Communication

Question 1

Define modulation

Answer 1

the process where audio signal is added onto a carrier signal

Question 2

Define AM

Answer 2

amplitude of the carrier wave is made to vary in synchrony with the displacement of the information signal

Question 3

Strengths and weaknesses of AM

Answer 3

weakness: subject to noise
strength: cheaper cost for receiver

Question 4

Define FM

Answer 4

frequency of the carrier wave is made in synchrony with the displacement of the information signal

F signal = F carrier + V*frequency deviation

Question 5

strengths and weaknesses of FM

Answer 5

strength: noise is eliminated
weaknesses: extra cost, line of sight only

Question 6

Advantages of using digital signal

Answer 6

1. regeneration removes noise
2. cheaper to produce and more reliable
3. can have extra info
4. if there is an error in reception it can be sent again

Question 7

Differences between amplification and regeneration

Answer 7

Both analogue and digital signal experiences attenuation

For analogue signal, repeater amplifier is used, but also amplify noises

For digital signal, regeneration remove noise

Question 8

Binary system (each digit is known as? MSB? V levels?)

Answer 8

• each digit in the binary number is known as ‘bits’
• the bit on the left hand side is MSB, and has highest value
• the no. of bits per sample limits the no. of possible V levels
  4 bits, 2^4, 16 levels

Question 9

Define sampling

Answer 9

measurement of the analogue signal at regular interval

Question 10

How to improve sampling

Answer 10

1. increase no. of bits at each sampling — step height is reduced
2. increase rate of sampling — step width reduced

Question 11

what is the maximum sampling rate?

Answer 11

twice the highest frequency produced

Question 12

Characteristics of geostationary and polar orbits

Answer 12

Geo:

1. period 24 hrs
2. very high orbit

Polar:

1. travel from pole to pole
2. lower orbit

Question 13

Advantages and disadvantages of geostationary orbit

Answer 13

adv:

1. easy to track down
2. can be used for continuous communication

dis:

1. high power input required, large attenuation
2. large delay time

Question 14

Advantages and disadvantages for polar orbit

Answer 14

adv:

1. detect small details
2. small delay time
3. passes over every area on earth

dis:

1. hard to track
2. cannot use for continuous comm.

Question 15

Problem with swamping when use geo and polar orbit

Answer 15

The satellite re-transmit the signal it receives with higher power and on another frequency.

If the downlink and uplink frequencies were the same, the signal from Earth would swamp the signal sent from satellite.

Question 16

frequency and wavelength used in wire pairs

Answer 16

f: 500kHZ
wavelength:

Question 17

frequency and wavelength used in coaxial cables

Answer 17

f: 50MHZ

Question 18

frequency used in radio and micro waves

Answer 18

surface wave: up to 3MHZ
sky wave: 3-30MHZ
space wave: 30-300+MHZ
microwave: 1-300GHZ

all travels at speed of light

Question 19

communication method and waveband of micro and radiowave

Answer 19

surface: LW and MW radio, e.g. AM, in LF band
sky: SW radio in HF band
space: FM radio in VHF band, TV and mobile phones in UHF band
microwave: microwave in SHF band and EHF bands

Question 20

Compare a fibre optic cable to metal cable

Answer 20

• much greater bandwidth
• less attenuation, repeater and regeneration amplifiers can be further apart
• does not suffer from crosstalk
• cheaper than same length of copper wire

Question 21

Principles involved in ADC

Answer 21

• v is measured at regular intervals
• convert to digits (binary numbers)
• transmitted as a series of pulses
• sample at twice max f
• use parallel to series converter