# ANALOG COMMUNICATION INTERVIEW QUESTIONS WITH ANSWERS

1. What are the main components needed for a communication?
• Sender, Transmission line and Receiver.
•  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
• 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.

• 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)