←
Computer Science and Electronics for Digital Assistant · Chapter 9

Communication Engineering

What to remember

  • A communication system has a transmitter, a channel and a receiver. Noise is added in the channel; the aim is to deliver the message with the least loss.
  • Modulation puts a message on a high-frequency carrier. AM changes amplitude, FM changes frequency, PM changes phase.
  • Digital communication uses sampling, quantisation and coding. Bandwidth, noise and signal-to-noise ratio decide quality.

Basic communication system

Elements: information source, input transducer, transmitter, channel (medium), receiver, output transducer and destination. A microphone is an input transducer; a loudspeaker is an output transducer. Communication can be one-way (broadcast) or two-way. Noise is unwanted signal added during transmission. Types: thermal noise (from random electron motion), shot noise, atmospheric noise and man-made noise (motors, sparks). Signal-to-noise ratio (SNR) = signal power / noise power, often given in decibels: SNR(dB) = 10 log10 (Ps/Pn). A ratio of 100 is 20 dB.

The audio frequency range is about 20 Hz to 20 kHz. Voice needs about 300 Hz to 3.4 kHz for the telephone. A signal can be analog (continuous) or digital (discrete levels, binary).

Why modulation is needed

Direct sending of audio signals is not practical because:

  • Antenna length would be too large. A practical antenna is about a quarter of the wavelength (λ/4).
  • Many stations would mix with each other.
  • Noise and loss are higher at low frequency.

Relation: wavelength λ = c / f, where c = 3 × 10⁸ m/s. Example: for f = 3 MHz, λ = 100 m, and a quarter-wave antenna is 25 m.

Modulation is varying a property of a high-frequency carrier wave with the message signal. The reverse process at the receiver is demodulation or detection.

Analog modulation

TypeWhat changesNotes
AM (Amplitude)Carrier amplitudeSimple, narrow bandwidth, more noise
FM (Frequency)Carrier frequencyBetter noise immunity, wider bandwidth
PM (Phase)Carrier phaseRelated to FM

AM details: modulation index m = Vm / Vc (message amplitude / carrier amplitude). For no distortion, m should be 1 or less. Over-modulation (m above 1) gives distortion. The AM wave has a carrier and two sidebands: the upper sideband (fc + fm) and the lower sideband (fc − fm). Bandwidth = 2 × fm. Example: with a 5 kHz message, AM bandwidth is 10 kHz. Total power Pt = Pc (1 + m²/2). Example: carrier power 100 W and m = 1 gives Pt = 100 × 1.5 = 150 W. Two-thirds of the power is wasted in the carrier at m = 1. The variants are DSB-SC (carrier suppressed) and SSB (one sideband only), which save power and bandwidth.

FM details: the carrier frequency deviates in proportion to the message amplitude. Frequency deviation is the maximum shift from the carrier frequency. Modulation index mf = deviation / message frequency. Carson's rule: bandwidth ≈ 2 (deviation + fm). Commercial FM broadcast has a maximum deviation of 75 kHz and channel spacing of 200 kHz. FM is less affected by amplitude noise, so it gives higher quality than AM.

Broadcast bands (standard values): AM medium wave is about 530 to 1600 kHz; FM broadcast is 88 to 108 MHz; TV uses VHF and UHF bands. Short wave (HF, 3 to 30 MHz) bounces off the ionosphere for long distance.

Radio receivers

The superheterodyne receiver is the standard receiver. Its stages are: antenna, RF amplifier, mixer with local oscillator, IF amplifier, detector, audio amplifier and loudspeaker. The intermediate frequency (IF) is fixed: 455 kHz for AM and 10.7 MHz for FM. The local oscillator frequency = signal frequency + IF. Example: to receive 1000 kHz with a 455 kHz IF, the oscillator runs at 1455 kHz. The advantages are good selectivity and sensitivity. Selectivity is the ability to pick one station. Sensitivity is the ability to receive weak signals. Fidelity is the faithful reproduction of the original sound. The image frequency = signal frequency + 2 × IF. AGC (automatic gain control) keeps the output level steady.

