Tells Flashcards

1
Q

What is Bandwidth ?

A

Its a range of freq within a frequency spectrum.

Perfect square wave:
BW = 1/2d Or 1/T

NON Square wave:
BW = 1/d

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2
Q

Whats the Freq ranges for the following?

HF.
VHF.
UHF.
SHF.

A

HF. 3-30Mhz
VHF. 30-300Mhz
UHF. 300-3Ghz
SHF. 3-30Ghz

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3
Q

In the time domain…

By only increasing or reducing the pulse duration (d), will have what effect on the frequency domain….?

A

Increasing (d)
- Will reduce your lobe width/bandwidth

Reducing (d)
- Will increase your lobe width/bandwidth

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4
Q

In the freq domain…

By only increasing or reducing the Lobe width (1/d), will have what effect on the time domain….?

A

Increasing (1/d) - (higher range of freq)
- Will reduce your pulse width

Reducing (1/d) - (lower range of freq)
- Will increase your pulse width

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5
Q

In the time domain..

By only increase or reducing the time period (T), will have what effect on the frequency domain….?

A

Increasing (T)

  • Reduces PRF (1/T)
  • Allowing for more spectral lines/ Harmonics to be present within the lobe

Reducing (T)

  • Increasing PRF(1/T)
  • Allowing for less spectral lines/ Harmonics to be present within the lobe
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6
Q

The frequency domain…

By only increasing or reducing the PRF (1/T), will have what effect on the time domain….?

A

Increasing PRF:
- Reduce the time period (T)

Reducing PRF:
- Increase the time period (T)

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7
Q

What is the PRF

A

Pulse repetition frequency

1/T

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8
Q

What is the formula for S/N ratio in dB for Voltage?

A

20Log10(Vsignal/Vnoise)

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9
Q

What is the formula for S/N ratio in dB for Power?

A

10Log10(Psignal/Pnoise)

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10
Q

What is termed as a complex Waveform?

A

Any wave that consists of more than 2 Sine waves

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11
Q

What is the relationship between Bandwidth and Bitrate?

A

Bitrate = 2 x Bandwidth

OR

Bandwidth = Bitrate / 2

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12
Q

What does a perfect SQUARE wave consist of?

A

The fundamental frequency and all the odd harmonics

Duty cycle = 50% or 1/2 or 0.5

1/T ,1/d, 3/T ,2/d, 5/T

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13
Q

What makes up a NON-Perfect SQUARE wave?

A

The fundamental frequency and all the odd and even harmonics

Duty cycle = 33% or 1/3 or 0.33

1/T , 2/T ,1/d, 4/T , 5/T ,2/d

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14
Q

Name 3 Electrical Characteristics of a Wire/Link ?

A
  • Resistance
  • Inductance
  • Capacitance
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15
Q

What is the formula for your Duty cycle?

A

Duty cycle = d/T

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16
Q

What do we know about the relationship between MARK SPACE ratio and the Duty cycle if

Mark space ratio is 1:4

A

Means the Duty cycle is 1:5

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17
Q

What is a signal prone to, when traveling through a link?

A
  • Distortion
  • Attenuation
  • Niose
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18
Q

S/N difference for comms links

Fibre optics

Attenuation?
Electrical noise pickup?
S/N?

A

Attenuation:
- Very low

Electrical noise pickup:
- None

S/N:
- Excellent

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19
Q

S/N difference for comms links

twisted pair/coax

Attenuation?
Electrical noise pickup?
S/N?

A

Attenuation:
- Moderate

Electrical noise pickup:
- Low

S/N:
- Good

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20
Q

S/N difference for comms links

wireless

Attenuation?
Electrical noise pickup?
S/N?

A

Attenuation:
- Severe over long ranges

Electrical noise pickup:
- No screening

S/N:
- Worst case

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21
Q

Characteristics of a good communications system

Transmitter requirements?

A

Power
Modulation
Bandwidth

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22
Q

Characteristics of a good communications system

Receiver requirements?

A

Selectivity (selecting wanted signals)
Sensitivity (good S/N ratio)
Bandwidth

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23
Q

Advantages of using digital signals compared to analogue

A

Error correction
Remove noise
Encryption

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24
Q

Name a few error correction techniques?

