response?
6. Draw the magnitude response or amplitude spectrum of DSBSC signal?
7. The signal generated by balanced modulator is———–
8. Draw the wave form of DSBSC wave and AM wave, and differentiate those two waveforms?
9. Give the equation of DSBSC signal?
10. What are the generation methods of DSBSC?
11. What are the demodulation methods of DSBSC?
12. What is the bandwidth of DSBSC signal?
13. Define Costas loop and it’s operation?
14. Amount of power saving in DSBSC signal is————
15. Coherent detection means?
16. Give the practical applications of balanced modulator?
17. Explain the operation of product modulator?
18. Why the circuit is called balanced modulator?
19. If the circuit is operating in balanced state, the modulation index value is——- –.
20. Explain the working procedure of 1496 IC for the generation of DSBSC wave?
21. As message signal amplitude increases, carrier suppression in dB’s ———
22. Plot message, carrier and DSBSC waves and explain each wave clearly.
23. How do you differentiate modulation by demodulation?
24. Explain the significance of local oscillator frequency in modulators and in
25. Differentiate synchronous and non synchronous detection techniques in analog modulators?
26. The phase shift at zero crossings in DSBSC wave is——- –.
27. What is Quadrature carrier multiplexing?
28. How DSBSC is different from SSB?

## Frequency Modulation:

1. Define Frequency modulation? How it is different from phase modulation?
2. Write equation of FM wave, explain each parameter in it?
3. Draw the amplitude spectrum of FM wave?
4. Give the Carson’s rule in FM?
5. Define modulation index β, frequency deviation?
6. Differentiate Narrow band FM with Wide band FM?
7. Explain the FM operation using 8308IC?
8. Draw message, carrier and FM waves and explain each wave clearly?
9. Explain the methods for generation of FM and its demodulation?
10. How FM wave is different from PM wave?
11. Give the practical applications of FM?
12. State advantages and disadvantages of FM?
13. The range of speech signals is——— –.
14. Type of Modulation used in radios is——- –.
15. Type of modulation used for voice signals in T.V — and for video signals in V is—- –.
16. Noise immunity is more in which analog modulation technique———– –.
17. FM is more robust to noise compared to AM, why?
18. Carson’s rule is for———- –.
19. In commercial FM broadcasting, the audio frequency range handled is only up to—- –.
20. The transmission band width required for commercial FM broadcasting is——– –.
21. Define Hilbert transform?
22. Explain capture effect in FM broadcasting?

## Pre-emphasis and De-emphasis:

1. Define pre-emphasis and De-emphasis processes in
2. Why Pre-emphasis is used at Transmitter of FM and de-emphasis at FM receiver?
3. Draw the pre-emphasis circuit and explain its working in detail?
4. Draw de-emphasis circuit and explain its working in detail?
5. Draw the frequency response characteristics of pre-emphasis and de-emphasis explain each one in detail?
6. Calculate the cut-off frequencies of pre-emphasis and de-emphasis circuits practically
7. Pre-emphasis circuit operation is similar to——— –.
8. De-emphasis circuit operation is similar to——— –.
9. What is the necessity of boosting up high frequencies in frequency modulation communication system?
10. Define 3dB frequencies?