Digital communication

Steps in converting an analog signal to digital are sampling, quantisation and encoding. The sampling theorem (Nyquist): the sampling frequency must be at least twice the highest frequency in the signal, fs ≥ 2 fmax. Example: a 4 kHz voice signal needs at least 8 kHz sampling. Telephone PCM uses 8000 samples per second with 8 bits per sample, giving 64 kbps. A CD uses 44.1 kHz sampling.

Number of quantisation levels = 2ⁿ for n bits. 8 bits give 256 levels.

Bit rate = sampling rate × bits per sample.

Common techniques: PCM (pulse code modulation), delta modulation. Digital keying: ASK (amplitude shift), FSK (frequency shift), PSK (phase shift). Advantages of digital over analog: better noise immunity, easy error detection, easy storage and encryption. Disadvantage: larger bandwidth. Multiplexing sends many signals over one channel: FDM divides the frequency, TDM divides the time. A parity bit is a simple error check bit. Channel capacity (Shannon): C = B log2 (1 + SNR), so more bandwidth or better SNR means higher capacity.

Antennas and wave propagation

An antenna changes electrical signals to radio waves and back. Types: dipole, monopole (Marconi), Yagi-Uda (rooftop TV antenna), parabolic dish (satellite and microwave links), horn antenna. Gain is the focusing power of the antenna. Radio waves travel as:

  • Ground (surface) wave: low frequency, follows the earth.
  • Sky wave: reflected by the ionosphere, long distance (HF).
  • Space (line of sight) wave: VHF, UHF and microwave; needs a clear path, so towers are used.

Line-of-sight range from a tower of height h is about d = √(2 R h), with R as the earth radius. Higher towers give a longer range.

Modern systems

  • Mobile cellular: the area is divided into cells with a base station each; frequencies are reused. Generations are 2G (digital voice, SMS), 3G (mobile data), 4G (LTE, broadband), 5G (very high speed and low delay). GSM and CDMA are access technologies.
  • Satellite: a geostationary satellite stays above one point of the equator at about 36,000 km height with a period of 24 hours. Indian systems include INSAT and GSAT for communication, and NavIC for navigation. Uplink and downlink use different frequencies.
  • Optical fibre: uses total internal reflection; very high bandwidth, low loss, no electrical interference.
  • Radar: uses reflected radio waves to find range and speed. Range = c × t / 2.
  • Wi-Fi and Bluetooth: short-range wireless using the 2.4 GHz band (and 5 GHz for Wi-Fi).
  • Doordarshan and All India Radio: national TV and radio broadcasters.

Mobile and internet services reach the villages that Sachivalayam staff serve, so a basic idea of towers, fibre and networks is useful.

Comparing AM and FM

PointAMFM
Quantity variedAmplitude of the carrierFrequency of the carrier
Bandwidth2 × fm (narrow)Wide (Carson's rule)
Noise effectEasily disturbed by noiseMuch less affected
Receiver and circuitSimple and cheapMore complex
Typical useMedium wave and short wave radioVHF radio broadcast, TV sound

Worked example: a carrier of 100 kHz is modulated by a 5 kHz tone. The sidebands are at 105 kHz and 95 kHz, and the bandwidth is 10 kHz. In FM, a 15 kHz audio signal with 75 kHz deviation needs about 2 × (75 + 15) = 180 kHz by Carson's rule, which fits inside the 200 kHz channel.

Other terms to know: a carrier is a steady high-frequency wave that carries the message. Attenuation is the loss of signal strength along the path. Distortion is an unwanted change in the shape of the signal. A decibel (dB) is a logarithmic unit to compare two powers; a gain of 3 dB means the power doubles. A transducer converts one form of energy to another. A repeater or amplifier is used on long links to restore the signal. Simplex, half duplex and full duplex describe the direction of communication; broadcast radio is simplex and a mobile call is full duplex.