A

Parity bit

Hamming code

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25
How can any repetitive signal be created electronically (Square wave)?
By adding a number of sin waves comprising of: Fundamental freq Harmonics Creating a steep curve with a flat top
26
What is meant by the BASEBAND signal
The original signal that had no amplification or any modulation added, ie. just the original voice signal.
27
What can we include in a PAM to recover the baseband signal / audio signal ?
Include a LPF ( low pass filter)
28
Comment on the following Nyquist rate relating to a Sampled (PAM) signal spectrum Fs < 2.Fm(max) Fs = 2.Fm(max) Fs > 2.Fm(max)
Fs < 2.Fm(max) - Does NOT satisfy Nyquist or practicle Fs = 2.Fm(max) - Satisfies Nyquist but not practical Fs > 2.Fm(max) - Satisfies Nyquist and practical
29
What does Fs need to be to allow no Aliasing when using a LPF? Fs < 2.Fm max Fs = 2.Fm max Fs > 2.Fm max
Fs > 2.Fm max
30
Name 3 solutions to prevent aliasing?
Increase sampling freq (Fs) Use a Band limiting filter (anti-aliasing filter) reduce Fm Improve the quality of receivers LPF
31
Formula to work out LPF component value for Fm
Fc = 1/ 2.Pi.R.C Fc = Fm(max)
32
Explain what causes Aliasing
When the LSB of (Fs) overlaps with the original signal (Fm), causing aliasing inside the LPF
33
Whats is Quantising
Splitting up a waveforms amplitude into levels, and giving each level a binary number
34
What is the biggest Quantising ERROR you can have What is the formula?
Q Error = Q Step size(v)/2
35
What is meant by Quantising noise
Its when a signals voltage is in between a quantising level and has to be rounded up or down
36
How do we work out the no. of Q Levels for a 8 bit code and vice versa? Formula
8 bit code 2 to the power of 8 = 256 (Q levels) Alternatively: Log2(256) = 8 bits
37
What is the Quantising step size(v) formula
Quantising Lv. size(v) = Voltage range(v)/no. of Q Levels
38
How do you work out your Sampling interval(t) for a PCM system? Formula
Sampling interval(t) = 1 / Fs(hz)
39
How do you work out your BIT time(t) on a PCM system? Formula
Bit time(t) = sampling interval(t)/ No. of bits(n)
40
How to work out Bit rate on a PCM system? Formula
Bit rate = 1/ bit time(t) Or Bit rate = Fs(hz) X no. of bits(n)
41
How do you work out your Bandwidth/ Fundamental freq on a PCM/TDM system?
Bandwidth/fund freq = Bitrate(hz)/2
42
For each extra BIT we add to our quantising Lv. How does that effect our Quantising noise?
For each extra BIT we add we get a 6dB improvement in our quantisation Lv. which reduces our quantising noise
43
Describe the process of PAM and PCM?
PAM - Sampled signal which becomes a series of pulses with their amplitudes in line with the signal being sampled PCM - Splitting up a signals voltage into levels, and giving each level a binary number (quantisation added)
44
How does regeneration work on a PCM system?
A regenerative circuit converts a signal above the threshold to a 1 and bellow the threshold to a 0, thus removing noise
45
EXPLAIN whats is meant by a PCM Sync/Framing bit
PCM Sync/Framing is used to separate each channel on a TDM line
46
Explain how NON linear companding works?
Its a non linear quintising method which incodes lower amplitude Lv. with higher accuracy And bigger amplitude Lv. with less accuracy without compromising on the no. of bits used
47
What companding method uses more bandwidth Non linear Linear
Linear Because it includes high accuracy encoding on higher amplitude levels which is not necessarily needed
48
Differences between Linear and NON Linear Companding
Linear: - Levels equally spaced - Quantisation noise is constant throughout Non Linear: - Lv. closer together for smaller amp signals, further apart on higher lv. amp - Quantisation noise is reduced for low amplitudes signals
49
Advantages / Disadvantages between Linear and NON Linear Companding
Linear: A- Simple D- Poor S/N low level amplitudes Non Linear: A- Improved S/N ratio D- Expensive/complicated
50
What is Multiplexing and give an example
Many different channels can be shared over a single link ie. a fibre optic cable This is called multiplexing And a popular technique is called TDM (time devision multiplexing)
51
What is companding mostly used for?