# Sampling and reconstruction:

1. Define sampling theorem? What is the need for sampling?
2. What are the necessary and sufficient condition for sampling and reconstruction of a signal?
3. Define Nyquist rate and Nyquist interval in sampling theorem?
4. If message frequency is 2 KHz and sampling frequency is 2 KHz,4 KHz, 8 KHz and 16 KHz in each case the number of samples are—————————– –.
5. What are different types of sampling techniques?
6. What was the effect on sampled signal if fs < 2 fm ?
7. Draw the amplitude spectrum of sampled signal if fs < 2 fm, fs =2 fm, fs > 2 fm.
8. What is aliasing effect in sampling? How to avoid it?
9. Why do we use pre-filtering in sampling?
10. What do you mean by reconstruction of sampling theorem?
11. What are the types of filters used in reconstruction?
12. Define sample and hold process?
13. Differentiate second order, fourth order and sixth order low pass filters in reconstruction process.
14. Explain the sampling and reconstruction process in detail by using the trainer
15. Define band pass sampling?
16. How sampling is different from PAM?
17. Define a continuous time signal or an analog signal. Give some examples of analog signals.
18. Define a discrete time signal. Give some examples of discrete
19. What is the difference between discrete and a digital signal?
20. Define a digital signal? Give some
21. What is the need for converting a continuous signal into a discrete
22. Explain about zero-order hold circuit.
23. How to convert an analog signal into digital signals?

Digital signal processors operates———— as inputs.As the number of samples increases, the reconstruction of original signal becomes?

# Pulse Amplitude Modulation:

1. What is the basic principle of PAM?
2. Name some Pulse Modulation techniques?
3. Define PAM?
4. How PAM is different from AM?
5. Can we produce a PAM signal using a sampling circuit?
6. Differentiate PAM output with sampling output?
7. Does PAM come under Analog modulation technique or Digital Modulation technique?
8. What is the Bandwidth of PAM?
9. Compare BW of PAM and AM?
10. Draw waveforms of PAM. explain each one briefly.
11. What are the advantages of PAM over AM?
12. What are the advantages and Disadvantages of PAM?
13. Explain the working procedure PAM kit?
14. Can we use PAM technique in TDM?
15. Differentiate uni-polar and bi-polar PAM.
16. What do you mean by zero order holding? And draw the circuit diagram of zero-order hold circuits?
17. What are the drawbacks of PAM?
18. Explain the working of PAM demodulation circuit?
19. Define Flat-Top sampling?
20. Draw the circuit diagram of Flat-Top Sampled circuit?
21. What was the roll off characteristics of sinc pulse?

## Pulse Width Modulation (PWM):

1. Define PWM?
2. Differentiate PWM, PAM and PPM?
3. Name the applications of Mono-stable multivibrator?
4. What is a Multivibrator?
5. Differentiate Monostable, Bi stable and Astable Multivibrators?
6. How a Monostable Multivibrator produces a PWM signal?
7. What are the other names of PWM?
8. Define Pulse Duration Modulation?
9. What is Pulse Time Modulation?
10. Draw PAM and PWM signals and each one in detail.
11. Draw PWM signal with respect to message signal?
12. In PWM —————- of Pulse carrier signal is changing with respect to message signal.
13. Explain the operation of PWM circuit.
14. 555 timer in Monostable mode produces————.
15. 555 timer in Astable mode produces——————.
16. What are the advantages of PWM over PAM?
17. What is the difference between PWM and FM?
18. Which type of noise is affecting the amplitude of PWM signal?
19. Which system is more immune to noise (PWM or PAM)?
20. What are the disadvantages of PWM?
21. What are the applications of PWM?
22. Band Width of PWM is—————-.
23. Band width of PAM is—————-.

## Pulse Position Modulation (PPM):

1. Define PPM?
2. The information is conveyed by ————- of Pulses in PPM.
3. In PWM information is conveyed by————— of pulses.
4. In PAM information is conveyed by————— of pulses.
5. What are the advantages and disadvantages of PPM?
6. Compare PPM with Phase Modulation.
7. PAM is similar to————————–.
8. PWM is similar to————————–.
9. PPM is similar to————————–.
10. Differentiate Analog Modulation Techniques with Pulse Modulation Techniques.
11. What are the applications of PPM?
12. Draw PPM signal with respect to message signal.
13. Draw PPM signal with respect to PWM signal.
14. Explain the operation of PPM Modulator?

