Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
67 Cards in this Set
- Front
- Back
What is Nyquists Sampling Theorem |
A signal can be reconstructed without error when sampled at the Nyquist Rate |
|
What is AntiAliasing Filtering |
A filter located before the systems sampling circuit. It limits the frequencies in the pre sampled signal to the folding frequency 2 Folding Frequency fH <= fS / 2To minimise aliasing the filter must have a sufficiently rapid roll-off and sufficient attenuation above the city off frequency. 2To minimise aliasing the filter must have a sufficiently rapid roll-off and sufficient attenuation above the city off frequency. To minimise aliasing the filter must have a sufficiently rapid roll-off and sufficient attenuation above the city off frequency. |
|
Continuous signal, PAM signals, PCM signal diagrams. |
|
|
Signal-to-quantisation noise ratio (linear quantization, uniform pdf) -Average Signal Power -Quantization Noise Power -Signal to quantization noise ratio |
|
|
Signal-to-quantisation noise ratio (linear quantization, uniform pdf) |
|
|
Signal-to-quantisation noise ratio (linear quantization, non-uniform pdf) |
|
|
Why is non-linear quantization / commanding used? |
When the pdf of an analogue signal is not uniform, we should ensure that the most frequently used quantization levels introduce the least quantization noise. |
|
What does Non-linear commanding do |
Creates a uniform pdf and optimum SNqR isbthe signal pdf is known.
Creates a constant SNqR if the signal pdf is unknown. |
|
What is the A law graph and what do the bits do |
Using the 8 bits for a given amplitude and the A-law graph an output amplitude can be found. The bit sequence is A-law encoded and can be quantized to send as PCM data For 8 but commanding: 1 bit gives the sign 3 bits specify the segment 4 bits give a location on the segment. |
|
How is non-linear quantization used for speech |
Speech has an unpredictable mean amplitude so this is used to fix the SNqR. |
|
How does centre point detection work |
Finds the bit by taking a measurement at the center of the bit and then compares this value to a reference value. |
|
Generic probability equation for noise |
|
|
Unipolar rectangular pulse signal equations |
|
|
Polar rectangular pulse signal equations |
|
|
Unipolar baseband probability equations |
|
|
Polar baseband probability equation |
|
|
Probability equation for multi level baseband signals |
|
|
What is the Shannon-hartley capacity theorem |
|
|
Multi-hop link with linear amplifiers diagram and probability equation |
|
|
Multi-hop link with regenerative repeater diagram and probability equation |
|
|
What do regenerative repeaters do |
They perform a demodulation to decouple noise between sections. |
|
Why are line codes used |
|
|
What criteria are line codes selected for |
-Presence or Absence of a DC level -Power spectral density -Spectral occupancy (BW size) -Noise immunity -Transparency -Easy of clock recovery for symbol timing -Inherent error detection properties |
|
Advantages of Unipolar signals |
-Require only a single positive supply voltage |
|
Disadvantages of Unipolar signals |
- The DC component is wasted power -Requires AGC at its receiver to ensure correct threshold detection -Cannot be used with AC coupled lines - The signal is distorted due to forcing of spectral density to 0 at DC. |
|
What is the distortion due to AC coupling of Unipolar signals |
Sending a long chain of 1s results in charge developing on the capacitor. This results in signal droop. |
|
What is Bipolar signalling / AMI |
It is a Pseudo-ternary format that denotes a binary 0 as zero volts and a binary 1 as +V and -V alternating. |
|
Advantages of Bipolar signals |
It ensures a DC null in the signal spectrum thus allowing AC coupling It prevents a long term DC build up due to many 1s, thus avoiding signal droop. |
|
Disadvantages of Bipolar signals |
Reduces the signal bandwidth Long strings of 0s may result in the loss of receiver clock synchronisation. |
|
What is HDB3 |
A form of Bipolar RZ signalling where a string of four 0s is substituted with a 0001 or 1001, where the last mark violates the AMI rules allowing the code word to be detected. |
|
What is nBmT coding |
Where n binary symbols are mapped to m ternary symbols. This increases the length of transmitted symbols and decreases the bandwidth of the resulting ternary signal. |
|
What is Coded mark inversion |
It uses a dipolar pulse for a digital 0 and a NRZ bipolar pulse for a digital 1. |
|
What is the Spectral Efficiency equation |
|
|
Why filter at the transmitter |
Because rectangular pulses have a sinc spectrum which has a large bandwidth. |
|
What is an ISI free signal |
A signal which passes through zero at all but one sampling instant. |
|
What is Nyquists Vestigial Symmetry Theorem |
|
|
Why do we not use sinc pulse signalling |
-Sinc goes to infinity on both ends
-Has no ISI
-We only need a small timing error before ISI occurs as sinc pulses have steep sides
-Unrealistic demands on symbol timing to do a good job. |
|
What is Raised Cosine filtering and what does it allow us to do |
A modification of th3 frequency response of a brick wall filter where delta f of excess bandwidth is taken from the edges of a rectangular wave and added to the sides.