Exam traps

  • AM and FM: AM varies amplitude; FM varies frequency.
  • AM bandwidth: it is 2 × fm, not fm.
  • Carrier and sideband power: at m = 1 only one-third of the total power is in the sidebands.
  • Modulation and demodulation: modulation is at the transmitter; demodulation at the receiver.
  • Sampling rate: at least twice the highest frequency, not equal to it.
  • IF values: 455 kHz for AM, 10.7 MHz for FM.
  • Selectivity and sensitivity: selectivity picks one station; sensitivity picks weak signals.
  • FDM and TDM: FDM shares frequency; TDM shares time.
  • Geostationary and polar satellites: a geostationary one appears fixed over a place.
  • Analog and digital noise immunity: digital is better.

One-liners

  • 1. A quarter-wave antenna has length λ/4.
  • 2. AM wave has a carrier and two sidebands.
  • 3. FM broadcast band is 88 to 108 MHz.
  • 4. Superheterodyne receivers use a fixed intermediate frequency.
  • 5. Nyquist rate is twice the highest signal frequency.
  • 6. Telephone PCM runs at 64 kbps.
  • 7. Shannon's formula gives channel capacity.
  • 8. TDM shares a channel by time slots.
  • 9. SSB saves power and bandwidth.
  • 10. Geostationary orbit is about 36,000 km above the equator.
  • 11. Optical fibre works on total internal reflection.
  • 12. Radar range is c × t / 2.

Practice questions

  1. A signal has a frequency of 3 MHz. Its wavelength in air is (speed of light 3 x 10^8 m/s)

    1. 1000 m
    2. 1 km
    3. 10 m
    4. 100 m
    Answer

    D. 100 m

    Wavelength = c/f = 3 x 10^8 / 3 x 10^6 = 100 m.

  2. The length of a quarter-wave antenna for a 3 MHz signal is

    1. 12.5 m
    2. 100 m
    3. 50 m
    4. 25 m
    Answer

    D. 25 m

    Wavelength is 100 m, and a quarter of it is 25 m.

  3. A 5 kHz message signal is used for AM. The bandwidth of the AM wave is

    1. 20 kHz
    2. 10 kHz
    3. 5 kHz
    4. 2.5 kHz
    Answer

    B. 10 kHz

    AM bandwidth = 2 x fm = 10 kHz.

  4. A 1000 kHz carrier is amplitude modulated by a 4 kHz tone. The upper sideband frequency is

    1. 1008 kHz
    2. 996 kHz
    3. 1000 kHz
    4. 1004 kHz
    Answer

    D. 1004 kHz

    Upper sideband = fc + fm = 1004 kHz.

  5. A carrier of amplitude 6 V is modulated by a message of amplitude 3 V. The modulation index is

    1. 2
    2. 0.25
    3. 1
    4. 0.5
    Answer

    D. 0.5

    m = Vm/Vc = 3/6 = 0.5.

  6. An AM transmitter has carrier power 100 W and modulation index 1. The total power is

    1. 200 W
    2. 150 W
    3. 100 W
    4. 125 W
    Answer

    B. 150 W

    Pt = Pc(1 + m^2/2) = 100 x 1.5 = 150 W.

  7. An AM transmitter has carrier power 100 W and modulation index 0.5. The total power is

    1. 106.25 W
    2. 112.5 W
    3. 125 W
    4. 150 W
    Answer

    B. 112.5 W

    Pt = 100 x (1 + 0.25/2) = 112.5 W.

  8. A superheterodyne AM receiver with IF 455 kHz is tuned to 1000 kHz. The local oscillator frequency is

    1. 455 kHz
    2. 545 kHz
    3. 1910 kHz
    4. 1455 kHz
    Answer

    D. 1455 kHz

    LO = signal + IF = 1000 + 455 = 1455 kHz.

  9. For a 1000 kHz station and IF 455 kHz, the image frequency is

    1. 545 kHz
    2. 2000 kHz
    3. 1910 kHz
    4. 1455 kHz
    Answer

    C. 1910 kHz

    Image = signal + 2 x IF = 1000 + 910 = 1910 kHz.