## Phase Locked Loop (PLL):

1. What are the applications of PLL?
2. Why this circuit is called Phase Locked Loop?
3. What are the three components of PLL circuitry?
4. Explain the operation of PLL by using a Block Diagram?
5. Define free-running frequency?
6. Define Lock range and Capture range of a PLL?
7. What is meant by Frequency synthesizer?
8. Why PLL is used in FM Receivers/
9. How PLL is used in FSK demodulation circuits?
10. What do you mean by Lock state in a PLL?
11. What is meant by Pull in time in PLL?
12. Phase Detector or Phase Comparator is used for ——————.
13. Why VCO is used in feedback loop of PLL?
14. What are the input and Output signals of a VCO in a PLL?
15. What are the advantages and Disadvantages of PLL?
16. Why Lock range is greater than Capture range in a PLL?

## Time Division Multiplexing (TDM):

1. Define the concept of Time Division Multiplexing?
2. Differentiate Multiplexing and Sampling?
3. What are the different types of Multiplexing Techniques?
4. Does TDM come under analog Multiplexing or Digital Multiplexing?
5. Define a frame in a TDM?
6. Why synchronization is required in TDM?
7. Why multiplexing is required?
8. What do you mean by inter-leaving gaps in TDM frame?
9. If two signals of frequencies 2KHz and 4 KHz are multiplexed in time –domain then draw TDM signal
1. Without inter-leaving gap.
2. with inter-leaving gap of 10 ms.
10. What are the advantages and Disadvantages of TDM?
11. Differentiate TDM with FDM?
12. What are the advantages of TDM over FDM?
13. Explain the operation of TDM using trainer kit used.
14. Synchronous TDM means?

1. What is a filter?
2. Differentiate Active and Passive Filters?
3. Why LPF is used in Demodulation Circuits?
4. Why pre-filtering is required in sampling?
5. Anti-aliasing is achieved by using———– in Sampling Circuits?
6. Define Single-tone modulation in AM and FM?
7. Why FM receivers are more immune to AM receivers?
8. BW of Narrow band FM is ——————.
9. BW of AM is ———————————-.
10. How to calculate Image frequency of a Radio Receiver?
11. Define Power Spectral Density.
12. Define AWGN noise.
13. Define SNR.
14. Where do we use Hilbert Transform?
15. Over modulation in AM means….
16. What is µ value when AM wave is similar to DSBSC wave?

## Digital Communications Slip Test Questions

Slip test-1

1. In a PCM system, if the Quantization levels are increased from 2 to 8,                 i. Find the change in Signal to Quantization Noise ratio.                                ii.Find the change in Transmission Bandwidth.
2.  i. Convert the following signal x(t) = 10 cos (200πt) to a discrete signal x[n] if sampling rate is 1000Hz.                                                                                       ii. Write the advantages of Digital Communication system over Analog Communication system.

PART-B

1. Draw the Basic Block Diagram of Digital Communication System and explain each block in detail.
2. Deduce the expression for Signal to Quantization noise ratio in PCM system.                                                                  Slip test-2
1. A PCM source transmits four samples (messages) with a rate 2B samples /second. The probabilities of occurrence of these 4 samples (messages) are equally likely Find out the information rate of the source.
2. A source produces 26 symbols with equal probability what is the average information produced by this source?

PART-B

1. State Mutual information. Prove any three properties of Mutual information.
2. A source alphabet has 10 symbols with the given probabilities 0.02, 0.04, 0.17, 0.02, 0.16, 0.06, 0.03, 0.27, 0.20, 0.03 construct Shannon-Fano coding and calculate the efficiency.