Voice
52
How to work out Sampling interval on a TDM system? Formula
Sampling interval = 1 / Fs (hz)
53
How to work out Channel time on a TDM system? Formula
Channel time(t) = Sampling interval(t) / no. of channels (N)
54
How to work out Bit time(t) on a TDM system? Formula
Bit time(t) = Channel time(t) / no. of bits(n)
55
How to work out Bit rate on a TDM system? Formula
Bit rate = 1/ bit Bit time (t) Or Bit rate = Fs(hz) X no. of bits(n) X no. of Channels (N)
56
LAN is used over what distances?
10-100m
57
WAN is used over what distances?
100s of km
58
Name 2 types of nodes?
- Computers | - Switches/routers
59
List a few LAN topologies?
Point to point - Line fails, the network fails Bus - Line fails, Network splits in two Star - Central Hub fails the hole network fails
60
Name a WAN topology?
Mesh Its ROBUST due to having many Routes across the WAN to find its destination
61
Framing and Packets are used on which types if networks?
LAN WAN
62
How is each and every NODE identifiable on WAN/LAN
Via a MAC ADDRESS 6 bytes
63
Name 6 parts that make up a ETHERNET Frame Format?
Preamble: - Establish sync by allowing Rx clock to sync to the Tx clock (8 bytes) Dest. MAC address: - Physical address of the network adaptor (6 bytes) Source MAC address: - Senders address needed for a response (6 bytes) Type/length: - Protocol that produces data (2 bytes) Message Data: - Data information being sent with padding to insure MIN send (46min - 1500 bytes) CRC (cyclic redundancy check): - Error checking, if CRC check fails frame is discarded (4 bytes)
64
How many bits does a MAC address hold?
48 bits Or 6 Bytes
65
What is the min-max amount of Bytes inside the DATA FIELD?
46-1500 Bytes
66
What does CRC stand for and what is it used for?
CRC- cyclic redundancy code Its used to check the packet/frame has no errors when received Fire and forget packets
67
Explain how PREAMBLE works?
Preamble establishes sync: - Uses 8 bytes - Establish sync with alternating ‘1’ and ‘0’ s - Last two bits being ‘1’ ‘1’ to mark the end of Preamble sync
68
Why and how is Manchester II Coding used?
Manchester II coding - Maintains sync - By combining the clock pulses to the Ethernet bits
69
What is the main difference between PREAMBLE and Manchester II coding?
Preamble - established synchronisation Manchester II coding - maintains synchronisation
70
What does CSMA/CD stand for?
CS- carrier sense - Sense the presence of a carrier signal MA- multiple access - All computers are connected to a transmission line CD- collision detection - Detecting a collision
71
Expand on CS (carrier sense)?
Senses for a carrier signal and waits for it to STOP. In order to become the carrier signal
72
Expand on MA (multiple access)
All the computers/nodes connected to that transmission line
73
Expand on CD (collision detection)
When a collision is detected a jamming signal is sent to the sender to STOP
74
Draw or describe a collision detection ?
1. Station A and B listens 2. A starts to Tx a message 3. Before A’s message arrives at B. B also Tx a message 4. B detects a collision and Tx a jamming signal 5. Station A receives the jamming signal and stops transmitting
75
Give example of cable terminology? 10 BASE T / 5
10 BASE T / 5 - (10) 10/100Mbps - (Base) Baseband signal - (T) Twisted pair - (5)100m intervals = 500 meter
76
What is differential signalling?
The potential noise difference in a TWISTED PAIR cable, which effectively cancels out and rejects noise.
77
How is differential signalling used in twisted pair cables?
a “1” or “0” can be sent over a twisted pair cable using +1V -1V in order to reduce EMI or electronic noise pickup
78
A TRANSMISSION CABLE should have the following desirable characteristics?
- Does NOT Tx - (EMI) electro magnetic interferance | - Does NOT Rx - Electronic noise pickup
79
What is a collision domain and give an example of one?
HUB Any shared link with 2 or more nodes Tx/Rx where a collision can occur
80
What are the differences between ? HUBs Bridges Switches
HUB - Collision domain Bridge - Uses an Ethernet card to manage collisions between networks Switches - Uses an Ethernet card and a Micro Processor to manage collision between all connections
81
How does a switch improve network performance?
