Stuff

Question 1

properties of line code

Answer 1

Self synchronisation

Low probability of bit error

Spectrum has to be suitable for the channel

Transmission bandwidth as small as possible

Question 2

Sequence for FM demodulation

Answer 2

Phase comparator

Loop filter

VCO

Question 3

What does the phase comparator do in a FM demodulator

Answer 3

Produces an error voltage that depends on the difference in phase between input signal and output from VCO

Question 4

What does the loop filter do in an FM demodulator

Answer 4

Smooths error voltage signal and this is applied to the input of the VCO

Question 5

What does the VCO do in an FM demodulator

Answer 5

The VCO has a certain output frequency when error voltage is zero. The error voltage varies the frequency above or below this value.

Question 6

When an FM signal is applied to the PLC what changes occur with the VCO

Answer 6

The VCO follows the variation in the instantaneous frequency. The error voltage at the VCO input corresponds to the voltage change that caused the change in frequency

Question 7

What causes atmospheric noise

Answer 7

Ionosphere effect during electrical storms and lightning

Question 8

What causes sky noise

Answer 8

Caused from oxygen and water vapour resonance in the earths atmosphere (this can affect all forms of radio transmission and the design of a radio system)

Question 9

What is the cause of man made noise

Answer 9

Man made devices such as electric motors and ignition systems

Question 10

What is the cause of thermal noise

Answer 10

Thermal agitation of electrons or charged particles present in all forms of electrical systems

Question 11

What is the cause of shot noise

Answer 11

Due to random arrival of electron packets at the potential barrier of forward biased PN junctions. Always associated the a DC flow in diodes and BJTs. It is frequency dependant white noise well into the GHz region

Question 12

What is flicker noise

Answer 12

Due to contamination and crystal defects found in all active devices is inversely proportional to frequency
(Also called 1/f noise)

Question 13

What is multiplexing

Answer 13

Transmission of signals from multiple sources via a single transmission medium

Question 14

What is demultiplexing

Answer 14

Restoration of original individual values from multiplexing

Question 15

What is FDM (frequency division multiplexing)

Answer 15

Signals are combined by modulating onto different carrier frequencies - modulated signals are combined onto a single signal that can be transported by the link - carrier frequencies are chosen as to leave sufficient guard band to minimise interference from adjacent channels.

Question 16

what is TDM (time division multiplexing)

Answer 16

Signals are interleaved in time. Each signal source transmits in turn in its own time slot repeating at different intervals. Transmitter and reliever need to be synchronised in order for the signals to arrive at the correct destination

Question 17

What is Statistical TDM

Answer 17

The allocation of time slots dependent on demand. No synchronisation required

Question 18

What is CDM (code division multiplexing)

Answer 18

A channel access method used by various radio communication technologies. Allowing numerous signals to occupy a single transmission channel, optimising the use of available bandwidth

Question 19

What is WDM (wavelength division multiplexing)

Answer 19

Multiplexes multiple optical signals on one optical fibre by utilising different wavelengths

Question 20

What is PDH (plesiochronous digital hierarchy)

Answer 20

An earlier system that is being replaced by SDH, it is used in Europe and is the E- carrier system

Question 21

What is SDH (synchronous digital hierarchy)

Answer 21

The synchronous optical network SONET

Question 22

Explain the E carrier system

Answer 22

E1 consists of 30 voice channels, each voice channel sampled is at 8KHz and encoded using 8bits yielding a 64Kbits/s , additional time slot 0 and 16 are used for synchronisation

Question 23

What is a transmission line

Answer 23

Medium used to carry electrical signals fro one place to another

Question 24

What must a balanced transmission line contain

Answer 24

Two conductors not earthed (twisted pair)

Question 25

What must an unbalanced transmission line contain

Answer 25

One conductor is earthed while the other conductor is not (coaxial cable)

Question 26

In a mismatched transmission line if the impedance at the end of a finite line is not equal to Z0 what happens to the energy at the end of the line

Answer 26

Not all energy is then absorbed by the load and some energy is reflected back resulting in voltage and current waves travelling in the reverse direction.

Question 27

In a mismatched transmission line what would happen in the interaction of incident wave and reflected wave

Answer 27

They would add together producing a standing wave. This is a variation of voltage or current that has a maxima and minima at certain lengths across the line

Question 28

What are the advantages of optical fibre

Answer 28

High bandwidth Low attenuation No electromagnetic interference Low cost Small size/ low weight

Question 29

Optical fibre is made of a core of glass surrounded by cladding and coating - is the refraction index of the glass higher or lower than the glass - explain why

Answer 29

It is lower so waves that travel further than that inside the glass travel faster so the signal arrives together

Question 30

Explain angle refraction and state how it relates to Snells law

Answer 30

The angle of refraction at the interface between two media is governed by snells law N1sineX1 = N2sineX2

Question 31

What is total internal reflection

Answer 31

When a wave hits the crossing of two mediums of different refractive index's at a certain angle the wave is bounced back into the medium from which it came

Question 32

What is the critical angle

Answer 32

The angle which forces a wave to travel along the boarder of two mediums of different refractive index's

Question 33

What is single mode optical fibre

Answer 33

Diameter of 8.3 to 10 microns fairly narrow will propagate typically 1310 or 1550 nm High transmission rate( 50x's more than multimode) Cost more than multimode

Question 34

What is step-index multimode fibre

Answer 34

Made of glass Diameter 50 to 100 microns Multiple paths of light can cause signal distortion at receiving end, resulting in unclear or incomplete data transmission

Question 35

What is plastic optic fibre

Answer 35

Strong and difficult to bend Not suitable for long distance transmission Transmitted very little infra red light

Question 36

What is chromatic dispersion in optical divers

Answer 36

Variation in refractive index with different wavelengths

Question 37

What is multimode dispersion in optical fibres

Answer 37

The single speed in time because the propagation velocity of the optical signal is not the same for all modes More oblique waves travel shorter distances More zigzag waves take longer due to further distance

Question 38

Causes of attenuation in optical fibre

Answer 38

Rayleigh scattering: scattering due to imperfections in fibre material Absorption in fibre material Macro bending : light passing into cladding when bent too much Micro bending: scattering of light due to localised stress on fibre

Question 39

What makes an isotropic radiator antenna

Answer 39

A point source radiating equally in all directions

Question 40

What defines a half wire dipole antenna

Answer 40

Ideal length of half wave dipole is half a wave length The true length is shorter due to wire diameter and capacitive effects Real length is expressed as a correction factor

Question 41

What defines a folded dipole antenna

Answer 41

A dipole with its ends connected It's impedance is about 4x's larger than that of an ordinary dipole and is approximately 300 ohms

Question 42

What defines a monopole antenna

Answer 42

Using a single conductor with the signal applied between conductor and earth Monopole has a typical length of a quarter wavelength