## Digital Communications Quiz with solutions

UNIT-1

QUIZ

1. The band width  needed to transmit Television video plus audio signals of bandwidth 4.2 MHz using Binary PCM  quantization level of 512 is (  c     )         a. 2 MHz            b. 25.6MHz                        c.37.8MHz                    d.75.6MHz
2. A signal m(t)has a bandwidth of 1.0 KHz and exhibits a maximum rate of change of 2.0 volts/sec. The signal is sampled at a sampling frequency of 20 KHz and quantized using delta modulator. The minimum step size to avoid slope overload is (  b    )                                                                                                                      a. 1.0mV        b. 0.1mV                c.10.0mV                      d.0.01mV
3. For a 10 bit PCM system, the signal to Quantization noise ratio is 62 dB. If the number of bits are increased by 2, then the signal to Quantization noise ratio will be (  c      )                                                                                                                    a. Increased by 6 dB                                   b.  Decrease by 6 dB                                       c. Increased by 12 dB                                  d. Decrease by 12 dB
4. Write the condition to eliminate slope overload error in a Delta modulation system—————————
5. Write the condition to eliminate slope overload error in a Delta modulation system if the input is a single-tone signal—————————
6. For which value of A , A-law has linear transfer characteristics——————–.
7. For which value of µ , µ-law has linear transfer characteristics——————–.
8. In a PCM system if the step size is 5V , then the Quantization noise in dB is   (  d    )                                                                                                                                             a. -5 dB.          b. -3.18dB       c. -10dB                      d. 3.18dB.
9. Draw the characteristics of Mid-rise and Mid-tread type Quantizers ————–
10. The maximum slope of the signal ———————-
11. The sampling rate of the signal ————————–
12. The step size in a 8- bit pcm system if the input signal to PCM is oscillating between [+4V,-4V] is—————————–

UNIT-2

QUIZ 2

1. Entropy is a measure of ——————————– 3M.
2. The capacity of a band – limited AWGN channel in terms of kbps if the average received signal power to noise power spectral density is 1000 and the bandwidth is approximately infinite is (  a    )   [Hint: Shannon’s bound Cinfinity =  S/No]                                                                                                                        a.  1.44        b. 1.08                                c. 0.72                           d. 0.36
3. If Y= g(X) where g denotes a deterministic function, then the conditional entropy H(Y/X) is (  b ) 3M.                                                                                                           a. ≠ 1         b.  = 0                           c. = 1                           d. ≠ 0
4. A Source generates three symbols with probability of 0.25, 0.25, and 0.50 at a rate of 3000 symbols per second. Assuming the symbols are generated independently from the source, the most efficient source encoder would have average bit rate of ( b ) 4M.                                                                                             a. 6000 bits/sec                                                         b. 4500 bits/sec                                 c. 3000 bits/sec                                                         d. 1500 bit/sec.
5. A source generates four equi-probable symbols. If the source coding is adopted, the average length of the code for 100% efficiency is ( c )                       a. 6 bits / symbol                                                          b.   3 bits / symbol                           c.  2 bits / symbol                                                        d.  4 bits / symbol
6. Draw the channel diagram of Binary Erasure channel and write its channel matrix ………………………..
7. For a Binary Symmetric Channel the entropies of X & Y are if the conditional probability p=0.5, where X & Y are input & output random variables of BSC  (   d     )3M                                                                                                                          a. 1,0           b. 0,0               c. 0,1                           d. 1,1
8. Match the following   (  d    )                                                                                                       a. Lossless channel                                           1.  only one non-zero element in each row                                                                                                                                        b. Deterministic channel                                2. Only one non-zero element in each row and each column.                                                                                                   c. Noiseless channel                                           3. Only one Non-zero element in each column.                                                                                                                                d. for a noiseless channel                                  4. H(Y/X)=0 and H(X/Y)= 0. a. none of these                                                  b.   b-3,c-4,a-1,d-2                                    c. c-4,d-3,a-1,b-2                                             d.  b-1,c-2,a-3,d-4.
9. Given a Binary Symmetric Channel, the expression for Entropy is (pis conditional probability of error   ——————- 3M.

