Tells Flashcards
What is Bandwidth ?
Its a range of freq within a frequency spectrum.
Perfect square wave:
BW = 1/2d Or 1/T
NON Square wave:
BW = 1/d
Whats the Freq ranges for the following?
HF.
VHF.
UHF.
SHF.
HF. 3-30Mhz
VHF. 30-300Mhz
UHF. 300-3Ghz
SHF. 3-30Ghz
In the time domain…
By only increasing or reducing the pulse duration (d), will have what effect on the frequency domain….?
Increasing (d)
- Will reduce your lobe width/bandwidth
Reducing (d)
- Will increase your lobe width/bandwidth
In the freq domain…
By only increasing or reducing the Lobe width (1/d), will have what effect on the time domain….?
Increasing (1/d) - (higher range of freq)
- Will reduce your pulse width
Reducing (1/d) - (lower range of freq)
- Will increase your pulse width
In the time domain..
By only increase or reducing the time period (T), will have what effect on the frequency domain….?
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
The frequency domain…
By only increasing or reducing the PRF (1/T), will have what effect on the time domain….?
Increasing PRF:
- Reduce the time period (T)
Reducing PRF:
- Increase the time period (T)
What is the PRF
Pulse repetition frequency
1/T
What is the formula for S/N ratio in dB for Voltage?
20Log10(Vsignal/Vnoise)
What is the formula for S/N ratio in dB for Power?
10Log10(Psignal/Pnoise)
What is termed as a complex Waveform?
Any wave that consists of more than 2 Sine waves
What is the relationship between Bandwidth and Bitrate?
Bitrate = 2 x Bandwidth
OR
Bandwidth = Bitrate / 2
What does a perfect SQUARE wave consist of?
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
What makes up a NON-Perfect SQUARE wave?
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
Name 3 Electrical Characteristics of a Wire/Link ?
- Resistance
- Inductance
- Capacitance
What is the formula for your Duty cycle?
Duty cycle = d/T
What do we know about the relationship between MARK SPACE ratio and the Duty cycle if
Mark space ratio is 1:4
Means the Duty cycle is 1:5
What is a signal prone to, when traveling through a link?
- Distortion
- Attenuation
- Niose
S/N difference for comms links
Fibre optics
Attenuation?
Electrical noise pickup?
S/N?
Attenuation:
- Very low
Electrical noise pickup:
- None
S/N:
- Excellent
S/N difference for comms links
twisted pair/coax
Attenuation?
Electrical noise pickup?
S/N?
Attenuation:
- Moderate
Electrical noise pickup:
- Low
S/N:
- Good
S/N difference for comms links
wireless
Attenuation?
Electrical noise pickup?
S/N?
Attenuation:
- Severe over long ranges
Electrical noise pickup:
- No screening
S/N:
- Worst case
Characteristics of a good communications system
Transmitter requirements?
Power
Modulation
Bandwidth
Characteristics of a good communications system
Receiver requirements?
Selectivity (selecting wanted signals)
Sensitivity (good S/N ratio)
Bandwidth
Advantages of using digital signals compared to analogue
Error correction
Remove noise
Encryption
Name a few error correction techniques?
Parity bit
Hamming code
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
What is meant by the BASEBAND signal
The original signal that had no amplification or any modulation added, ie. just the original voice signal.
What can we include in a PAM to recover the baseband signal / audio signal ?
Include a LPF ( low pass filter)
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
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
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
Formula to work out LPF component value for Fm
Fc = 1/ 2.Pi.R.C
Fc = Fm(max)
Explain what causes Aliasing
When the LSB of (Fs) overlaps with the original signal (Fm), causing aliasing inside the LPF
Whats is Quantising
Splitting up a waveforms amplitude into levels, and giving each level a binary number
What is the biggest Quantising ERROR you can have
What is the formula?
Q Error = Q Step size(v)/2
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
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
What is the Quantising step size(v)
formula
Quantising Lv. size(v) = Voltage range(v)/no. of Q Levels
How do you work out your Sampling interval(t) for a PCM system?
Formula
Sampling interval(t) = 1 / Fs(hz)
How do you work out your BIT time(t) on a PCM system?
Formula
Bit time(t) = sampling interval(t)/ No. of bits(n)
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)
How do you work out your Bandwidth/ Fundamental freq on a PCM/TDM system?