  10. A voice signal has a highest frequency of 4 kHz. The minimum sampling frequency is

    1. 2 kHz
    2. 4 kHz
    3. 16 kHz
    4. 8 kHz
    Answer

    D. 8 kHz

    Nyquist rate is twice the highest frequency.

  11. Telephone PCM uses 8000 samples per second and 8 bits per sample. The bit rate is

    1. 8 kbps
    2. 64 kbps
    3. 16 kbps
    4. 128 kbps
    Answer

    B. 64 kbps

    8000 x 8 = 64000 bps.

  12. The number of quantisation levels with an 8-bit code is

    1. 256
    2. 8
    3. 128
    4. 64
    Answer

    A. 256

    2^8 = 256.

  13. A signal-to-noise power ratio of 100 corresponds to

    1. 2 dB
    2. 10 dB
    3. 20 dB
    4. 100 dB
    Answer

    C. 20 dB

    10 log10(100) = 20 dB.

  14. A power gain of 1000 equals

    1. 3 dB
    2. 300 dB
    3. 10 dB
    4. 30 dB
    Answer

    D. 30 dB

    10 log10(1000) = 30 dB.

  15. For FM with a maximum deviation of 75 kHz and a highest audio frequency of 15 kHz, Carson's rule gives a bandwidth of about

    1. 90 kHz
    2. 30 kHz
    3. 180 kHz
    4. 150 kHz
    Answer

    C. 180 kHz

    2 x (75 + 15) = 180 kHz.

  16. A radar echo returns after 200 microseconds. The target distance is about (speed of light 3 x 10^8 m/s)

    1. 3 km
    2. 30 km
    3. 15 km
    4. 60 km
    Answer

    B. 30 km

    d = c x t / 2 = 3 x 10^8 x 200 x 10^-6 / 2 = 30,000 m.

  17. A 100 MHz FM carrier has a wavelength of

    1. 3 m
    2. 300 m
    3. 30 m
    4. 0.3 m
    Answer

    A. 3 m

    3 x 10^8 / 10^8 = 3 m.

  18. A channel has bandwidth 1 kHz and a signal-to-noise ratio of 15 (as a ratio). By Shannon's formula, capacity is

    1. 4 kbps
    2. 15 kbps
    3. 16 kbps
    4. 1 kbps
    Answer

    A. 4 kbps

    C = 1000 x log2(16) = 4000 bps.

  19. Varying the amplitude of a carrier in step with the message is called

    1. Phase modulation
    2. Pulse demodulation
    3. Amplitude modulation
    4. Frequency modulation
    Answer

    C. Amplitude modulation

    This defines AM.

  20. Varying the frequency of a carrier in step with the message is called

    1. Pulse counting
    2. Frequency modulation
    3. Amplitude modulation
    4. Delta demodulation
    Answer

    B. Frequency modulation

    This defines FM.

  21. The FM broadcast band in India and worldwide is about

    1. 530 to 1600 kHz
    2. 3 to 30 MHz
    3. 2.4 to 2.5 GHz
    4. 88 to 108 MHz
    Answer

    D. 88 to 108 MHz

    FM broadcasting is in the VHF range of 88 to 108 MHz.

  22. The standard intermediate frequency of an AM receiver is

    1. 1 MHz
    2. 455 kHz
    3. 10.7 MHz
    4. 88 MHz
    Answer

    B. 455 kHz

    AM receivers use 455 kHz; FM receivers use 10.7 MHz.

  23. The standard intermediate frequency of an FM receiver is

    1. 108 MHz
    2. 1.07 MHz
    3. 10.7 MHz
    4. 455 kHz
    Answer

    C. 10.7 MHz

    FM receivers use 10.7 MHz IF.

  24. The ability of a receiver to pick one station out of many is called

    1. Sensitivity
    2. Modulation index
    3. Fidelity
    4. Selectivity
    Answer

    D. Selectivity

    Selectivity separates stations; sensitivity is about weak signals.