Digital Communications Unit-3 Quiz with Solutions

UNIT3

QUIZ3[CO3]

1. The number of Parity check bits in an (n, k) Linear Block codes are (    b      )      a.n                b. (n-k)                       c.  (n+k)                       d. k
2. The Hamming Weight of the following code words 10011101 & 00111100 is      (   c   ) 2M.                                                                                                                                       a. None of these      b. 4, 5              c.5, 4                           d.3,4
3. A cyclic code can be generated using———————— and A block can be generated using——————-.(   c   )                                  a.Generator matrix & Generator polynomial.                                                          b.Generator matrix & Generator matrix.                                                            c.Generator polynomial & Generator matrix.                                                            d.None of the above.
4. The rate of a Block code is the ratio of(    c     )                                                                a.Message length to Block length.                 b.Block length to message length.  c.Message weight to Block length.                 d.None of the mentioned.
5. The syndrome in LBC is calculated using , where Y represents received code word   (     a       ) 2M                                                                                                                      a. S= Y HT          b. S = YH                    c. S= YT H                  d. S= YT HT
6.  A non-Zero value of Syndrome in a Block code represents ( b    ) 2M.                  a.No error during transmission.     b.An error occurred during transmission.   c.Both a and  b                                         d.None of the above.
7. The transmitted code word(X) in an LBC can be obtained from received code word( Y) by using the equation (  a   ), where E represents error vector.              a. X= E + Y           b.  X = X.Y                  c. X= E.Y                    d. X= X/Y
8. The parity check matrix of a (6,3) block code if Generator matrix is G  3M. ——————-.
9. In the above question find the Code words corresponding to message vectors  and ——————-. Ans: , .
10. For the Q.8. Find the syndrome value when the received code word is 001111—-4MAns: syndrome value [0 0 1]
11. For a (7,4) cyclic code , the generator polynomial is given as find the codeword  for the data 1100       Non systematic codeword is—————- .Ans:  1011100              Systematic codeword is——————–.Ans: 1011100.

UNIT-4

QUIZ-4

1. The modulation technique that provides minimum probability of error is (  b   ). a. ASK                  b. PSK                   c. DPSK                        d.FSK
2. At a given probability of error, binary coherent FSK is inferior to binary coherent PSK by (  c  )                                                                                                         a. 6 dB                b. 2 dB           c.  3 dB           d.  0 dB             e. None of these.
3. If Eb, the energy per bit of a binary digital signal, is 10-5watt-sec and the one-sided power spectral density of the white noise, No= 10-6 W/Hz, then the output SNR of the matched filter is  (   d    )                                               a. 26 dB         b.  20 dB          c. 10 dB          d. 13 dB          e. None of these
4. In which system, bit stream is portioned into even and odd stream (   c   )  a. BPSK                  b.  MSK            c. QPSK                     d.  FSK
5. Optimum filter can be called as———when the input noise is white noise (  a   ) 2M.                                                                                                                        a. Matched filter   b.  High pass filter        c. Low pass filter      d. None of these
6. The probability of error of ASK is ————————————.
7. The probability of error of FSK is ————————————.
8. Write the expression for QPSK modulation —————————————-2M.
9. Draw the block diagram of DPSK system————————————.
10. A pulse g(t) = A cos(πt/2T) for 0 ≤ t ≤ T is transmitted over an AWGN channel with two sided noise power spectral density No/2 Watts/Hz. The impulse response of the matched filter is (  a     )                                                           a. A sin (πt/2T)                                                            b.   A rec (πt/2T)                           c. A rec (π(T-t)/2T)                                                    d. A sin (π(T-t)/2T)
11. If the probability of error function of a modulation scheme is Pe = (½) erfc(x) thenthe same Pe interms of Q-function is ( b   )                                             a. Q ( 31/2 * x)                    b. Q (21/2 * x)                c. Q(x)             d. none of these.
12. ## Analog Communications Important Questions unit 2 (FAQ)

UNIT 2:

1. What is meant by Carson’s rule? Problem on calculation of Bandwidth of FM signal using Carson’s rule.(imp)
2. Distinguish between NBFM and WBFM.(imp)
3. Problems on instantaneous phase and frequency.
4. What is the basic principle involved in Phase discriminator?
5. Define capture range and lock range of a PLL.
6. Problem on Armstrong method of generation of a WBFM signal?(imp)
7. Capture effect and threshold effects in FM (very very imp).
8. Problems on carrier swing, frequency deviation, BW, modulation index etc.
9. Phasor diagrams of NBFM and AM.
10. What is the need for limiter in FM receivers? (Very very imp).
11. Find the maximum Frequency deviation of Frequency Modulated signal given by  S(t) = 10 cos(1000000 *pi*t + 5 sin(2000*pi*t)).
12. A carrier signal 10 cos(8x 106πt) is modulated by a modulating signal               5 cos(30x 103πt)  then i. Find the Band width of Frequency Modulated signal assuming  kf  =15KHz.   ii.     Calculate Highest Frequency and lowest Frequency of FM signal.  iii. Find  Modulation index of FM signal.
13. Compare Frequency Modulation with Phase Modulation.

1. Explain the operation of balanced slope detector in detail.
2. Explain in detail about frequency spectrum of WBFM signal using Bessel functions.
3. write in detail about generation methods of FM signal
4. Direct method of generation
5. Indirect method of generation (Armstrong method).(very very imp)
6. What is ratio detector? Explain its working in detail with neat circuit diagram. Explain how ratio detector provides amplitude limiting?
7. What is amplitude limiter? Draw its circuit diagram explain its working. Why amplitude limiter is needed in FM receivers justify.(very very imp)
8. Compare and contrast foster seeley discriminator with ratio detector.
9. What is PLL? What are the applications of PLL? Explain the working of PLL with neat block diagram.(imp)
10. How audio frequency signal is demodulated by using PLL? Explain in detail.
11. Write about pre-emphasis and de-emphasis circuits in detail. Give the significance of pre-emphasis and de-emphasis.(imp)
12. In an FM system if the AF is 500 Hz and its amplitude is 2.4V with a frequency deviation of 4.8 KHz. If the AF voltage is increased to 7.2V then find the modified frequency deviation calculate BW in above two cases.
13. Design Armstrong FM generator for the generation of WBFM signal with frequency deviation=75 KHz and using the NB carrier as 100 KHz and second carrier as 10 MHz Find the suitable multiplying factors. Assume the message signal is defined in the range 100Hz-15 KHz.
14. Draw the block diagram of single-tone NBFM signal if message signal is.
15. Derive an expression for an FM signal with carrier frequency fc and a modulating signal Obtain an expression for its spectrum.
16. Why an FM system is preferred over an AM system?

Unit 2

Assignment 2

1. Explain the generation techniques of FM signal using Direct and Indirect methods in detail.
2. Explain the working of the following demodulator circuits of FM                        i. Balanced slope Detector for FM modulation.                                                            ii. Foster-Seeley discriminator.                                                                                         iii. Ratio-detector.
3. Explain the detection of FM signal using PLL.
4. Explain in detail about pre-emphasis and De-emphasis circuits in FM.

Unit 2

Assignment 2

1. Illustrate the working of PLL using a Block Diagram. Define capture range and Lock range in PLL.                                                                                                                 5M.
2. Sketch the circuit diagram of Ratio-detector and explain working of it in detail. 5M.
3. Sketch the frequency response curves & compare the working of pre-emphasis and De-emphasis circuits in FM communication System.

TUTORIAL TEST 2

Answer any three of the following

1. In an FM system if the AF is 500 Hz and its amplitude is 2.4V with a frequency deviation of 4.8 KHz. If the AF voltage is increased to 7.2V then find the modified frequency deviation calculate BW in above two cases.
2. Explain in detail about frequency spectrum of WBFM signal.
3. Draw the block diagram of single-tone NBFM signal if message signal is
4. Write any four differences between NBFM and WBFM.

