Basic block diagram of analog communication system


Communications refer to sending, receiving and processing of information by electrical means, that is it means exchanging information between transmitter and receiver.

In early 1840’s the type of communication used was Wire telegraphy later on the forms are as telephony, Radio communication (possible with the invention of triode tube, Satellite communications and fibre optics(with the invention of transistors and IC’s and semi-conductor devices), that means communications become more advanced with increasing emphasis on computer and other data communications.

A modern communication system is concerned with

before transmission:- 

  • sorting:- sorting for the right message.
  • Processing:- processing is to make that message more suitable for transmission.
  • storing:- storing that message before transmission.

then the actual transmission of that message takes place (processing and filtering  noise)

at the receiver:-

  • decoding:-decoding the original message.
  • storage:-storing a copy of that message.
  • interpretation:-and analyzing for the correctness of that message.

the different forms of modern communication systems includes Mobile communications,Computer communications, Radio telemetry etc.

to become familiar with communication systems one needs to know about amplifiers and oscillators that means fundamentals of electronic circuits must be known, with these concepts as a background the every day communication concepts like noise, modulation and information theory as well as various types of systems may be studied.

The most general form of Communication system ( one or two blocks may differ) is shown in the figure basic terminology used in Communication systems is message signal /information/data,channel,noise,modulation, encoding and decoding. Communication system is meant for communicating messages between Transmitter and Receiver (or) source & destination.


source or information source is the primary block in communication system which generates original message / actual message. 

i.e, selecting one message (actual message) from a group of messages itself is called as sorting data (or) information. Source generates message which may be in any form like words, code , symbols, sound signal, images, videos etc.among these the desired message has been selected and conveyed.

A transducer is one which converts one form of energy into electrical energy because the message from information source may not be always in electrical form, a transducer is used in between source and transmitter as a separate block sometimes (or) may be a part of Tx r.


Txr is meant for the following tasks

  • restriction of range of audio frequencies (i.e, limiting the bandwidth of the message signal).
  • Amplification.
  • Modulation. 

In general modulation is said to be the main function of the transmitter.


The medium that exists between transmitter and receiver is called as channel. The function of channel is to provide connection between transmitter  and receiver, two types of channels are  there wired/point to point  and wireless/broadcasting channels.

Point to point channels are generally wired channels(i.e, a physical medium exists) like Microwave links, optical fibre links etc. 

Microwave links:- these links are used in telephone transmission.In these type of links guided EM waves are used to transmit from Txr to Rxr.

optical fibre links:- used in low-loss high speed data transmission and uses optical fibers as the medium .

Broadcast channels:- the medium or channel is wireless here, in broadcasting a single transmitter can send information to many receivers simultaneously, satellite broadcasting system is one such system.

during the process of transmission and reception, the signal gets distorted due to noise in the channel, noise may interfere with the signal at any point but noise in the channel has greatest effect on the signal.


The main function of the receiver is to reproduce the message signal in electrical form from the distorted received signal. This reproduction process is called demodulation (or) detection , in general this demodulation may be assumed as the reverse process of modulation carried out in transmission. 

there are a great variety of receivers in communication systems, the type of receiver chosen depends on type of modulation, operating frequency ,its range  and type of destination required. Most common receiver is superheterodyne receiver .

                            crystal receiver with head phones
                                  Radio receiver

so many types of receivers are available from a very simple crystal receiver with headphones to radar receiver etc.

Destination:- It is the final stage of any communication system. it would be a loud speaker / a display device/simply a load etc depending up on the requirement of the system.

analog communications lab viva questions


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?

            Additional Questions:

            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?





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