1. What are the main components needed for a communication?
  • Sender, Transmission line and Receiver.
  1. What is Radio communication?
  •  Radio communication uses electrical energy to transmit information. Radio transmitter converts audio (sound) signals to electrical signals that are sent over wires or through space. A radio receiver converts the electromagnetic waves so that the information can be found out.
  1. Range of Audio Frequency?
  • Ø 20Hz to 20 KHz
  1. Range of Radio Frequency?
  • Ø 10 KHz to 1000GHz.
  1. What is modulation?
  • Ø Modulation is the process of changing the characteristics of the carrier signal with respect to that of the message signal.
  1. What are the principle forms of modulation?
  • Ø Amplitude Modulation(AM)
  • Ø Frequency Modulation(FM)
  • Ø Phase Modulation(PM)
  1. What are the types of angle modulations?
  •  FM and PM
  1. What is a mixer?
  •  An electronic circuit that combines two frequencies.
  1. What is a product detector?
  • It is a detector whose audio frequency output is equal to the product of the Beat Frequency Oscillator (BFO) and the RF input signal.
  1. What is a Beat Frequency Oscillator (BFO)?
  •  An oscillator whose output frequency is approximately equal to the transmitter’s carrier frequency.
  1. What is known as Bandwidth?
  •  Bandwidth is the frequency range, in hertz (Hz), between the upper and lower frequency limits.

          12. What is Harmonics?

  • Harmonics is the signals with frequencies that are an integral multiple of the fundamental frequency.

         13. What is the range of frequency in commercial AM broadcasting?

  • 540 KHz to 1600 KHz has a 10 KHz bandwidth.

          14. Range of frequency in AM baseband?

  •  100KHZ to 5 KHz.

          15. What is baseband?

  •  It is a band of frequencies occupied by a message signal.

          16. What you mean by frequency translate?

  •  It is the process of displaying the frequency contents of a signal to another place in the frequency spectrum.

           17. Modulation index?

  • Modulation index (m) is the ratio between the amplitude of the message signal to the amplitude of the unmodulated carrier signal.

            18. What is percentage of modulation?

  •  The modulation index (M) is expressed as a percentage.(%m)

           19. Transmission efficiency?

  •  It is the ratio of the total AM power to that of the sideband.

            20. What is Splatter?

  •  Adjacent channel interference due to over modulation of carrier signal by abrupt peak message signal.

21. What is the selectivity of a radio receiver? The ability of a radio receiver to select a desired signal frequency while rejecting all others is called selectivity.           22. What are the main components of a RF receiver?

  • RF filter, RF amplifier, mixer. Local Oscillator, IF filter, IF amplifier and Envelope Detector.

23. What is RF stage?

  •  It is the first input stage in which primary selection, filtering and amplification of the input RF signal is performed.

24. What is IF stage?

  •  It is the section of the receiver between the mixer and the detector. The IF stage operates at a fixed intermediate frequency (FIF) and it is where most of the amplification and filtering occurs.
  •   24. What is local oscillator?
  • An oscillator in which a piece of electronic equipment that is usually used as a source of electromagnetic wave frequencies for mixing with other frequencies that the equipment handles.

25. What is envelope detector?

  •  A circuit containing a diode in series with an RC network, used to perform demodulation. An envelope detector, which demodulates an AM signal, cannot demodulate an SSB signal

26. What is a diode detector?

  •  A diode detector is a nonlinear charging circuit formed by a diode in series with a parallel with RC network.

27. What is a balance modulator?

  •  Balance modulators translate a message signal to two sidebands and suppress the carrier signal frequency to produce a DSB signal.

28. What is up conversion?

  •  The process of increasing the frequency of a signal to a higher frequency for transmission is up conversion. The mixer, which is a balanced modulator, performs the up-conversion.

29. What is the function of AGC circuit?

  • A circuit that maintains the output volume of a receiver, regardless of the variations in the received signal power.

30. What is centre frequency?

  •  Centre frequency is the frequency of the unmodulated FM carrier signal.

31. What is frequency deviation?

  • Frequency deviation – the maximum frequency change between a modulated and unmodulated carrier signal.

32. What is discriminator?

  • Discriminator is a device that demodulates an FM signal.

33. What is Quadrature?

  • Quadrature is the phase relationship between two equal frequency signals that are 90° out of phase.

34. FM modulation index?

  •  FM modulation index – the ratio of frequency deviation to the message signal frequency.

35. What is signal to noise ratio?

  •  A signal-to-noise ratio is the ratio of the magnitude of the signal to that of noise (often expressed in decibels).

36. What are varactor diode and schottky diode?

  •  Varactor diode is a diode that changes capacitance with a change in input voltage.
  • Schottky diode is diodes that have a low forward voltage drop in the range of 0.2V to 0.4V.

37. What is phase-locked loop (PLL)?

  • Phase-locked loop (PLL) is a circuit that locks onto a phase relationship between an input signal and a VCO signal, and produces an error signal. The error signal is fed back to control the VCO frequency so that it equals the input frequency.

38. What is Frequency-Shift Keying (FSK)?

  • Frequency-Shift Keying (FSK) is shifting an analog signal’s frequency between two values to represent the binary states high and low of a digital signal. A modem uses FSK to transmit digital signals over analog telephone lines.

39. What is voltage controlled oscillator (VCO)?

  •  Voltage controlled oscillator (VCO) an oscillator that uses an input voltage to control its output frequency.

40.  What are the advantages and applications of PLL?

  •  PLLs have high degree of noise immunity and a narrow bandwidth.
  •  Do not require complex coil adjustments.
  • PLLs are also used for Frequency-Shift Keying (FSK) operations in digital communications.
  •  A PLL operates by producing a voltage controlled oscillator (VCO) frequency (f vco) that matches the RF frequency (fi).
  • PLLs are used in FM and AM demodulation because they do not require complex coil adjustments.

41. What is noise?

  • An unwanted signal that will disturb the transmission or processing of signals in communication systems. Eg.  Shot noise, Thermal noise.

42. What is shot noise?

  •  A noise arises from the discrete nature of diodes and transistors.

          E.g., a current pulse is generated every time an electron is emitted by the cathode. 43. What is thermal noise?

  • A noise arises from the random motion of electrons in a conductor.

44. What is white noise?

  • A noise is white if its PSD (Power Spectral Density) equals constant for all frequencies.

45. What is modulation?

  •  A process by which some characteristic of a carrier is varied in accordance with a modulating wave (baseband signal).

      There are,

  • Amplitude modulation
  •  Angle modulation (Phase modulation, Frequency modulation).

46. What is Transmission Bandwidth?      

Transmission bandwidth of an AM wave,

  • For positive frequencies, the highest frequency component of the AM wave equals fc + W, and the lowest frequency component equals fc W.
  •  The difference between these two frequencies defines the transmission bandwidth BT for an AM wave.

47. Limitations of Amplitude Modulation (DSB-C)?

  • Ø Waste of power in the information-less “with-carrier” part.
  • Ø Wasteful of power and bandwidth

48. How to generate SSB signal?

  • Ø Product modulator to generate DSB-SC signal
  • Ø Band-pass filter to pass only one of the sideband and suppress the other.

49. Vestigial Sideband Modulation?

  •  Instead of transmitting only one sideband as SSB, VSB modulation transmits a partially suppressed sideband and a vestige of the other sideband.

Eg: Television Signals

50. What is Angle Modulation?

  •  The angle of the carrier is varied in accordance with the base band signal.

        Commonly used angle modulation

  •  Phase modulation (PM)

Frequency modulation (FM)


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