Bandwidth/fund freq = Bitrate(hz)/2
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
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)
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
EXPLAIN whats is meant by a PCM Sync/Framing bit
PCM Sync/Framing is used to separate each channel on a TDM line
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
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
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
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
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)
What is companding mostly used for?
Voice
How to work out Sampling interval on a TDM system?
Formula
Sampling interval = 1 / Fs (hz)
How to work out Channel time on a TDM system?
Formula
Channel time(t) = Sampling interval(t) / no. of channels (N)
How to work out Bit time(t) on a TDM system?
Formula
Bit time(t) = Channel time(t) / no. of bits(n)
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)
LAN is used over what distances?
10-100m
WAN is used over what distances?
100s of km
Name 2 types of nodes?
- Computers
- Switches/routers
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
Name a WAN topology?
Mesh
Its ROBUST due to having many Routes across the WAN to find its destination
Framing and Packets are used on which types if networks?
LAN
WAN
How is each and every NODE identifiable on WAN/LAN
Via a MAC ADDRESS
6 bytes
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)
How many bits does a MAC address hold?
48 bits
Or
6 Bytes
What is the min-max amount of Bytes inside the DATA FIELD?
46-1500 Bytes
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
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
Why and how is Manchester II Coding used?
Manchester II coding
- Maintains sync
- By combining the clock pulses to the Ethernet bits
What is the main difference between PREAMBLE and Manchester II coding?
Preamble
- established synchronisation
Manchester II coding
- maintains synchronisation
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
Expand on CS (carrier sense)?
Senses for a carrier signal and waits for it to STOP.
In order to become the carrier signal
Expand on MA (multiple access)
All the computers/nodes connected to that transmission line
Expand on CD (collision detection)
When a collision is detected a jamming signal is sent to the sender to STOP
Draw or describe a collision detection ?
- Station A and B listens
- A starts to Tx a message
- Before A’s message arrives at B. B also Tx a message
- B detects a collision and Tx a jamming signal
- Station A receives the jamming signal and stops transmitting
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
What is differential signalling?
The potential noise difference in a TWISTED PAIR cable, which effectively cancels out and rejects noise.
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
A TRANSMISSION CABLE should have the following desirable characteristics?
- Does NOT Tx - (EMI) electro magnetic interferance
- Does NOT Rx - Electronic noise pickup
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
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
How does a switch improve network performance?
By using a micro processor(uP) which delays frames IOT avoid colisions
In a switch what appears like a collision could instead be a……..?
Delay of frames being stored or held back by the micro processor
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
An IP address is split in 2 parts, which is ?
Network field
Host field
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
How many bit values does an IP (v4) have
32 bits
Or 4 Bytes
For automatic IP configuration, what protocol is used and in what type of network
For Bowman
DHCP (dynamic host control protocol)
LAN
BNAU
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
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
BEC is responsible for what?
Bowman Ethernet Card:
Responsible for ALL Switching and Routing inside the BNAU
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
Name 2 different variations/ types of DELTA modulators?
- Simple Delta modulation
- up-down DELTA modulation
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)
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
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
Name 2 ways to overcome Slope Overload Distortion?
- Increase the Clock Speed
- Increase the Step size of the Approximating waveform
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
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
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)
What are the essential requirements for a Key Sequence Generator ?
- Must appear random(Pseudo random)
- The Bit stream must be extremely long
State the equations for:
Enciphering
De-ciphering
Enciphering:
Ci = Mi + Ki
De-ciphering:
Mi = Ci + Ki
What Modulo 2 Addition is used during encryption?
Xor Gate
0 0 = 0
1 0 = 1
0 1 = 1
1 1 = 0
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
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
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
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
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
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)
State the Formula to work out your Bit rate for a PSK system?
Bit rate = Bits(bits/sec) x Baud rate(symbols/sec)
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
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)
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
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)
PLL (Phase Locked Loop) Synthesiser
Standard FORMULA :
Fref = Fvco / M.N
Or
Fvco = Fref. M. N
PLL (Phase Locked Loop) Synthesiser:
Formula for?
Fch (Frequency channel)
Fch = M. Fref
Or
Fvco = N. Fch
PLL (Phase Locked Loop) Synthesiser:
Formula for?
Number of channels
No. Of Channels = (Nmax - Nmin) + 1
OR
No. Of Channels = (Fvcomax - Fvcomin / Fch) + 1
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
Power Gain needed to Jam FH (Frequency hopping) channel ?
PG = 10log(n)
n = no. Hopping ch (1024)
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)
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
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