## Basic Electronics Important Questions Unit 5 (FAQ)

UNIT-5

Data Acquisition systems: Study of transducer (LVDT, Strain gauge, Temperature, Force).

Photo Electric Devices and Industrial Devices: Photo diode, Photo Transistor, LED, LCD, SCR, UJT Construction and Characteristics only.

Display Systems:  Constructional details of C.R.O and Applications.

ASSIGNMENT- UNIT5

1. Explain with neat sketch, the working principle of LVDT.
2. Give the applications of CRO. Explain the Block diagram of Cathode ray tube in detail. 10M.
3. Write a short note on LED and LCD.

1. Distinguish between photodiode and LED.
2. Why is SCR known as negative resistance device?
3. What are the different types of transducers used for the measurement of temperature. Explain the principle of any of these.
4. Draw the symbol of a SCR and VI characteristics .explain its principle of operation.
5. What is meant by sensitivity in a CRO and explain the necessity of a saw tooth generation of a CRO.
6. What are the important characteristics of an LCD?
7. What is an LVDT? By means of a neat sketch explain how a LVDT is used in measurements.
8. Give the block diagram of a CRO, Explaining the importance of each block.
9. Explain about an instrumentation amplifier.
10. Write a note on UJT.
11. How are DIAC and TRIAC different from operational point of view.
12. What is a transducer?
13. List out the applications of a SCR.
14. Explain the principle involved in a strain gauge.
15. Draw the symbols of SCR, TRIAC, DIAC and UJT.

1. Explain the working of Cathode Ray Tube of CRO with neat block diagram in detail.
2. What are the applications of CRO?
3. Differentiate Photo Diode and Photo Transistor in detail in terms of operation and applications.
4. Differentiate LED and LCD in detail in terms of operation and applications.
5. Explain the working of SCR and draw the characteristics of it.
6. Explain the working of UJT and draw the characteristics of it.

ALL THE BEST

Prepared by         P.Lakshmi Prasanna

## Basic Electronics Important questions units 4 (FAQ)

Unit-4:  Operational Amplifiers – Introduction to OP Amp, characteristics and applications – Inverting and Non-inverting Amplifiers, Summer, Integrator, Differentiator, Instrumentation Amplifier.

Digital System: Basic Logic Gates, Half, Full Adder and Subtractors.

ASSIGNMENT- UNIT4

1. What are the four basic building blocks of Op-Amp explain each block in detail. 10M.
2. Write in detail about Instrumentation amplifier.
3. Design and realize Full Subtractor (Give the necessary expression and Truth table and Circuit diagram)

1. What are the ideal characteristics of OP-AMP?
2. What is an operational amplifier? Mention some of its applications.
3. A 5mV, 1 kHz sinusoidal signal is applied to the input of an OP-amp integrator for which R=100KΩ and C=1µF.Find the output voltage.
4. What are the different parameters of an OP-amp? Sketch the circuit of a summer using OP-amp 741to get Vo= -(-V1+2V2-3V3).
5. Write a note on basic logic gates.
6. Draw the truth table of an EX-OR gate.
7. State and prove De-Morgan’s theorems. Discuss the working of half adder and full adder and give their truth tables.
8. Write a note on universal gates.
9. Realize OR gate using NAND gates.
10. What is half Subtractor? Realize full Subtractor using using NAND gate only.
11. What is an op-amp? List the four building blocks of an op-amp.
12. Draw the schematic symbol of an op-amp in detail. Write the expression for output voltage of an op-amp?
13. What are the applications of op-amp?
14. What are the advantages of op-amp?
15. Why op-amp is named as op-amp?
16. What are universal gates and why those are called as universal gates?
17. Draw EX-OR and EX-NOR gates and give the truth tables of each gate.
18. Name some basic logic gates draw their symbols and give the corresponding truth tables.
19. Draw half adder and half Subtractor.

