#5: Transmission Lines & Fibre Optic Flashcards
Why do different coaxial terminations have specific impedances?
Source and load impedances must be matched to prevent reflections.
What does UTP stand for? Give an example of a UTP cable.
Unshielded Twisted Pair - e.g. cat 5e.
Why are there two pairs in UTP? Why are they twisted?
UTP uses differential encoding (i.e. difference in voltage used to represent change in state). Therefore, one core for +1V, and another for -1V. Twisted to reduce inteference.
Maximum power is transferred when the load impedance _____ the source impedance. However, in this case, the efficiency is __%.
equals; 50%
What causes reflection in a line?
Unmatched source/load/cable impedances.
How might one prevent reflection?
Z(load) = Z(cable) = Z(source)
What is Time Domain Reflectonomy?
Uses pulse reflections to detect location of a break in a cable.
What is the formula for the voltage of a reflected waveform?
V(reflected) = [(load impedance - cable impedance) / (load impedance + cable impedance)] * V(incident)
A reflected wave is _____ of _____ with the incident wave.
Out of phase
In an open-circuit, the reflected wave is (in / out) of phase with the incident wave.
In phase
What is a standing wave, and how can it happen?
If two waves of identical frequency move in an opposite direction to each other, they can add on to each other and create a wave that doesn’t really propagate; it just oscillates from up and down.
Why are standing waves dangerous in a telecommunications setting?
The high peaks (where both waves are added together) generates dangerously high voltages/currents that can damage equipment and cable.
What is CSMA/CA, and how does it work?
Carrier Sense, Multiple Access, Collision Avoidance. Used in wireless networks; devices must request a timeslot to ‘talk’; other devices expected to honour it.
What are the standard equations for an incident + reflected waveform travelling along a transmission line?
Why is the ‘wait-for-ACK’ concept inefficient?
Throughput is limited by network delay (how long it takes for a bit to travel to the router) rather than bandwidth.
Why can the impedance of a line (at high frequency) be said to equal 0? Explain; include a diagram + equation of characteristic impedance.
If we take a small section of line, its impedance is made of an inductor and capacitor in parallel. Through some nifty maths, the line impedance is found to be independant of length and frequency, and dependant on a ratio of its inductance/capacitance:
Z = sqrt(L / C)
What is the ISM band in the context of wireless transmission? What are its advantages/disadvantages?
Industrial, Scientific & Medical band.
- no license required
- lots of other users + interference (i.e. microwaves)
- anyone can listen (security concern)
- not standardized internationally
- power restrictions in some countries.
Why is ADSL’s downlink bigger than its uplink?
People use more downlink (e.g. watching videos, looking at websites, etc) than uplink.
What is the ‘VSWR’? Explain; include its equation.
Voltage Standing Wave Ratio. Max/min voltage levels of reflected waveform/
What does this symbol mean: ‘𝞀’. Include both equations (load/impedance resistance + incident wave).
Reflection co-efficient. Ratio of original (incident) wave to reflected wave.
Rowe = V(incident) / V(reflected)
What would a 𝞀 = 0 scenario mean? What would the VSWR mean, and what would it say about the load/source impedance ratio?
No reflection - all power used by load.
VSWR = 1
Z(load) = Z(characteristic impedance of line)
What is the ‘Return Loss’? Include its equation. What value do we want it to be?
ratio of reflected power to incident power.
We want it as high as possible (i.e. rowe = 0, no reflection)
What is ‘DCT’ in the context of image compression?
Discrete Cosine Transform. A type of discrete fourier transform that seperate the high and low frequency parts of image blocks (8x8 pixels). It then discards unecessary high frequency elements.