By using a micro processor(uP) which delays frames IOT avoid colisions
82
In a switch what appears like a collision could instead be a........?
Delay of frames being stored or held back by the micro processor
83
Draw and name what makes up an basic IP PACKET and expand on SEQUENCE number?
- Source IP address - Destination IP address - SEQUENCE number: Allows the destination to determine whether all the packets have been received - Data - Error Detection
84
An IP address is split in 2 parts, which is ?
Network field Host field
85
Name 3 types of classes when it comes to IP addressing
Class A - Extremely large Network - 8 bits Network - 24 bit Host Class B - Moderate size networks - 16 bit Network - 16 bit Host Class C - Small sized networks - 24 bit Network - 8 bit Host
86
How many bit values does an IP (v4) have
32 bits Or 4 Bytes
87
For automatic IP configuration, what protocol is used and in what type of network For Bowman
DHCP (dynamic host control protocol) LAN BNAU
88
Expand on DNS(Domain name server) is?
A IP address converted to a written address for ease of use googles IP address 0.0.0.0 DNS address www.google.com
89
How is Data sent from a software application over a network, explain the SEQUENCE? How is Data encapsulated?
- Data is created via a Software Application Program - The data is then encapsulated within the Data Field of a IP PACKETS - The hole IP PACKET is then incapsulated in the Data Field of a ETHERNET FRAME within a LAN - Network card (BNAU) extracts the IP PACKET and sends it over the WAN
90
BEC is responsible for what?
Bowman Ethernet Card: Responsible for ALL Switching and Routing inside the BNAU
91
What is DELTA modulation?
Its a type of ADC(analogue to digital conversion) we use with HF and VHF radios Its a Digital O/P that only represents and tracks the rate of change of a analogue signal
92
Name 2 different variations/ types of DELTA modulators?
- Simple Delta modulation | - up-down DELTA modulation
93
Explain how a Simple Delta modulator works?
Simple Delta modulator: - an APPROXIMATING waveform(🔺M) is used as feedback going into a COMPARATOR - The approximating waveform tries to track the INPUT SIGNAL (playing catch-up) - I/P signal > approximating WF = 1 - I/P signal < approximating WF = 0 - The digital O/P from the Comparator is then fed into the Modulator - Which becomes a Delta Modulated signal (1 bit code)
94
Explain how a Up-Down COUNTER Delta modulator works?
Up-Down COUNTER Delta modulator: - Uses a Up-Down Counter staircase generator - Comparator “1” = Produces Positive going steps - Comparator “0” = Produces Negative going steps - Fed into a modulator and producing a Delta Modulated signal
95
What is a Major limitation to DELTA Modulators?
Slope overload distortion: When the rate of change of the I/P signal is too fast for the Approximation waveform
96
Name 2 ways to overcome Slope Overload Distortion?
- Increase the Clock Speed | - Increase the Step size of the Approximating waveform
97
Compare PCM(pulse code modulation) with Delta Modulation?
PCM: - encodes actual signal levels to binary - encode DC levels - Complex Delta Modulation: - Coding only represents the RATE of CHANGE of the signal - Cannot encode DC levels - Simple
98
How is a Delta Modulated Signal Received, Draw diagram ? Precise 3 page 5 (figure. 6)
- The Delta Modulated Signal is received and INTEGRATED - Then passed through a LPF(low pass filter) which smooths the sharp edges - O/P being the original analogue signal
99
Name 2 generic ENCRYPTION schemes?
Block Cipher: - Used in internet browsers to Encrypt Data (Existing block data) DES or AES256 Key Stream Cipher: - Used to Encrypt continuous data like Digital voice ( Real time)
100
What are the essential requirements for a Key Sequence Generator ?
- Must appear random(Pseudo random) | - The Bit stream must be extremely long
101
State the equations for: Enciphering De-ciphering
Enciphering: Ci = Mi + Ki De-ciphering: Mi = Ci + Ki
102
What Modulo 2 Addition is used during encryption?
Xor Gate 0 0 = 0 1 0 = 1 0 1 = 1 1 1 = 0
103
State the Min Length Sequence equation?
M - Sequence = 2 to the power of 4(no. of bits) - 1 4 Stage PRSG (Pseudo Random Sequence Generator) will have 16 possible STATES - 0 0 0 0 will cause a all zero state and LOCK UP - Hence why we (- 1) minus 1 to avoid a lock up state