1. Draw and explain block diagram of op-amp in detail. Or what are the basic building blocks of an op-amp explain in detail.
2. Write in detail about
4. Full Subtractor/ Half Subtractor.
5. Explain the operation of inverting and non-inverting amplifier circuits using op-amp and calculate gain of each circuit in closed loop configuration.
6. Define common mode rejection ratio (CMRR).a differential amplifier has a differential mode gain of 100 and common mode gain of 0.01 then find CMRR in dB?
7. Write in detail about instrumentation amplifier.
8. Explain the operation of the following circuits of op-amp in open loop configuration.    a. Non –inverting amplifier.        b.   Inverting amplifier  c.  Differential amplifier.
9. What are the characteristics of practical op-amp?
10. Define slew rate of op amp? Problem on calculation of Slew rate.
11. Explain how op-amp is used as  Summer and difference amplifier or Subtractor.

Prepared by                                                                                       P.Lakshmi Prasanna

## Basic Electronics Important Questions Unit 3 (FAQ)

UNIT – 3

Unit-3:  Feedback concepts: Properties of negative feedback amplifiers, Classification, Parameters. Oscillators: Barkhausen’s Criterion, LC Type and RC type oscillators and crystal oscillator. (Qualitative treatment only)

ASSIGNMENT- UNIT3

1. Draw the basic block diagram of feedback amplifier and explain each block in detail. 10M.
2. What is the effect of negative feedback on input and output impedances of a voltage series feedback amplifier? Give necessary expressions.
3. Write a short note about RC Phase Shift Oscillator.

1. What is meant by feedback?
2. What is tank circuit? How it is used in oscillators?
3. State Barkhausen’s criterion.
4. Differentiate positive feedback with negative feedback.
5. If open loop voltage gain of feedback amplifier is 200 and feedback factor is 1/5 then find
6. Gain with negative feedback.
7. BW with negative feedback if open loop BW is 100 KHz.
8. What are the advantages of negative feedback?
9. What is the effect of negative feedback on bandwidth?
10. Explain how gain stability is improved by negative feedback?
11. Draw block diagrams of voltage- series and current- shunt feedback topologies?
12. What are four topologies of feedback amplifiers?
13. What is positive feedback .Give the advantage of positive feedback?
14. Write the definition of oscillator?
15. Differentiate oscillators with the frequency of operation?
16. Draw under damped over damped and un-damped oscillatory signals.
17. .In the forward path of an oscillator, two amplifiers of equal gain are cascaded. If the feedback path transfer function is 1/81, find the gain of each amplifier.

1. What is the effect of negative feedback on input and output impedances in voltage-series feedback amplifier give necessary expressions?
2. Derive the general equation for LC oscillator circuits used in oscillators.
3. Distinguish between positive feedback and negative feedback in amplifiers. State the advantages of negative feedback in amplifiers.
4. Draw the circuit diagram of Hartley’s oscillator and explain its operation in detail. Derive the expression for frequency of oscillation.
5. Draw the circuit diagram of Colpitt’s oscillator and explain its operation in detail. Derive the expression for frequency of oscillation.
6. What is the effect of negative feedback on Bandwidth give explanation with necessary equations?
7. Write in detail about four topologies of feedback amplifiers and compare the topologies in terms of gain, BW, input impedance and output impedance.
8. Write the working operation of the following circuits and derive the expression of frequency of oscillation. i. RC-Phase shift oscillator. ii. Wein-bridge oscillator. iii. Crystal oscillator.
9. Draw a neat circuit diagram of an RC phase shift oscillator using BJT and explain its working principle
10. A Hartley oscillator is designed with L1=2mH, L2=20mH, and a variable capacitance. Determine the range of capacitance value, if the frequency of oscillation is varied between 950 kHz and 2050 kHz.

ALL THE BEST

Prepared by       P.Lakshmi Prasanna

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