It is used to avoid ISI |
|
How do we find the bandwidth of a raised cosine filter |
|
|
What is the bandwidth of the modulated raised cosine filter |
|
|
What is traded off during raised cosine filtering |
Excess bandwidth is traded off for more rapidly decaying sidelobes in the received pulses. |
|
How can centre point detection be improved upon |
We can take multiple samples of a single pulse, this will help solve the issue of noise. |
|
What is the integrate and dump method of detection for rectangular pulses |
An integration over one pulse period is performed. This gives an integration value to evaluate. The integrator can be made easily in analogue using a resistor and capacitor. The value on the integrator is then dumped by shorting the switch/capacitor, thus resetting the value to zero. This method makes the effect of noise miniscule. |
|
What does an integrate and dump detection graph look like |
|
|
What is the circuit realisation of the integrate and dump detection |
|
|
What is Matched Filtering |
|
|
What are the mathematical representations of a matched filter in the frequency and time domains |
|
|
How does matched filtering work in the time domain |
The impulse response of the filter is a time reversed version of the pulse to which it is matched, it is then delayed by a time equal to the duration of the pulse. |
|
What is the output of the matched filter |
This is the autocorrelation of the expected symbol pulse. |
|
What is correlation |
|
|
What is the signal power and noise power of a correlate at sampling instant To |
Signal Power: Es ^2
Noise Power: ( N0 / 2 ) Es |
|
What is the sampling instant SNR of a correlator |
S/N = 2Es / N0 |
|
Probability of error for a single channel binary-symbol correlator |
Where p = 0 for unipolar signals And P = -1 for polar signals |
|
What is the formula for the sampling instant voltage difference |
|
|
What is the formula for the noise power at the matched filter output |
|
|
What is the formula for the rms noise voltage at the filter output |
|
|
Differences between matched filtering and correlation detection |
Their output waveforms
Their sensitivity to timing errors, with timing errors we could easily go from positive to negative. |
|
What are eye diagrams and what are they used for |
These are used to see if everything is working at the put it.
The eye diagram has many overlayed pulses / data symbols and the colour of said pulses determine their probability.
They can be used to determine: -ISI -The optimum sampling time -Jitter - The noise margin |
|
What is the linear quantization of a PAM signal |
It is a quantization that converts an analogue signal to a digital signal. |
|
What do the pdf graphs of analogue and digital signals look like |
|
|
What is the relationship between PAM, quantized PAM, PCM signal |
|
|
What are the formulas to the peak signal-to-quantisation noise ratio for a linear quantization and uniform pdf |
|
|
What is the probability of error for a binary polar NRZ signal |
|
|
What are the formulas for quantization noise and detection noise for a PCM decoder output |
|
|
What is the input and output SNR graph for PCM |
|
|
What are the diagrams of the probability degradation due to multiple hops |
|
|
What are the pdf graphs of binary symbols, noise and signal plus noise |
|