104
Explain with the aid of a diagram how voice is Encrypted over a VHF radio transmitter? Précis.3 Page 18 (Figure.13)
- Analogue voice is Digitised (Delta modulated)= Mi - PRSG(Pseudo Random Sequence Generator) creates a Key Stream = Ki - Ki + Mi gets encrypted together using a Modulo 2(Xor Gate) inside the Encryption Unit - Ci (cipher txt) is produced and sent over the air via the Radio Transmitter
105
Draw a spectral diagram of an ASK and state the bandwidth? Précis 3 page.25 (Figure.20)
ASK: Amplitude Shift Keying Where the Modulates RF Signal is turned on for a “1” and off for a “0” Bandwidth: BW = Fc + 2/d
106
Draw a spectral diagram of an FSK and state the bandwidth? Précis 3 page.25 (Figure.21)
FSK: Frequency Shift Keying The FSK Carrier has 2 separate Frequencies, a LOWER Freq make up a “1” and a HIGHER Freq make up a “0” Bandwidth: BW = 2🔺F + 2/d
107
Draw a spectral diagram of an BPSK and state the bandwidth? Précis 3 page.26 (Figure.22)
BPSK: Bi-Phase Shift Keying The Bi-PSK Carrier uses the same frequency, the Frequency only phase shifts a 180 degrees giving one phase a “1” and the other a “0” Bandwidth: BW = Fc + 2/d
108
Draw a Signal space (constellation) diagram of the following and their Grey Code: BPSK (Bi-Phase Shift Keying) QPSK (Quad Phase Shift Keying) 8-Phase PSK (8 Phase Phase Shift Keying) Précis 3 page.26-27 (Figure.23-25)
Précis 3 page.26-27 (Figure.23-25)
109
State the Formula to work out your Bit rate for a PSK system?
Bit rate = Bits(bits/sec) x Baud rate(symbols/sec)
110
Compare the following in terms of Bandwidth and S/N ratio ASK FSK PSK
Bandwidth: ASK - Same FSK - Worst PSK - Same S/N Ratio: ASK - Worst FSK - Moderate PSK - Best
111
Draw a Signal space (constellation) diagram of a V.29 and its Grey Code: Précis. 3 Page. 29-30 (Figure.26-Table.6)
Précis. 3 Page. 29-30 (Figure.26-Table.6)
112
Calculate the Bit Rate for the following Schemes at 2400 Baud Rate? BPSK QPSK 8-PHASE PSK V.29
Log2(no of states) = no. Bits Bit Rate = no. Bits x Buad Rate BPSK - 1 Bits = 2400bps QPSK - 2 Bits = 4800bps 8-PHASE PSK - 3 Bits = 7200bps V.29 - 4 Bits = 9600bps
113
Draw and explain a Practical Phase Locked Loop Synthesiser with a detailed description of each block Précis. 3 Page. 35 (Figure.32)
Crystal oscillator: Provides a highly stable fixed frequency Fixed Divider (P): Does not change to scale down the crystal Osc. freq VCO: Fvco(voltage controlled oscillator) Fixed Divider (M): Just a fixed no. to scale down the Fvco going to (N) and to protect its circuitry Variable Divider (N): Just a variable no. to channel select via switches Freq/Phase Comparator: Compares Fref and Fvco frequencies and scales a Dc voltage +/- iot keep the 2 frequencies the same Loop filter: LPF smoothes/filters and ensures freq changes to happen as fast as possible(ideally damped)
114
PLL (Phase Locked Loop) Synthesiser Standard FORMULA :
Fref = Fvco / M.N Or Fvco = Fref. M. N
115
PLL (Phase Locked Loop) Synthesiser: Formula for? Fch (Frequency channel)
Fch = M. Fref Or Fvco = N. Fch
116
PLL (Phase Locked Loop) Synthesiser: Formula for? Number of channels
No. Of Channels = (Nmax - Nmin) + 1 OR No. Of Channels = (Fvcomax - Fvcomin / Fch) + 1
117
FREQUENCY HOPPING Formulas: - No. Of FH Channels - HOP RATE
No. of FH channels (n): - 2 to the power 10(K) no. of bits = 1024 HOP RATE: - Hr = Clock rate(Rc)/ Shift Register length (K) - Hr = Rc/K
118
Power Gain needed to Jam FH (Frequency hopping) channel ?
PG = 10log(n) n = no. Hopping ch (1024)
119
Compare by aid of a diagram the differences between Conventional radio FF(Fixed Frequency) DS-SS (Direct Sequence Spread Spectrum) FF-SS (Frequency Hopping Spread Spectrum Précis. 3 Page. 44 (Figure. 37)
Précis. 3 Page. 44 (Figure. 37)
120
What are the limitation between FH(freq hopping) Radios and DS-SS (Direct Sequence Spread Spectrum)?
Limitations are that the Hop Rate is restricted by the Low Pass Loop in the PLL Synthesiser
121
How is the Data recovered from a DS-SS (Direct Sequence Spread Spectrum) signal?
A process called CORRELATION where the Receiver Demodulator must have the same Pseudo random sequence as the Transmitter