  25. The theorem that sets the minimum sampling rate is the

    1. Nyquist sampling theorem
    2. Gauss theorem
    3. Thevenin theorem
    4. Superposition theorem
    Answer

    A. Nyquist sampling theorem

    Sampling must be at least twice the highest frequency.

  26. Which multiplexing method shares a channel by dividing it into time slots?

    1. TDM
    2. DSB
    3. AM
    4. FDM
    Answer

    A. TDM

    Time Division Multiplexing assigns time slots; FDM assigns frequency bands.

  27. A satellite that appears fixed over one point on the equator is

    1. polar
    2. low earth orbit
    3. elliptical
    4. geostationary
    Answer

    D. geostationary

    Its period is 24 hours at about 36,000 km height.

  28. Optical fibre guides light by

    1. diffraction
    2. refraction only
    3. polarisation
    4. total internal reflection
    Answer

    D. total internal reflection

    Light is trapped in the core by total internal reflection.

  29. Radio waves reflected by the ionosphere for long-distance HF communication are

    1. surface waves
    2. sky waves
    3. space waves
    4. ground waves
    Answer

    B. sky waves

    Sky waves are reflected by the ionosphere.

  30. Statement 1: In AM, the amplitude of the carrier is varied. Statement 2: FM is less affected by amplitude noise than AM. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  31. Statement 1: Modulation is done at the receiver. Statement 2: Demodulation is done at the transmitter. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Modulation is at the transmitter and demodulation at the receiver; both are wrong.

  32. Statement 1: SSB saves power and bandwidth compared with ordinary AM. Statement 2: AM bandwidth equals the message frequency. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    AM bandwidth is twice the message frequency.

  33. Statement 1: Digital signals have better noise immunity than analog signals. Statement 2: Digital transmission usually needs more bandwidth. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  34. Statement 1: FDM divides the channel by frequency. Statement 2: TDM divides the channel by frequency. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    TDM divides by time.

  35. Statement 1: Sampling frequency must be at least twice the highest signal frequency. Statement 2: Quantisation converts a sampled value into one of a finite set of levels. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  36. Statement 1: The local oscillator frequency in a superheterodyne AM receiver is signal frequency plus IF. Statement 2: The IF is different for each station tuned. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The IF is fixed; the oscillator tracks the station.

  37. Statement 1: 2G brought digital voice and SMS. Statement 2: 4G (LTE) gave mobile broadband. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  38. Statement 1: A parabolic dish antenna is used for satellite links. Statement 2: A Yagi-Uda antenna is a common rooftop TV antenna. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  39. Statement 1: Over-modulation in AM (index above 1) gives distortion. Statement 2: In DSB-SC the carrier is transmitted at full power. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    DSB-SC suppresses the carrier.

  40. Statement 1: Noise is an unwanted signal added in the channel. Statement 2: A loudspeaker is an input transducer. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A loudspeaker is an output transducer.

  41. Statement 1: Line-of-sight waves are used by VHF, UHF and microwave links. Statement 2: A taller tower gives a longer line-of-sight range. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  42. Statement 1: Ground waves are mainly used at low frequencies. Statement 2: Wi-Fi uses the 2.4 GHz band. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  43. Which of these is an input transducer in a communication system?

    1. Loudspeaker
    2. Microphone
    3. Antenna array only
    4. Video screen
    Answer

    B. Microphone

    A microphone converts sound to an electrical signal.

  44. Which technique shifts the phase of a carrier to represent digital data?

    1. ASK
    2. PSK
    3. PCM
    4. FSK
    Answer

    B. PSK

    Phase Shift Keying changes the phase.

  45. A cellular network reuses frequencies by dividing the area into

    1. spheres
    2. zones of one tower
    3. rings of one cable
    4. cells
    Answer

    D. cells

    Each cell has a base station, so frequencies are reused in distant cells.

Page 1 of 1
‹
›