Aero 250 Flashcards
Wavespeed, c
fxlambda
Angular frequency, w
2πf
Difficulty of transmitting radio waves
Antenna must be similar size to wavelength
Channel
Transmission medium conveying the electrical signal. There’s two types: wired and unwired.
Distortion
Undesirable change in a signal that disappears when the signal is turned off.
Interference
Contamination of the channel by external signals.
Noise
Random, unpredictable and undesirable electrical signal from natural sources.
Transmitter
Modifies the input signal in order to cope with the limitations imposed by the channel.
Receiver
Processes the received signal by reversing the signal modifications made at the transmitter.
Analogue message
Data that varies continuously and smoothly over a continuous range of time.
Digital message
An ordered combination of finite symbols.
Reasons digital has replaced analogue
- Enhanced immunity to noise and interference. 2. Regenerative repeater stations can be set up for long-distance transmission.
SNR definition
The ratio of signal power to noise power. A certain min SNR is required at the receiver for successful communication.
SNR equation
SNR(dB) = 10Log(Ps/Pn)
Signal bandwidth, B, definition
The maximum range of frequencies a signal occupies; the amount of frequency needed to sustain the unmodified signal.
Signal bandwidth equation
B = fmax - fmin
Baseband bandwidth
The range of frequencies from zero to the highest frequency present in a signal.
Noise power
Equals the variance of the noise.
Channel bandwidth
The range of frequencies that a communication channel can transmit with reasonable accuracy. It limits the signal bandwidth that can pass through.
Channel capacity, C, definition
The amount of information that can be reliably transferred. In a digital system, this is the number of error-free bits that can be transferred per second.
Channel capacity, C, equation
C = BcLog2(1+SNR) = BcLog2(1+(P/N0Bc))
Features of a good communication system
High C, high SNR, low Ps, low Bs
Modulation
Uses the baseband signal to modify amplitude, phase or frequency of the carrier signal generated by the transmitter. Allows for the simultaneous transmission of multiple signals.
Demodulation
The opposite of modulation. The baseband signal is reconstructed at the receiver.
Amplitude modulation with suppressed carrier
Carrier signal, c(t) = cos(Wct), baseband message signal = m(t), so modulated signal, s(t) = m(t)c(t). The demodulator extracts m(t) from s(t). Also called DSB-SC.
Low-pass filter
Removes the high frequency component of a signal, usually around 2wc.
Amplitude modulation with large carrier
Modulated signal, s(t) = AcCos(wct) + kam(t)AcCos(wt). Also called DSB-LC.
Modulation factor, mu
mu = kaAm = Am/A
Envelope detection
An AM demodulation technique
Condition for envelope detection
0 < mu < 1
Upper sideband
Spectral content lies outside of the carrier frequency.
Lower sideband
Spectral content lies within the spectral frequency.
DSB-LC total power
Ptotal = Pcarrier + Plsb + Pusb
DSB-SC total power
Ptotal = Plsb + Pusb
Carrier power
Pcarrier = 0.5(Ac)^2
Relationship between Plsb and Pusb
They’re equal.
Power efficiency
nu = Ps/(Pc + Ps), where Ps is sideband power and Pc is carrier power.
Comparison of DSB-LC and DSB-SC
DSB-LC is less power efficient since the carrier components doesn’t convey any information. However, DSB-SC requires a more complex demodulator.
DSB disadvantage
DSB doubles the baseband bandwidth, which becomes a disadvantage for crowded frequency bands.
Single sideband, SSB
Only transmits either the upper or lower pair of sidebands. Requires energy gap at low frequencies.
SSB total power
Ptotal = Pusb or Ptotal = Plsb
SSB generation
Generate a DSB-SC signal, the suppress one of the sidebands by filtering.
SSB advantages
Saves power, and reduces the bandwidth by 50%.
SSB disadvantages
Complex circuits required for frequency stability and filtering.
Vestigial sideband, VSB
For signals with little/no energy gap at low frequencies, VSB transmits a pair of sidebands, plus a small amount of the other pair of sidebands.
If a generalised signal is Acos(wct), then what’s the generalised phase?
Generalised phase is wct = thetai(t)
Instantaneous angular frequency
wi(t) = dthetai(t)/dt
Power of an angle modulated signal
(A^2)/2
Max phase deviation
delta theta = (max(thetai(t) - wct) - min(thetai(t) - wct))/2
Max frequency deviation
delta w = (max(wi(t) - wc) - min(wi(t) - wc))/2
Bandwidth of an angle modulated signal
Infinite
Carson’s rule (angle modulation bandwidth)
Banglemod = 2(deltaf + B) = 2B(beta + 1), where beta = =deltaf/B
Zero-crossing demodulation
Only uses points of the modulated signal where it crosses zero.
Sample rate, fs
fs = 1/Ts, where Ts is how often the value of the signal is measured.
What are the three types of sampling?
Critical sampling, over-sampling and under-sampling.
Critical sampling
fs = 2B
Over-sampling
fs > 2B. Easy reconstruction, but generates more than necessary number of samples.
Under-sampling
fs < 2B. Data-reduction technique. Expensive. Normally an anti-aliasing filter is used.
Nyquist rate
2B
Aliasing
Spectral overlap that occurs in under-sampling.
How can a signal be recovered from its sample?
Use an ideal low-pass filter.
Quantisation
Maps samples of a continuous amplitude waveform to a finite set of amplitudes. This introduces quantisation noise.
Pulse amplitude modulation
Varies the amplitude of a constant width, constant position pulse, according to the position of the sample of the analogue signal.
Pulse position modulation
Varies the position of a constant width pulse within a prescribed time slot according to the amplitude of the sample of the analogue signal.
Pulse width modulation
Varies the width of a constant amplitude pulse, proportional to the amplitude of the analogue signal at the time the signal is sampled.
Line coding
Converts the bit stream produced by a source encoder to electrical pulses.
On-off line coding
1 is transmitted as a positive pulse, 0 by no pulse.